AS1768-2007 - Lightning Protection

AS/NZS 1768:2007

AS/NZS 1768:2007

Australian/New Zealand Standard™ Lightning protection

For DOD Internal Use ONLY

AS/NZS 1768:2007 This Joint Australian/New Zealand Standard was prepared by Joint Technical Committee EL-024, Protection Against Lightning. It was approved on behalf of the Council of Standards Australia on 13 September 2006 and on behalf of the Council of Standards New Zealand on 6 October 2006. This Standard was published on 10 January 2007.

The following are represented on Committee EL-024: Association of Consulting Engineers Australia Australasian Corrosion Association Australasian Railway Association Australian Chamber of Commerce and Industry Australian Electrical and Electronic Manufacturers Association Australian Institute of Petroleum Ltd Bureau of Meteorology CSIRO Industrial Physics Department of Defence (Australia) Department of Natural Resources and Mines (QLD) Department of Primary Industries, Mine Safety (NSW) Energy Networks Association Engineers Australia ITU NSG5 Master Builders Australia Ministry of Economic Development (New Zealand) National Electrical and Communications Association Telstra Corporation Limited The University of Queensland Transpower New Zealand UniQuest

Keeping Standards up-to-date Standards are living documents which reflect progress in science, technology and systems. To maintain their currency, all Standards are periodically reviewed, and new editions are published. Between editions, amendments may be issued. Standards may also be withdrawn. It is important that readers assure themselves they are using a current Standard, which should include any amendments which may have been published since the Standard was purchased. Detailed information about joint Australian/New Zealand Standards can be found by visiting the Standards Web Shop at www.standards.com.au or Standards New Zealand web site at www.standards.co.nz and looking up the relevant Standard in the on-line catalogue. Alternatively, both organizations publish an annual printed Catalogue with full details of all current Standards. For more frequent listings or notification of revisions, amendments and withdrawals, Standards Australia and Standards New Zealand offer a number of update options. For information about these services, users should contact their respective national Standards organization. We also welcome suggestions for improvement in our Standards, and especially encourage readers to notify us immediately of any apparent inaccuracies or ambiguities. Please address your comments to the Chief Executive of either Standards Australia or Standards New Zealand at the address shown on the back cover.

This Standard was issued in draft form for comment as DR 06132.


AS/NZS 1768:2007

Australian/New Zealand Standard™ Lightning protection

Originated in Australia as MC1—1969. Originated in New Zealand as NZS/AS 1768:1991. Previous edition AS/NZS 1768(Int):2003. This edition AS/NZS 1768:2007.

COPYRIGHT © Standards Australia/Standards New Zealand All rights are reserved. No part of this work may be reproduced or copied in any form or by any means, electronic or mechanical, including photocopying, without the written permission of the publisher. Jointly published by Standards Australia, GPO Box 476, Sydney, NSW 2001 and Standards New Zealand, Private Bag 2439, Wellington 6020 ISBN 0 7337 7967 0


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PREFACE This Standard was prepared by the Joint Standards Australia/Standards New Zealand Committee EL-024, Protection against Lightning, to supersede AS/NZS 1768(Int):2003, Lightning protection. This Standard is intended to provide authoritative guidance on the principles and practices of lightning protection for a wide range of structures and systems. It is not intended for mandatory application but, if called up in a contractual situation, compliance with this Standard requires compliance with all relevant clauses of the Standard such that the level of protection will be sufficient to achieve a tolerable level of risk as determined by the risk calculation. In general, it is not economically possible to provide total protection against all the possible damaging effects of lightning, but the recommendations in this Standard will reduce the probability of damage to a calculated acceptable level, and will minimize any lightning damage that does occur. Guidance is given on methods of enhancing the level of protection against lightning damage, if this is required in a particular situation. Where a new structure is to be erected, the matter of lightning protection should be considered in the planning stage, as the necessary measures can often be affected in the architectural features without detracting from the appearance of the building. In addition to the aesthetic considerations, it is usually less expensive to install a lightning protection system during construction than afterwards. The decision to provide lightning protection may be taken without carrying out a risk assessment or regardless of the outcome of any risk assessment, for example, where there is a desire that there be no avoidable risk. Any decision not to provide lightning protection should only be made after considering the advice provided in this Standard. Where doubt exists as to the need for lightning protection, further advice should be sought from a lightning protection designer or installer. Unless it has been specified that lightning protection must be provided, the first decision to make is whether the lightning protection is needed. Section 2 provides guidance to assist in this decision. Section 3 provides advice on the protection of persons from lightning, mainly relating to the behaviour of persons when not inside substantial buildings. Once a decision is made that lightning protection is necessary, Section 4 provides details on interception lightning protection for the building or structure. This includes information on the size, material, and form of conductors, the positioning of air terminals and downconductors, and the requirements for earth terminations. Persons and equipment within buildings can be at risk from the indirect effects of lightning and Section 5 gives recommendations for the protection of persons and equipment within buildings from the effects of lightning. Section 6 describes methods of lightning protection of various items not covered in earlier sections, such as communications antennas, chimneys, boats, fences, and trees. A clause is included on methods for protecting domestic dwellings and assorted structures in public places, where a complete protection system may not be justified, but some protection is considered desirable. Section 7 sets out recommendations for the protection of structures with explosive or highly-flammable contents. Section 8 gives advice on precautions to be taken during installation, inspecting, testing, and maintaining lightning protection systems. A number of appendices are included that provide additional information and advice. The appendices form an integral part of this Standard unless specifically stated otherwise. i.e. appendices identified as ‘informative’ only provide supportive or background information and are therefore not an integral part of this Standard.


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CONTENTS Page SECTION 1 SCOPE AND GENERAL 1.1 SCOPE ........................................................................................................................ 5 1.2 APPLICATION ........................................................................................................... 5 1.3 INTRODUCTION ....................................................................................................... 5 1.4 REFERENCED DOCUMENTS .................................................................................. 6 1.5 DEFINITIONS ............................................................................................................ 6 SECTION 2 ASSESSMENT AND MANAGEMENT OF RISK DUE TO LIGHTNING — ANALYSIS OF NEED FOR PROTECTION 2.1 INTRODUCTION ..................................................................................................... 11 2.2 SCOPE OF SECTION ............................................................................................... 11 2.3 CONCEPT OF RISK ................................................................................................. 12 2.4 DAMAGE DUE TO LIGHTNING ............................................................................ 13 2.5 RISKS DUE TO LIGHTNING .................................................................................. 17 2.6 PROCEDURE FOR RISK ASSESSMENT AND MANAGEMENT ......................... 21 2.7 RISK MANAGEMENT CALCULATION TOOL..................................................... 23 SECTION 3 PRECAUTIONS FOR PERSONAL SAFETY 3.1 SCOPE OF SECTION ............................................................................................... 28 3.2 NEED FOR PERSONAL PROTECTION.................................................................. 28 3.3 PERSONAL CONDUCT........................................................................................... 29 3.4 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING ...... 31 SECTION 4 PROTECTION OF STRUCTURES 4.1 SCOPE OF SECTION ............................................................................................... 32 4.2 PROTECTION LEVEL ............................................................................................. 32 4.3 LPS DESIGN RULES ............................................................................................... 32 4.4 ZONES OF PROTECTION FOR LIGHTING INTERCEPTION .............................. 34 4.5 METHODS OF PROTECTION................................................................................. 42 4.6 MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION ................ 44 4.7 MATERIALS ............................................................................................................ 47 4.8 FORM AND SIZE OF CONDUCTORS.................................................................... 51 4.9 JOINTS...................................................................................................................... 52 4.10 FASTENERS............................................................................................................. 52 4.11 AIR TERMINALS..................................................................................................... 53 4.12 DOWNCONDUCTORS ............................................................................................ 55 4.13 TEST LINKS............................................................................................................. 58 4.14 EARTH TERMINATIONS........................................................................................ 58 4.15 EARTHING ELECTRODES ..................................................................................... 59 4.16 METAL IN AND ON A STRUCTURE..................................................................... 61 SECTION 5 PROTECTION OF PERSONS AND EQUIPMENT WITHIN BUILDINGS 5.1 SCOPE OF SECTION ............................................................................................... 66 5.2 NEED FOR PROTECTION....................................................................................... 66 5.3 MODES OF ENTRY OF LIGHTNING IMPULSES ................................................. 66 5.4 GENERAL CONSIDERATIONS FOR PROTECTION ............................................ 69 5.5 PROTECTION OF PERSONS WITHIN BUILDINGS ............................................. 70 5.6 PROTECTION OF EQUIPMENT ............................................................................. 73


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Page SECTION 6 PROTECTION OF MISCELLANEOUS STRUCTURES AND PROPERTY 6.1 SCOPE OF SECTION ............................................................................................... 90 6.2 STRUCTURES WITH ANTENNAS......................................................................... 90 6.3 STRUCTURES NEAR TREES ................................................................................. 90 6.4 PROTECTION OF TREES........................................................................................ 91 6.5 CHIMNEYS, METAL GUY-WIRES OR WIRE ROPES .......................................... 91 6.6 PROTECTION OF MINES ....................................................................................... 92 6.7 PROTECTION OF BOATS....................................................................................... 94 6.8 FENCES .................................................................................................................... 97 6.9 MISCELLANEOUS STRUCTURES ........................................................................ 97 6.10 PROTECTION OF HOUSES AND SMALL BUILDINGS ....................................... 99 6.11 PROTECTION OF METALLIC PIPELINES .......................................................... 100 SECTION 7 PROTECTION OF STRUCTURES WITH EXPLOSIVE OR HIGHLYFLAMMABLE CONTENTS 7.1 SCOPE OF SECTION ............................................................................................. 101 7.2 GENERAL CONSIDERATIONS............................................................................ 101 7.3 AREAS OF APPLICATION ................................................................................... 102 7.4 EQUIPMENT APPLICATION................................................................................ 102 7.5 SPECIFIC OCCUPANCIES .................................................................................... 104 SECTION 8 INSTALLATION AND MAINTENANCE PRACTICE 8.1 WORK ON SITE..................................................................................................... 109 8.2 INSPECTION .......................................................................................................... 109 8.3 TESTING ................................................................................................................ 109 8.4 RECORDS............................................................................................................... 110 8.5 MAINTENANCE .................................................................................................... 110 APPENDICES A EXAMPLES OF LIGHTNING RISK CALCULATIONS ...................................... 111 B THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION ........................................................................................................ 133 C NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF EARTH IMPEDANCE .......................................................................................................... 145 D THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND REQUISITE SEPARATION DISTANCES FOR ISOLATION OF A LIGHTNING PROTECTION SYSTEM ........................................................................................ 165 E EARTHING AND BONDING ................................................................................ 174 F WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OF EQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TO LIGHTNING ............................... 182 G REFERENCED DOCUMENTS .............................................................................. 186


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STANDARDS AUSTRALIA/STANDARDS NEW ZEALAND Australian/New Zealand Standard Lightning protection

SECT ION

1

SCOPE

AND

GENERA L

1.1 SCOPE This Standard sets out guidelines for the protection of persons and property from hazards arising from exposure to lightning. The recommendations specifically cover the following applications: (a)

The protection of persons, both outdoors, where they may be at risk from the direct effects of a lightning strike, and indoors, where they may be at risk indirectly as a consequence of lightning currents being conducted into the building.

(b)

The protection of a variety of buildings or structures, including those with explosive or highly-flammable contents, and mines.

(c)

The protection of sensitive electronic equipment (e.g. facsimile machines, modems, computers) from overvoltages resulting from a lightning strike to the building or its associated services.

The nature of lightning and the principles of lightning protection are discussed and guidance is given to assist in a determination of whether protective measures should be taken. This Standard is applicable to conventional lightning protection systems (LPSs) that comprise air terminals, downconductors, earth termination networks and surge protective devices (SPDs). Nothing contained within this Standard either endorses or implies the endorsement of non-conventional LPSs that comprise air terminals that claim enhanced performance or downconductors that claim enhanced magnetic screening over conventional systems. The performance of such systems is outside the scope of this Standard. Irrespective of claimed performance, air terminals shall be placed in accordance with Section 4 to comply with this Standard. 1.2 APPLICATION This Standard does not override any statutory requirements but may be used in conjunction with such requirements. Compliance with the recommendations of this Standard will not necessarily prevent damage or personal injury due to lightning but will reduce the probability of such damage or injury occurring. 1.3 INTRODUCTION Thunderstorms are natural phenomena and there are no proven devices and methods capable of preventing lightning flashes. Direct and nearby cloud-to-ground lightning discharges can be hazardous to persons, structures, installations and many other things in or on them. Consideration should always be given to the application of lightning protection measures.

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Realization that it is possible to provide effective protection against lightning began with Franklin and for over a hundred years national and international manuals and standards have been developed to provide guidance on the principles and practice of lightning protection. Until about ten years ago, risk assessment was used to determine if there was a need to provide lightning protection. However, the modern approach is that of risk management, which integrates the determination of the need for protection with the selection of adequate protection measures to reduce the risk to a tolerable level. This selection takes into account both the efficiency of the measures and the cost of their provision. In the risk management approach, the lightning threats that create risk are identified, the frequencies of all risk events are estimated, the consequences of the risk events are determined, and if these are above a tolerable level of risk, protection measures are applied to reduce the risk (R) to below the tolerable level (Ra). This involves a choice from a range of protection level efficiencies for protection against direct (d) strikes to the structure and decisions about the extent of other measures for protecting low-voltage and electronic equipment against indirect (i) lightning stresses incident from nearby strikes. In summary— R = ∑ R x = ∑ Rd + ∑ R i Rx = Nx Px δx Px = kx px R ≤ Ra where N x is the frequency of dangerous events, P x is the probability of damage or injury, δ x is the relative amount of damage or injury with any consequential effects, and k x is a reduction factor associated with the protection measure adopted and which equals 1 in the absence of protection measures when P x = p x . The lightning protection measures include an LPS for the structure and its occupants, protection against the lightning electromagnetic pulse (LEMP) caused by direct and nearby strikes, and transient protection (TP) of incoming services. The LPS for the structure comprises an air terminal network to intercept the lightning strike, a downconductor system to conduct the discharge current safely to earth and an earth termination network to dissipate the current into the earth. The LEMP protection includes a number of measures to protect sensitive electronic equipment such as the use of a mesh of downconductors to minimize the internal magnetic field, the selection of lightning protection zones, equipotential bonding and earthing, and the installation of SPDs. The TP for incoming services includes the use of isolation devices, the shielding of cables and the installation and coordination of SPDs. 1.4 REFERENCED DOCUMENTS The documents referred to in this Standard are listed in Appendix G. 1.5 DEFINITIONS For the purpose of this Standard, the definitions below apply. 1.5.1 Air terminal A vertical or horizontal conductor of an LPS, positioned so as to intercept a lightning discharge, which establishes a zone of protection. 1.5.2 Air terminal network A network of air terminals and interconnecting conductors, which forms the part of an LPS that is intended to intercept lightning discharges.

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1.5.3 Base conductors Conductors placed around the perimeter of a structure near ground level interconnected to a number of earth terminations to distribute the lightning currents amongst them. 1.5.4 Bond (bonding conductor) A conductor intended to provide electrical connection between the LPS and other metalwork and between various metal parts of a structure or between earthing systems. 1.5.5 Damage (δ) Mean relative amount of loss consequent to a specified type of damage due to a lightning event, when damage factors are not taken into account. 1.5.6 Direct lightning flash A lightning discharge, composed of one or more strokes, that strikes the structure or its LPS directly. 1.5.7 Downconductor A conductor that connects an air terminal network with an earth termination. 1.5.8 Earth impedance (Z) The electrical impedance of an earthing electrode or structure to earth, derived from the earth potential rise divided by the impulse current to earth causing that rise. It is a relatively complex function and depends on— (a)

the resistance component (R) as measured by an earth tester;

(b)

the reactance component (X), depending on the circuit path to the general body of earth; and

(c)

a modifying (reducing) time-related component depending on soil ionization caused by high current and fast rise times.

1.5.9 Earth potential rise (EPR) The increase in electrical potential of an earthing electrode, body of soil or earthed structure, with respect to distant earth, caused by the discharge of current to the general body of earth through the impedance of that earthing electrode or structure. 1.5.10 Earthing boss (terminal lug) A metal boss specially designed and welded to process plant, storage tanks, or steelwork to which earthing conductors are attached by means of removable studs or nuts and bolts. 1.5.11 Earthing conductor The conductor by which the final connection to an earthing electrode is made. 1.5.12 Earthing electrodes (earth rods or ground rods) Those portions of the earth termination that make direct low resistance electrical contact with the earth. 1.5.13 Earthing resistance The resistance of the LPS to the general mass of earth, as measured from a test point. 1.5.14 Earth termination (earth termination network) That part of an LPS intended to discharge lightning currents into the general mass of the earth. All parts below the lowest test link in a downconductor are included.

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1.5.15 Electricity supply service earthing electrode An earthing electrode installed for the purposes of providing the connection of the electrical installation earthing system to the general mass of earth. 1.5.16 Explosive gas atmosphere A mixture of flammable gas, vapour or mist with air in atmospheric conditions in which, after ignition, combustion spreads throughout the unconsumed mixture that is between the upper and lower explosive limits. NOTE: The term refers exclusively to the danger arising from ignition. Where danger from other causes such as toxicity, asphyxiation, and radioactivity may arise this is specifically mentioned.

1.5.17 Finial A term not used in this Standard owing to its confusion with architectural application but occasionally used elsewhere in other Standards as referring to short vertical air terminals. 1.5.18 Frequency of lightning flashes direct to a service (Nc) Expected annual number of lightning flashes directly striking an incoming service. 1.5.19 Frequency of lightning flashes direct to a structure (N d) Expected annual number of lightning flashes directly striking the structure. 1.5.20 Frequency of lightning flashes to ground near a service (NI) Expected annual number of lightning flashes striking the ground surface near an incoming service. 1.5.21 Frequency of lightning flashes to ground near a structure (N m) Expected annual number of lightning flashes striking the ground surface near the structure. 1.5.22 Hazardous area An area where an explosive atmosphere is, or may be expected to be present continuously, intermittently or due to an abnormal or transient condition (see AS/NZS 2430 series). 1.5.23 Incoming service A service entering a structure (e.g. electricity supply service lines, telecommunications service lines or other services). 1.5.24 Indirect lightning flash A lightning discharge, composed of one or more strokes, that strikes the incoming services or the ground near the structure or near the incoming services. 1.5.25 Internal installation An installation or the part of an incoming service that is located inside the structure. 1.5.26 Joint A mechanical and electrical junction between two or more sections of an LPS. 1.5.27 Lightning flash (lightning discharge) An electrical discharge in the atmosphere involving one or more electrically charged regions, most commonly in a cumulonimbus cloud, taking either of the following forms: (a)

Ground flash (earth discharge) A lightning flash in which at least one lightning discharge channel reaches the ground.

(b)

Cloud flash A lightning flash in which the lightning discharge channels do not reach the earth.

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1.5.28 Lightning flash density (N g) The number of lightning flashes of the specified type occurring on or over unit area in unit time. This is commonly expressed as per square kilometre per year (km−2 year −1). The ground flash density is the number of ground flashes per unit area and per unit time, preferably expressed as a long-term (>10 years) average value. 1.5.29 LPS (LPS Type I to IV) Complete system used to reduce the danger of physical damages and of injuries due to direct flashes to the structure. It consists of both external and internal LPSs and is defined as a set of construction rules, based on corresponding protection level. 1.5.30 Lightning protection zone (LPZ) With respect to the lightning threat, a zone may be defined, inside of which is sensitive equipment. Extra protection is applied at the zone boundary to minimize the risk of damage to equipment inside the zone. 1.5.31 Lightning strike A term used to describe the lightning flash when the attention is centred on the effects of the flash at the lightning strike attachment point, rather than on the complete lightning discharge. 1.5.32 Lightning strike attachment point The point on the ground or on a structure where the lower end of the lightning discharge channel connects with the ground or structure. 1.5.33 Lightning stroke A term used to describe an individual current impulse in a complete ground flash. 1.5.34 Loss Due to lightning strike, the loss can be of human life, loss of service to the public or economic loss. 1.5.35 Multiple earthed neutral (MEN) system A system of earthing in which the parts of an electrical installation are connected to the general mass of earth and in addition are connected within the electrical installation to the neutral conductor of the supply system. 1.5.36 Partial probability of damage (p) Probability of a lightning flash causing a specified type of damage to the structure, depending on one characteristic of the structure or of an incoming service. 1.5.37 Probability of damage (P) Probability of a lightning flash causing a specified type of damage to the structure. It may be composed of one or more simple probabilities of damage. 1.5.38 Protection level (I to IV) Four levels of lightning protection. For each protection level, a set of maximum (sizing criteria) and minimum (interception criteria) lightning current parameters is fixed, together with the corresponding rolling sphere radius. 1.5.39 Protection measures Protection measures taken to reduce the probability of damage. These may include an LPS on the building, isolation transformers and/or surge protection on incoming services (primary protection) and internal equipment (secondary protection).

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1.5.40 Resistibility Ability of equipment to withstand an overvoltage or an overcurrent without damage. 1.5.41 Risk (R) Probable average annual loss (humans and goods) in a structure due to lightning flashes. 1.5.42 Risk assessment The process of designing an LPS to achieve a probable frequency of damage and injury. It is based on determining the likely number of lightning discharges and also estimates the probability and consequences. A range of protection measures can be selected to reduce the risk to less than a target value. This process is also known as risk management. 1.5.43 Risk component Partial risk assessed according to the source of damage and the type of damage. 1.5.44 Side-flash A discharge occurring between nearby objects or from such objects to the LPS or to earth. 1.5.45 Special damage factors (k n ) Factors affecting the value of the damage δ, with respect to the existing peculiar conditions in the structure, that may decrease or increase the loss. 1.5.46 Striking distance (ds ) The distance between the tip of the downward leader and the eventual lightning strike attachment point at the moment of initiation of an upward intercepting streamer. 1.5.47 Structure or object Any building or construction, process plant, storage tank, tree, or similar, on or in the ground. 1.5.48 Surge protective device (SPD) A device that is intended to mitigate surge overvoltages and overcurrents. 1.5.49 Test link A joint designed and situated so as to enable resistance or continuity measurements to be made. 1.5.50 Thunderday A calendar day during which thunder is heard at a given location. Thunderstorm occurrence at a particular location is usually expressed in terms of the number of calendar days in a year when thunder was heard at the location, averaged over several years. 1.5.51 Tolerable risk (R a) Maximum value of the risk that can be tolerated in the structure to be protected. Also referred to as acceptable risk, being the maximum value of risk acceptable based on community expectations. 1.5.52 Zone of protection The portion of space within which an object or structure is considered to be protected from a direct strike by an LPS.

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SECT ION 2 ASSESSMENT AND MANAGEMEN T OF R I SK D UE TO L I GH TN I NG —ANA L YS I S OF N E E D FO R PRO T E CT I O N 2.1 INTRODUCTION Cloud-to-ground lightning discharges can be hazardous to structures, people and installations or equipment in, on or connected to the structure. Lightning can cause damage to all or part of a structure or to the contents of a structure, especially to electrical and electronic systems. Consequential effects of lightning damage may extend to the surroundings of a structure. To reduce lightning damage and its consequential effects, lightning protection measures may be required. The need for protection and the level of protection applied should be considered in terms of the assessment of risk due to lightning, and management of that risk to an acceptable level. The approach outlined in this section is based on the principles of the management of risk due to lightning outlined in initial work by IEC Committee TC 81 on this subject. The approach has been simplified by reducing the number of variables and options requiring selection to a minimum (based on assumptions for general conditions in Australia and New Zealand) and providing a Microsoft Excel calculation tool as an integral part of this Standard. The Microsoft Excel calculation tool provides only an estimate of the lightning risk. The risk assessment calculator is a simplified tool for the more common structure types. For specialized structures (e.g. telecommunication exchanges, substations), a detailed lightning risk assessment may be undertaken. This may involve the application of industry specific Standards. Where other information exists, such as the damage/hazard history of existing similar structures in the nearby area, then this should be taken into account when deciding whether or not to install lightning protection and the appropriate level of lightning protection required. A decision to provide lightning protection may be taken regardless of the outcome of any risk assessment, for example, where there is a desire that there be no avoidable risk. In such cases, the required protection level for the structure (Level I, II, III or IV – as defined in Section 4) should be specified. It may also be important to specify other protection measures such as SPDs on incoming conductive electrical service lines and internal equipment. Risk assessment for protection of specific conductive electrical services may also be undertaken in isolation based on specific Standards and performance criteria. For telecommunication overvoltages, AS 4262.1 deals with protection of persons, AS 4262.2 deals with protection of equipment and the ITU-T K series of recommendations provide requirements for protecting telecommunication networks. Before any decision is made not to install lightning protection to a structure, consideration should be given to the factors outlined in other sections of this Standard. 2.2 SCOPE OF SECTION This Section is applicable to the management of risk caused by lightning discharges to earth. The object of this Section is to give a procedure for evaluation of the risk to a structure, people and installations or equipment in, on or connected to the structure. This evaluation considers mechanical damage of the structure and contents, damage and failure of equipment, potential differences causing deaths of people and livestock from step and touch voltages, and fire damage that may result from the lightning discharge.

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The procedure involves the comparison of the evaluated risk to the tolerable or acceptable limit of the risk and allows for the selection of appropriate protective measures to reduce the risk to below the tolerable limit. This Standard does not consider the risk of personal injury when using telecommunication equipment during a lightning storm. 2.3 CONCEPT OF RISK 2.3.1 General considerations In this Standard, risk R is defined as the probable annual loss due to lightning. Expressed as a number, it represents the probability of loss occurring over the period of a year. Thus a risk of 10 -3 represents a chance of 1 in 1000 of a loss occurring during a year. To increase understanding of the risk concept, some risks associated with everyday living are provided in Table 2.1. Many human activities imply a judgement that the benefits outweigh the related risks. Table 2.1 gives a scale of risk of loss of human life associated with different activities. TABLE 2.1 COMPARATIVE PROBABILITY OF DEATH FOR AN INDIVIDUAL PER YEAR OF EXPOSURE (ORDER OF MAGNITUDE ONLY) * Risk

Activity

Chance of occurrence

Probability per year

1 in 400

2.5 × 10 −3

1 in 2000 1 in 8000 1 in 20 000

5 × 10

−4

1.3 × 10 5 × 10

3.3 × 10

1 in 100 000

1 × 10 −5 2 × 10

−6

1 in 1 000 000

1 × 10

−6

1 in 2 000 000

5 × 10 −7

1 in 500 000

All accidents −4

−5

1 in 30 000

Smoking (10 cigarettes per day)

Traffic accidents Leukaemia from natural causes

−5

Work in industry, drowning Poisoning Natural disasters Rock climbing for 90 s†, driving 50 miles (80 km) by road † Being struck by lightning

*

The source of this table is BS 6651:1999.

†

These risks are conventionally expressed in this form rather than in terms of exposure for a year.

2.3.2 Types of risk due to lightning The types of risk due to lightning for a particular structure or facility may include one or more of the following: (a)

R 1—risk of loss of human life.

(b)

R 2—risk of loss of service to the public. NOTE: Only applicable to structures involved in the provision of public service utilities (e.g. water, electricity, gas, telecommunications, rail).

(c)

R 3—risk of loss of cultural heritage.

(d)

R 4—risk of loss of economic value.

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2.3.3 Tolerable values of risk In order to manage risk, a judgement must be made of what is an acceptable or tolerable upper limit for the risk. In relation to human fatalities, various societal risk guidelines or criteria have been proposed. Generally for a single human fatality, risks of greater than 10–3 per year (i.e. chance of 1 in 1000 of occurrence in a year) are considered unacceptable. Public money would normally be spent to try to eliminate (or reduce to a level as low as reasonably practical) the causes of risks greater than 10–4 per year (i.e. chance of 1 in 10 000 of occurrence). Risks less than 10 –5 per year (i.e. chance of 1 in 100 000 of occurrence) are generally considered tolerable although public money may still be spent on an education campaign to reduce those risks regarded as avoidable. In terms of the risk of various types of losses due to lightning, a value of the tolerable risk, R a needs to be specified. For each type of loss due to lightning, R a represents the tolerable probability of that loss occurring over the period of a year. Regarding the potential types of risk due to lightning listed in Clause 2.3.2, typical values of the tolerable or acceptable risk, R a are given in Table 2.2. TABLE 2.2 TYPICAL VALUES OF TOLERABLE RISK, Ra Type of loss

Tolerable risk per year, Ra

Loss of human life

10 −5

Loss of service to the public

10 −3

Loss of cultural heritage

10 −3

For a loss of economic value, the tolerable risk, Ra may be fixed by the facility owner or user, often in consultation with the designer of the protection measures, based on economic or cost/benefit considerations. For example, at a particular facility, it may be considered that a chance of 1 in 1000 of economic loss due to lightning occurring over a period of a year is tolerable. Alternatively, this would mean that it is considered acceptable for such a loss to occur, on average, once every 1000 years. In such a case the tolerable risk, R a for loss of economic value would be set at 10 -3 . Similarly, if it were considered acceptable for such a loss to occur, on average, once every 100 years, Ra for loss of economic value would be set at 10-2. 2.4 DAMAGE DUE TO LIGHTNING 2.4.1 Sources of damage The current in the lightning discharge is the potential source of damage. In this Section, the following sources of damage, relating to the proximity of the lightning strike, are taken into account (see Table 2.3): (a)

S1—direct strike to the structure.

(b)

S2—strike to the ground near the structure.

(c)

S3—direct strike to a conductive electrical service line.

(d)

S4—strike to ground near a conductive electrical service line.

Conductive electrical service lines include electricity supply service lines (underground or overhead) and telecommunications service lines.

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The number of lightning strikes influencing the structure depends on— (i)

the dimensions and the characteristics of the structure;

(ii)

the dimensions and characteristics of the incoming conductive electrical service lines;

(iii) the environment around the structure; and (iv)

the density of lightning strikes in the region where the structure is located.

The greater the height and collection area, the more lightning strikes will influence the structure. Tall trees and surrounding buildings may shield a structure from lightning strikes. Incoming conductive electrical service lines add to the lightning collection area as they can conduct lightning current into the building. 2.4.2 Types of damage The type of damage that a lightning strike may cause depends on structure or facility characteristics such as— (a)

type of construction;

(b)

contents and application;

(c)

incoming conductive electrical service lines; and

(d)

measures taken for limiting the risk.

The damage may be limited to a part of the structure or may extend to the whole structure. Damage may also extend to the surrounding environment (e.g. contamination caused by consequential chemical spills or radioactive emissions). Direct strikes to the structure or to incoming conductive electrical service lines may cause mechanical damage, injury to people and animals and may cause fire and/or explosion. Indirect strikes as well as direct strikes may cause failure of electrical and electronic equipment due to overvoltages resulting from coupling of the lightning current. For practical applications of risk assessment, it is useful to distinguish between three basic types of damage that can appear as the consequence of a lightning strike. They are as follows: (i)

D1—Injury to people (shock of living beings) due to step and touch voltages and side-flash contact.

(ii)

D2—Fire, explosion, mechanical destruction, chemical release due to physical effects of the lightning channel (including dangerous sparking).

(iii) D3—Failure of electrical and electronic systems due to overvoltages. 2.4.3 Consequences of damage (types of loss) The value amount of damage caused by the consequential effects of lightning depends on factors such as— (a)

the number of people and the time they are in the facility;

(b)

the type and importance of the service provided to the public; and

(c)

the value of goods and/or services affected by the damage.

Some special hazard factors also need to be considered. For example, in theatres and halls there can be a significant risk of panic if a lightning strike causes loss of electricity supply or other mechanical or fire-related damage. As a result, people may be injured in the panic to evacuate the building. Museums and heritage listed buildings have a cultural value. There may be significant loss of revenue (economic loss) associated with damage to computer centres and communication nodes. COPYRIGHT


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For a particular facility or structure, the following consequences of damage due to lightning or types of loss should be taken into account. (i)

L1—Loss of human life.

(ii)

L2—Loss of services to the public. NOTE: Only applicable to structures involved in the provision of public service utilities (e.g. water, electricity, gas, telecommunications, rail).

(iii) L3—Loss of cultural heritage. (iv)

L4—Loss of economic value (structure, content and loss of activity).

Table 2.3 illustrates the relationship between the ‘sources of damage’, ‘types of damage’ and ‘types of loss’ selected according to the point of strike. TABLE 2.3 SOURCES OF DAMAGES (S1, S2, S3, S4), TYPES OF DAMAGES (D1, D2, D3) AND TYPES OF LOSS (L1, L2, L3, L4) SELECTED ACCORDING TO THE POINT OF STRIKE Point of strike

Source of damage

S1

S2

S3

S4 1) 2) 3)

Structure Type of damage

Type of loss

D1

L1, L4 1)

D2

Service Type of damage

Type of loss

L1, L2, L3, L4

D2

L1 2), L2, L4

D3

L1, L2, L4

D3

L2, L4

D3

L1 3), L2, L4

D1

L1, L4 1)

D2

L1, L2, L3, L4

D2

L1 2), L2, L4

D3

L1, L2, L4

D3

D2, D4

D3

L1 3), L2, L4

D3

L2, L4

In the case of agricultural properties (loss of animals). In the case of pipelines, with no metallic gasket on flanges, conveying explosive fluid. In the case of hospitals and of structures with risk of explosion.

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FIGURE 2.1 LOSSES, DAMAGES AND RISK COMPONENTS

Figure 2.1 illustrates the relationship between the ‘types of loss’, ‘types of damage’ and ‘components of risk’ (discussed in Clause 2.5.1) that can be associated with lightning discharges to earth.

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2.5 RISKS DUE TO LIGHTNING 2.5.1 Risk components For each type of loss relevant to the structure or facility, the total risk due to lightning, R, is the probability of that loss occurring over the period of a year. The total risk, R, is made up of the sum of a number of risk components associated with the four possible sources of damage (according to the point of strike) as listed below: (a)

S1 Lightning strikes directly to the structure These may generate: (i)

Component R h due to step and touch voltages outside the structure (mainly around downconductors) causing shock to living beings (D1).

(ii)

Component Rs due to mechanical and thermal effects of the lightning current or by dangerous sparking causing fire, explosion, mechanical and chemical effects inside the structure (D2).

(iii) Component Rw due to overvoltages on internal installations and incoming services causing failure of electrical and electronic systems (D3). (b)

S2 Lightning strikes to ground near the structure These may generate component R m due to overvoltages on internal installations and equipment (mainly induced by the magnetic field associated with the lightning current) causing failure of electrical and electronic systems (D3).

(c)

S3 Lightning strikes directly to conductive electrical service lines These may generate: (i)

Component R g due to touch overvoltages transmitted through incoming lines causing shock of living beings inside the structure (D1).

(ii)

Component Rc due to mechanical and thermal effects including dangerous sparking between external installation and metallic parts (generally at the pointof-entry of the incoming line into the structure) causing fire, explosion, mechanical and chemical effects on the structure and/or its content (D2).

(iii) Component R e due to overvoltages, transmitted through incoming lines to the structure, causing failure of electrical and electronic systems (D3). (d)

S4 Lightning strikes to ground near conductive electrical service line conductors These may generate component R l due to induced overvoltages, transmitted through incoming lines to the structure, causing failure of electrical and electronic systems (D3).

Figure 2.1 illustrates the relationship between the ‘types of loss’, ‘types of damage’ and ‘risk components’ that can be associated with lightning discharges to earth. Table 2.4 summarizes the various risk components and the ways that these can be summed to give the total risk.

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For each type of loss, the total value of the risk due to lightning, R, may be expressed in the following ways: (i)

With reference to the type of lightning strike— R

=

Rd + Ri

Rd

=

R h + Rs + R w

risk due to direct strikes to the structure

Ri

=

R g + Rc + R m + R e + R l

risk due to indirect strikes to the structure (including direct and indirect strikes to conductive electrical service lines)

. . . 2.5.1(1)

where

(ii)

With reference to the types of damage— R

=

Rt + Rf + Ro

Rt

=

Rh + Rg

risk due to shock to living beings (D1)

Rf

=

Rs + Rc

risk due to fire, explosion, mechanical destruction and chemical release (D2)

Ro

=

R w + R m + Re + R l

risk due to the failure of electrical and electronic systems due to overvoltages (D3)

. . . 2.5.1(2)

where

2.5.2 Calculation of risk components Each component of the risk R x depends on the number of dangerous events N x , the probability of damage P x and the damage factor δ x . The value of each component of risk R x may be calculated using an expression similar to that shown below: Rx

=

Nx Px δx

NOTE: Details of the parameters, factors and equations required to calculate each of the risk components are given in Appendix A.

For each risk component, the damage factor, δ x , represents the mean damage and takes into account the type of damage, its extent, and the consequential effects that may occur as the result of a lightning strike. Typical values of the damage factors for each type of loss are given in Appendix A and in the risk management calculation tool. NOTE: Where specific information is known regarding the function or use of a particular structure, alternative damage factor values may be selected based on these relations.

The damage factors are related to the structure’s function or use and may be determined from the following approximate relations below: Loss of human life (L1) =

n t × nt 8760

n

=

the number of possible victims from a lightning strike

nt

=

the expected total number of people associated with the structure

t

=

the time, in hours per year, for which the people are present in a dangerous place

δx

(relative number of victims)

. . . 2.5.2(1)

where

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Unacceptable loss of service to the public (L2) =

n t × nt 8760

n

=

the mean number of users not served

nt

=

the total number of users served

t

=

the annual period of loss of service, in hours.

δx

(relative amount of possible loss)

. . . 2.5.2(2)

where

Loss of cultural heritage (L3) =

c ct

c

=

the insured value of possible loss of goods (monetary amount)

ct

=

the total insured value of all goods present in the structure (monetary amount)

δx


. . . 2.5.2(3)

where

Economic loss (L4) δx

=

c ct


. . . 2.5.2(4)

where c

=

the mean value of the possible loss of the structure, its contents and associated activities (monetary amount)

ct

=

the total value of the structure, its content and associated activities (monetary amount)

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D3 Failure of electrical and electronic systems

D2 Physical destruction

D1 Injury of living beings

Type of damage

Cause of damage

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Risk due to direct strikes to the structure

R d = Rh + R s + R w

Ri = Rg + Rc + Rm + Re + R1

Component due to overvoltages transmitted through incoming conductive electrical service lines to the structure causing failure of electrical and electronic systems

Component due to induced overvoltages transmitted through incoming conductive electrical service lines causing failure of electrical and electronic systems

R1

S4 Strike to ground near incoming conductive electrical service line

Risk due to indirect strikes to the structure (including direct and indirect strikes to the conductive electrical service lines)

Component due to overvoltages on internal installations and equipment (induced by the magnetic field associated with the lightning current) causing failure of electrical and electronic systems

Re

Rw

Component due to overvoltages on internal installations and incoming services causing failure of electrical and electronic systems

Component due to mechanical and thermal effects or dangerous sparking from incoming conductive electrical service lines (mainly at the point-ofentry to the structure) causing fire or physical damage

Component due to mechanical and thermal effects or dangerous sparking causing fire or physical damage

Component due to touch voltages transmitted through incoming conductive electrical service lines causing shock to living beings inside the structure

Rg

Indirect S3 Strike to incoming conductive electrical service line

Rc

Rm

S2 Strike to ground near the structure

Lightning

Rs

Component due to step and touch voltages or side-flash arc from EPR outside the structure causing shock to living beings

Rh

Direct S1 Strike to the structure

POSSIBLE RISK COMPONENTS CAUSED BY DIFFERENT EFFECTS

TABLE 2. 4

R = Rd + R i Total risk due to lightning

R = Rt + R f + Ro Total risk due to lightning

Risk due to the failure of electrical and electronic systems from overvoltages

R o = R w + R m + Re + R1

Risk due to fire or physical damage

Rf = Rs + Rc

Risk due to shock to living beings

R t = Rh + Rg

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2.6 PROCEDURE FOR RISK ASSESSMENT AND MANAGEMENT The procedure for risk assessment and the subsequent selection of protection is outlined in flow chart form in Figure 2.2. 2.6.1 Procedure for risk assessment The procedure for the assessment of the risk requires: (a)

Identification of the structure or facility to be protected. This involves defining the extent of the facility or structure being assessed. In most cases the structure or facility will be a stand-alone building. The structure may encompass a building and its associated outbuildings or equipment supports. Under certain conditions, a facility that is a part of a building may be considered as ‘the structure’ for risk assessment purposes. An example of this might be a communications installation at the top of an office building. This segregation of a part of a building is only valid under the following conditions: (i)

There is no risk of explosion in the remainder of the building.

(ii)

Suitable fire barriers exist around the structure being considered (fire rating of not less than 120 min).

(iii) Overvoltage (SPD) protection is provided on all conductive electrical service lines at their point-of-entry to the structure being considered. (b)

Determination of all the relevant physical, environmental and service installation factors applicable to the structure.

(c)

Identification of all the types of loss relevant for the structure or facility. For most structures, only L1 and L4 will need to be considered. L3 will apply to museums, galleries, libraries and heritage listed buildings while L2 applies to structures involved in the provision of public service utilities such as water, gas, electricity and telecommunications.

(d)

For each type of loss relevant to the structure, determine the relevant damage factors δ x and special hazard factors.

(e)

For each type of loss relevant to the structure, determine the maximum tolerable risk, R a.

(f)

For each type of loss relevant to the structure, calculate the risk due to lightning by— (i)

identifying the components R x that make up the risk (see Figure 2.1);

(ii)

calculating the identified risk components R x ; and

(iii) calculating the total risk due to lightning, R. (g)

Compare the total risk R with the tolerable value R a for each type of loss relevant to the structure.

If R ≤R a (for each type of loss relevant to the structure) lightning protection is not necessary. If R >Ra (for any type of loss relevant to the structure) the structure shall be equipped with protection measures against lightning. The selection of the most suitable protection measures shall be made by the designer according to the contribution of each risk component to the total risk, and according to the technical and economic aspects of the different protection measures available. Technical considerations include addressing the highest risk components while economic considerations involve minimizing the total cost to achieve a suitable level of protection. COPYRIGHT


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It is appropriate to consider separately the risk Rd due to direct lightning strikes and the risk R i due to indirect lightning strikes. 2.6.2 Protection against direct lightning strikes if Rd > R a When the risk due to direct lightning strikes is greater than the acceptable risk (R d > Ra), then the structure shall be protected against direct lightning strikes with an LPS designed and installed in accordance with the recommendations given in Section 4. In Section 4, four protection levels (I, II, III, IV) with corresponding interception efficiencies (99%, 97%, 91%, 84%) and resulting LPS efficiencies, E (98%, 95%, 90%, 80%) are defined. To determine the required protection level, the final calculation for the protected structure may be repeated successively for the protection levels IV, III, II, I until the condition Rd ≤ R a is fulfilled. NOTE: A previous edition, AS 1768—1991, specified LPSs with protection equivalent to IEC Level III (interception efficiency ≈ 91%)—Rolling sphere with a = 45 m)

If an LPS of protection level I cannot fulfil this condition, consider surge protection on all incoming conductive electrical service lines at the point-of-entry to the structure or other specific protection measures according to the values of the risk components (refer to detailed calculations and assumptions in Appendix E). These may include— (a)

measures limiting step and touch voltages;

(b)

measures limiting fire propagation;

(c)

measures to mitigate the effects of lightning-induced overvoltages (e.g. additional, coordinated surge protection or isolation transformers); and

(d)

measures to reduce the incidence of dangerous discharges (e.g. bonding of structural elements).

2.6.3 Protection against indirect lightning strikes if R d ≤ R a but R i > R a When R d ≤ Ra, then the structure is protected against direct lightning strikes. However, if the risk due to indirect strikes is greater than the acceptable risk (R i > R a), then the structure must be protected against the effects of indirect lightning strikes. Possible protection measures include— (a)

suitable application of SPDs on all external conductive electrical service lines at the point-of-entry to the structure (primary or point-of-entry surge protection); and

(b)

suitable application of SPDs on all internal equipment (secondary surge protection at the equipment).

NOTE: Suitable application of surge protection requires correct installation, earthing and coordination of appropriately rated SPDs.

To determine the required protection, the final calculation for the protected structure shall be repeated with one or both of these protection measures in place until the condition R i ≤ R a is fulfilled. If the application of these protection measures cannot fulfil this condition, specific protection measures shall be provided according to the values of the risk components (refer to detailed calculations and assumptions in Appendix A). These may include magnetic shielding of the structure and/or of the equipment and/or of cable ways and/or by using cable screening. It may also be appropriate to have extra zones of protection around sensitive areas with an extra level of SPD protection at the boundary of that zone.

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2.6.4 Final check if R d + R i > Ra When Rd ≤ Ra and R i ≤ Ra it is still possible that the total risk R = R d + R i > Ra. In this case, the structure does not require any specific protection against direct lightning strikes or against overvoltages due to nearby strikes or transmitted through the incoming conductive electrical service lines. However, since R > Ra, protection measures shall be taken to reduce one or more risk components to reduce the risk to R ≤ Ra . Critical parameters have to be identified to determine the most efficient measure to reduce the risk R. For each type of loss, there are a number of protection measures that, individually or in combination, may make the condition R ≤ Ra . Those measures that make R ≤ Ra for all the types of loss must be identified and adopted with due consideration of the associated technical and economic issues. 2.7 RISK MANAGEMENT CALCULATION TOOL A Microsoft  Excel spreadsheet file has been included as a risk management calculation tool. This file (LIGHTNING RISK.XLS) is provided as an integral part of the Standard and is designed to operate using Microsoft  Excel 97 (or later versions). The spreadsheet implements the risk calculations detailed in Appendix A with the required inputs and outputs presented on a single page for ease of use. The risk calculations implemented represent a simplification of the approach outlined in initial work by IEC Committee TC 81 with the number of variables and options requiring selection reduced to a minimum based on assumptions for general conditions in Australia and New Zealand. In addition, a simplified form of the equation for risk component Rs (risk related to physical destruction) has been used, and the classification descriptions for fire risks based on structure type and content (ps) have been modified, in order to reduce the fire risk sensitivity of the draft IEC model. These modifications have been made to give more practical values based on experience in Australia and New Zealand. 2.7.1

General operation

When the file is opened using Microsoft Excel, a front page spreadsheet is displayed. This front page presents all of the inputs and final calculation outputs required in the risk management process. Other work sheets showing the calculated values of all of the individual risk components for each type of risk are also accessible if a more in depth analysis is required. On the front page, the required inputs are subdivided into various categories with input cells highlighted with a border. The possible input options are explained in a comment box, which is displayed when the cursor is positioned over the input cell. For most input cells, the input option is selected from a pull-down menu of key words that are defined in the associated comment box. Some inputs require numerical values (e.g. structure dimensions), which should be entered in the usual way from the keyboard. When all of the inputs have been entered, the output values in the ‘Risk’ section represent the calculated risk components and overall risk for the particular set of structure parameters and conditions specified.

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2.7.2 Using the calculation tool in the risk management procedure The calculation tool can be used in the following way to implement the risk assessment and management procedure outlined in Clause 2.6. (a)

Identify the structure and input the structure dimensions.

(b)

Input the structure attributes relating to fire risk, screening effectiveness and internal wiring.

(c)

Determine the average annual lightning ground flash density (N g ) for the structure location from the appropriate Ground Flash Density map (Figure 2.3 or 2.4) and input the value in the environment section. NOTES: 1

Earlier editions of AS/NZS 1768 provided thunderday maps, refer Appendix B2.3.

2

An approximate relationship between ground flash density (N g ) and thunderdays (T d ) for Australia is N g = 0.012 T d1.4.

(d)

Input the other environment attributes relating to surrounding feature height and service density.

(e)

Specify the details of the conductive electrical service lines associated with the structure in the following way: (i)

Input the type of electricity supply service line and identify whether or not a transformer is installed on this service line at the structure.

(ii)

Input the number and type of other overhead or underground conductive electrical service lines connected to the structure via divergent routes. NOTES: 1

Different service lines that follow the same physical route from the nearest distribution node to the structure should be considered as one service line connection.

2

Typically a structure will have one electricity supply service connection (overhead or underground) and one telecommunications service connection (overhead or underground) that could be considered as being connected via divergent routes.

(f)

Identify the loss types relevant to the structure and for each type input the damage factors and special hazard factors as appropriate.

(g)

Determine and input an appropriate value for the acceptable risk of loss of economic value as it applies to the structure.

(h)

Input details of any protection measures installed. The surge protection options offered are for: (i)

Suitable application of SPDs on all external conductive electrical service lines at the point-of-entry to the structure (primary or point-of-entry surge protection).

(ii)

Suitable application of SPDs on all electrical equipment inside the structure (secondary surge protection at the equipment).

NOTE: Suitable application of surge protection requires correct installation, earthing and coordination of appropriately rated SPDs.

For each type of loss relevant to the structure, compare the acceptable risk with the total risk calculated. Review the risk components and follow the Risk Management procedure detailed in Clause 2.6 and Figure 2.2. Use the spreadsheet to recalculate the risk components and total risk figures for any protection measures proposed. Successive calculations can be performed to observe the effects of various protection measures. A number of completed spreadsheet examples are provided for information in Appendix A. COPYRIGHT


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* Refer to Section 4. NOTE: A previous edition, AS 1768—1991, specified an LPS with protection equivalent to Level III—Rolling sphere with a=45 m.

FIGURE 2.2 RISK MANAGEMENT PROCEDURE FOR SELECTION OF LIGHTNING PROTECTION MEASURES

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FIGURE 2.3 AVERAGE ANNUAL LIGHTNING GROUND FLASH DENSITY MAP OF AUSTRALIA

NOTE: The Australian Ground Flash Density map has been compiled and kindly supplied by the Australian Bureau of Meteorology and the University of Queensland.

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NOTE: This figure has been derived from ground flash density data obtained from the Lightning Detection Network of New Zealand for the period January 1, 2001 through February 9, 2006. Data supplied by Transpower New Zealand Ltd and the Meteorological Service of New Zealand Ltd (MetService).

FIGURE 2.4 AVERAGE ANNUAL LIGHTNING GROUND FLASH DENSITY MAP OF NEW ZEALAND

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SE C T I O N

3

P R E CA U T I O N S SA FE T Y

FO R

PE RSO N A L

3.1 SCOPE OF SECTION This Section provides guidance for personal safety during thunderstorms. Measures for the protection of persons, which should be incorporated in LPSs for buildings and structures, are outlined in other sections. For shelters designed for the protection of persons during storm activity, reference should be made to Clause 6.9.1. 3.2 NEED FOR PERSONAL PROTECTION A hazard to persons exists during a thunderstorm. Each year a number of persons are struck by lightning, particularly when outdoors in open space such as an exposed location on a golf course, or when out on the water. Between six and ten people are killed by lightning in Australia each year. This is equivalent to a probability of about 5 × 10 –7 per year for an individual being killed by lightning in Australia. Lightning strikes to a person, or close by, may cause death or serious injury. A person touching or close to an object struck by lightning may be affected by a side-flash, or receive a shock due to step, touch or transferred potentials. There is a significant risk of side-flash for people in small, public structures such as picnic shelters, particularly those with unearthed metallic roofs. In built-up areas protection is frequently provided by nearby buildings, electricity supply service lines or street lighting poles. Persons within a substantial structure are normally protected from direct strikes, but may be exposed to a hazard from conductive electrical services entering the structure or from conductive objects within the structure that may attain different potentials. The first recorded ‘electrical accident’ involving the use of a telephone occurred in 1860 and was caused by lightning being conducted through the telephone system. Telephone related injuries include acoustic and/or electric shock. About 10% of injuries are severe. No telephone related deaths have been reported in Australia. This is probably because of warnings not to use the telephone, except in an emergency, during a lightning storm and the use of SPDs on telephone installations in lightning prone areas. Around 80% of incidents involve a lightning strike to or close to a building or a strike to the electricity supply service line all of which result in a rise of the local earth potential rather than surges on the telecommunications service line. This rise in local earth potential can result in a breakdown between the person and the telephone, (which is connected to a nominal remote earth via the telecommunications service line). In some workplaces employees who work within larger buildings may be unaware of the changing outside weather conditions, and may not be aware that it may be unsafe to use telephone systems. Where modern fixed line telephonist headsets are used, this increases the risk of human injury through external transients being conducted through to those wearing the headsets. When moderate to loud thunder is heard, persons out of doors should avoid exposed locations and should seek adequate shelter. Persons indoors should avoid using the telephone and contacting metallic structures. These warnings apply particularly if thunder follows within 15 s of a lightning flash (corresponding to a distance of less than 5 km).

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3.3 PERSONAL CONDUCT 3.3.1 General The threat to personal safety is greatest if a person is out of doors when the thunderstorm is local. In the absence of specific information from weather radar, a lightning location system or a specialized lightning warning device, the ‘30/30’ safety guideline (Paragraph G2, Ref. 1) should be used. An approaching thunderstorm is treated as local when the time interval between seeing a lightning flash and hearing the thunder is less than 30 s and then the appropriate safety measures described in Clause 3.3.2 should be implemented. A receding local thunderstorm is no longer a threat when more than 30 min have elapsed after the last thunder is heard. 3.3.2 Outdoors When outdoors, some of the measures for reducing the risk of injury that may be caused by lightning strikes to ground during a local thunderstorm are as follows: (a)

Seek shelter in a substantial building with at least normal headroom or within a totally enclosed, metal-bodied vehicle such as car or truck with metallic roof. If in a car, close the windows and avoid contact with metallic parts and remove any handsfree mobile telephone attachments from the body. Avoid driving the car as a strike to the car may blow out the tyres. Do not stay in open vehicles such as tractors, beach buggies, or any other type of open or enclosed vehicle without a metallic roof. Conventional fabric tents offer no protection; small sheds offer uncertain protection.

(b)

Do not ride or sit on horses, bicycles or motorcycles, or otherwise elevate the body above the surroundings.

(c)

Do not shelter under trees, particularly an isolated tree. If surrounded by trees, seek a position outside the foliage and crouch, keeping the feet together.

(d)

Do not shelter in small sheds, pagodas, walkways etc. with low unearthed metallic roofs supported on wooden or other electrically insulating materials.

(e)

Do not touch or stand close to any metallic structures, including wire fences and clothes lines. Do not stand on or under bridges or other elevated structures. Do not carry metallic objects such as umbrellas or golf clubs and remove metallic chains and other jewellery, particularly from the head and upper parts of the body.

(f)

If on open field or on the beach and remote from any shelters, keep as low and as small a profile as possible, i.e. crouch keeping the feet together and do not touch any objects or people near you. A dry ditch, valley or any depression in the ground is safer than an elevated or flat terrain. Do not lie on the ground as this could cause dangerous voltage to develop across the body by earth currents generated by a nearby strike. Footwear or a layer of non-absorbing, insulating material, such as plastic sheets, can offer some protection against earth voltages.

(g)

Do not swim or wade in the sea, lake, river, pool or any other body of water. Exit the water and move to a safe place.

(h)

If on a boat deck, keep a low profile and avoid contacts with or being close to masts, rails, stay wires or any other metallic objects. Avoid unnecessary contacts with communication or navigation equipment. Do not enter the water, and in general avoid contact with water. Additional protection may be gained by anchoring under relatively high objects such as jetties and bridges, provided that direct contact is not made with them. Isolated buoys and pylons should be avoided.

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In addition, the following checks should be made when planning outdoor activities: (i)

Check weather reports for likely thunderstorms.

(ii)

When engaged in outdoor activities, monitor the surroundings for indications of the onset of thunderstorms. These checks are particularly important when planning and undertaking activities involving groups and large numbers of people.

3.3.3 Indoor and outdoor swimming pools Certain locations are extremely hazardous during thunderstorms and should be avoided if at all possible. Statistics show that more than 10% of lightning-related injuries and deaths are water related (e.g. fishing, boating and swimming). Swimming pool facilities are connected to a large surface area via underground water pipes, gas lines, electric and telephone wiring, etc. Hence, lightning strikes to the ground anywhere on this metallic network may cause shocks elsewhere. Indoor and outdoor pools are treated the same with regard to lightning dangers. The following swimming pool safety procedures are recommended: (a)

A person should be designated as the pool’s weather safety lookout. That person should keep an eye on the weather and use the appropriate means to obtain localized, advanced weather information.

(b)

Identify in advance safe/not safe places—

(c)

(i)

Safe—dry areas inside large permanent buildings.

(ii)

Not safe—near electrical conductors, electrical equipment, metal objects (lifeguard stands, ladders, diving board stanchions) and water, including showers.

When thunder and/or lightning are first noticed, use the ‘30/30 method’ described in Clause 3.3.1. The pool should be evacuated in a time interval of less than 30 s and people should be directed to a safe shelter nearby.

3.3.4 Indoors When indoors, some of the measures for reducing the risk of injury that may be caused by lightning strikes to ground during a local thunderstorm are as follows: (a)

Avoid unnecessary use of telephones particularly in suburban and rural dwellings during local thunderstorms. If unavoidable, keep it brief and try not to touch electrical appliances, personal computers, metal pipes, stoves, sinks, and any other metallic objects at the same time. Mobile and cordless telephones are safe to use indoors. Where headsets are used for a large percentage of the time, or where operators may be unaware of local lightning storms, the risk of injury from lightning can be dramatically reduced by the use of wireless headsets.

(b)

Do not take a bath or a shower and do not wash hands or dishes. Do not use personal computers and other electronic and electrical equipment, and avoid contacts with sinks, stoves, refrigerators, metallic pipes and other large metallic objects in the house.

(c)

Disconnect television sets, personal computers, video recorders and other electronic and electrical appliances from antennas, conductive telecommunication connections and electricity supply outlets to avoid damage to them. This should be done before the storm is local to minimize risk of personal injury. NOTES: 1

Switching off an appliance does not disconnect neutral and earth wiring.

2

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3.4 EFFECT ON PERSONS AND TREATMENT FOR INJURY BY LIGHTNING The severity of the injuries inflicted on a person by a lightning strike will depend upon the intensity of the strike and for any given strike, on the fraction of the current that flows over the skin outside the body and the fraction that flows through the body, and its path. The worst situation would arise when a person is struck on the head, in which case the current through the body could cause fatal injuries to the brain, the heart and the lungs. A less dangerous situation is where the person is subjected to step or touch potentials, and only a small fraction of the total current passes through the body, although the pathway taken by this fraction is still important. The effects of lightning include burns to the skin, which are usually superficial, damage to various bodily organs and systems, unconsciousness and, most dangerously, cessation of breathing and cessation of heart beat. Independently of these electrically-related effects, temporary or permanent hearing impairment may be experienced as a consequence of the extremely high sound pressure levels associated with a nearby lightning strike. In the first aid treatment of a patient injured by lightning, it is essential that breathing be restored by artificial respiration and blood circulation be restored by external cardiac massage, if appropriate. These procedures should be continued until breathing and heart beat are restored, or it can be medically confirmed that the patient is dead. It should also be noted that the usual neurological criteria for death may be unreliable in this situation. There is no danger in touching a person who has been struck by lightning. Lightning strike victims are sometimes thrown violently against an object, or are hit by flying fragments of a shattered tree, so first aid treatment may have to include treatment for traumatic injury. Subsequent treatment of a lightning strike patient is a specialized area with important differences from the treatment of injuries inflicted by electric power current. For example, the nature of the burns and the extent of damage to underlying muscle tissue tend to be severe with electric power current, but mild with lightning current. Neurological and cardiac injuries also are different, and follow different courses. NOTE: For a more comprehensive treatment of the subject covered by this Clause—see Paragraph G2, Ref. 2.

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SECT I ON

4

P R OTE CT I O N

O F

ST RU CT U RE S

4.1 SCOPE OF SECTION This Section sets out recommendations for installation practices and for the selection of equipment to prevent or to minimize damage or injury that may be caused by a lightning discharge. The recommendations apply generally to the protection of structures using LPSs comprising air terminals, downconductors, equipotential bonding and earth terminations. If, after completing the LPS risk assessment, it is evident that surge protection is required to protect internal systems within the building and services at entry to the buildings then the requirements of Section 5 shall be applied. 4.2 PROTECTION LEVEL Four protection levels (PL) I, II, III, IV are used to define the efficiency with which the LPS is designed to protect the structure against physical damage and life hazard. The protection level efficiency (η) has two components–interception protection efficiency (η I), which characterizes the effectiveness of the air terminals, and sizing protection efficiency (η S), which characterizes the effectiveness of the downconductors and the earth terminations. Each is determined independently—by the minimum lightning current (I, kA) that will be intercepted, and by the maximum sizes of lightning current, charge (Q, C) and current steepness (di/dt, kA/µs) that will be discharged safely. The four protection levels are based on IEC TC 81 documents and are defined in Table 4.1. TABLE 4.1 PROTECTION LEVELS Protection level

Interception efficiency

Sizing efficiency

LPS efficiency

PL

ηI

ηS

η

I II III IV

0.99 0.97 0.91 0.84

0.99 0.98 0.97 0.97

0.98 0.95 0.90 0.80

4.3 LPS DESIGN RULES 4.3.1 General The following Clauses provide the details of the recommendations for the design and installation of all the LPS elements. This Clause lists the overriding design rules that shall normally be observed to provide minimum requirements for air terminals, downconductors and earth terminations. Observance of these rules will ensure that appropriate interception protection is provided by air terminals for the parts of structures most likely to be damaged by direct lightning strikes, that the conduction of the lightning current by the downconductors is adequate and that it is dissipated into the earth by the earth terminations. These rules are the first step in the process of the design of a complete LPS. The remaining steps are referred to in the design rules and their application is referred to in subsequent sections. NOTE: These design rules may not apply to some small structures.

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Field data of damage caused by lightning flashes terminating on structures (See Paragraph G2, Refs. 3 and 4) identify the parts that are vulnerable to strikes. The most vulnerable, associated with over 90% of observed lightning damage, are nearly always located on the upper parts of structures, such as— (a)

pointed apex roofs, spires and protrusions;

(b)

gable roof ridge ends; and

(c)

outer roof corners.

Other areas of vulnerability, in decreasing order, are— (d)

the exposed edges of horizontal roofs, and the slanting and horizontal edge of gable roofs (<10%);

(e)

lower horizontal edges and vertical edges on outer-sides just below corners (<5%);

(f)

flat surfaces near points and corners (<3%); and

(g)

intruding surfaces and other surfaces, particularly flat surfaces (<1%).

As discussed in later Sections, the vulnerability is caused mainly by the electric field intensification associated with exposed points and corners on the upper surfaces of structures. It is obvious that air terminals must be installed to provide interception for the most vulnerable parts. As well as providing conductive paths for the lightning current from the air terminals to the earth terminals, the downconductors should assist in preventing side-flashes to nearby metal elements (including reinforcing bars). This is best done by locating downconductors immediately below the air terminals used to protect the most vulnerable parts. 4.3.2 Rules for air terminals (a)

First, provide air terminals to protect the most vulnerable parts (points and corners); second, use the rolling sphere method (RSM) to check if the less vulnerable parts (edges) are protected and, if not, add more terminals to protect them; third, also check if the least vulnerable parts (such as flat surfaces) are protected and, if not, add more terminals.

(b)

Air terminals shall be placed on or close to the most vulnerable parts; if a strip conductor is used, it shall be directly on the part it is to protect; if a vertical rod is used, its length shall be not less than 500 mm, and it shall preferably be mounted on the part it is to protect or within 1 m or 1/2 its length, whichever is the smaller (this rule is supported by recent research – see Paragraph G2, Ref. 5). The maximum allowable length of a rod terminal is 6 m.

(c)

If the structure has horizontal or gently sloping upper parts that are essentially cylindrical or oval in shape, then the edges are the vulnerable parts and shall be protected by air terminals; if a strip conductor is used, it shall be run along the edge(s); if vertical rods are used, there shall be a minimum of two evenly spaced terminals.

(d)

Non-conventional LPS using air terminals that claim enhanced performance are outside the scope of this Standard. To comply with this Standard and irrespective of claimed performance, air terminals shall be placed in accordance with the relevant clauses of this Section.

4.3.3 Rules for downconductors (a)

Main conductors shall interconnect all air terminals and shall form one or more paths to earth via downconductors, such that the spacing between the downconductors does not exceed 20 m.

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A downconductor shall be connected directly below an air terminal used to protect the most vulnerable parts; if the air terminal is on an exposed roof corner, its downconductor will also act as a continuation of the air terminal to protect the vertical edge below it, as is required for tall structures.

4.3.4 Rules for earth terminations (a)

Low earth resistance is desirable and all practical measures should be taken to achieve 10 Ω or less for the whole interconnected LPS earth termination network. There shall be equipotential bonding at ground level for all metallic surfaces. If the risk assessment indicates a need for SPDs, these shall be installed and bonded in accordance with Section 5.

(b)

There shall be one earth termination per downconductor.

4.4 ZONES OF PROTECTION FOR LIGHTING INTERCEPTION 4.4.1 Basis of recommendations The selected interception protection efficiency against direct lightning strikes is achieved by installing an LPS in such a way that its air terminals establish zones of protection enclosing the whole structure. For the calculation of these zones of protection, the RSM, with a modification for large flat surfaces, is used. The RSM generally ensures that for lightning striking distances determined by the radius of the rolling sphere, the shortest distance between a lightning leader tip and any part of the structure is an air terminal. This method of analysis is suitable for conventional lightning terminals, which may be vertical rods, horizontal wires or strip conductors, railings, metal sheets, fascias and so on. 4.4.2 Rolling sphere method (with a modification for large flat surfaces) In the ‘rolling sphere’ technique of determining zones of protection, a sphere of specified radius (a) is theoretically brought up to and rolled over the total structure. All sections of the structure that the sphere touches are considered to be exposed to direct lightning strokes and would need to be protected by air terminals. In general, air terminals need to be installed so that the sphere only touches their interception surfaces. This is illustrated in Figure 4.1, which shows that the top corner/edge of the structure requires protection by an air terminal but the sides and lower section do not. The values of the rolling sphere radius (a) for the four protection levels (PL) I, II, III, IV are given in Table 4.2 together with the corresponding minimum lightning current (I min ) that will be intercepted. TABLE 4.2 ROLLING SPHERE RADIUS FOR EACH PROTECTION LEVEL Protection level

Sphere radius

Interception current †

PL

a, m (a i)*

I min , kA

I II III IV

20 (60) 30 (60) 45 (90) 60 (120)

2.9 5.4 10.1 15.7

*

The values in brackets are for increased radius (a i ), see below.

†

Values from IEC documents, which use distributions of lightning current parameters that differ slightly from those in Table B1 of Appendix B.

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FIGURE 4.1 ZONE OF PROTECTION ON A STRUCTURE ESTABLISHED BY A ROLLING SPHERE OF RADIUS a

It is common to consider that PL III using a sphere of radius a, 45 m provides ‘standard’ protection (as in AS/NZS 1768(Int):2003 and NFPA 780—2004). PL I and II with a, 20 and 30 m provide higher degrees of protection and should be used if required by the risk management calculations of Section 2 and Appendix A. Conversely, PL IV with a, 60 m provides a lower degree of protection. For PL III, the protection ensures that, for striking distances of 45 m or more, the shortest distance to the structure is to an air terminal. From Tables 4.1 and 4.2, such striking distances correspond from empirical observations to peak currents of 10 kA or more, and an interception efficiency of 91%, there being only of the order of 9% of strikes having a lower current. In the RSM, lightning is considered most likely to follow the path of shortest distance. This path will have the highest average electric field produced by the potential difference between the tip of the lightning leader (likely to be at more than 10 MV) and the structure (approximately at earth potential). The RSM produces a conservative design since it makes no allowance for field intensification at the edges and corners of structures. Using a constant radius for the rolling sphere the sides and tops of structures are assigned an equal probability of lightning strike to the corners and edges. In particular, the ‘rolling sphere’ method is unduly conservative for large flat surfaces, such as on the roof of a structure and on the sides of tall structures, both of which are unlikely to be struck by lightning. Further advice on the protection of roofs is given in Clause 4.11.2. A simple modification to the RSM can overcome the former problem (See Paragraph G2, Ref. 6). The basis of the modification is that the application of the RSM will be a two-step process in which— (a)

the air terminal network is first selected and positioned to provide interception protection for points, corners and edge surfaces using a rolling sphere of radius (a) selected from Table 4.2; and

(b)

the selected and positioned air terminal network is then used to determine if protection is provided to all plane (flat) surfaces using a rolling sphere with the corresponding increased radius (a i ) in Table 4.2; if not, more air terminals are added to protect the exposed plane surface(s) still using the rolling sphere of radius (a i ).

For the purposes of this modification to the RSM, a plane surface is defined as any large flat surface that has no projection from it exceeding 300 mm. Any flat surface is considered to be large if, after step (b), it is apparent that more air terminals are required to protect it.

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According to the RSM, an air terminal of height h above the edge of a flat roof or horizontal plane is considered to protect the edge of that roof or plane up to a horizontal distance r from a vertical rod or a horizontal conductor, where r is given by: r=

(2a h − h 2 )

. . . 4.4.2(1)

in which a is the sphere radius from Table 4.2 and r, a and h are in metres. For points on the flat surface, according to the modification to the RSM, the protection extends to a distance r i from the air terminal, given by: ri =

(2ai h − h 2 )

. . . 4.4.2(2)

in which a i is the increased radius from Table 4.2. For all but small horizontal surfaces, an array of air terminals is required at spacing distances that ensure that no parts of the surface are unprotected. If the array is a square grid, the spacings must not exceed d vr for vertical rods and d hc for horizontal conductors, given by: d ≤ 2 ri d ≤ r 2 and hc . . . 4.4.2(3) vr

i

Some numerical values are given to illustrate the magnitudes; for protection level III, a is 45 m and a i is 90 m, then when the air terminal of height h is 1 m, r is 9.4 m, r i is 13.4 m, d vr is 19 m and d hc is 26.8 m. Table 4.3 provides information on the spacings of air terminals required to protect roofs at each of protection levels I to IV for the three most common heights of air terminals used in Australia and New Zealand. Figures 4.2 and 4.3 illustrate the application of the RSM (with the modification for flat surfaces) to a rectangular structure using either vertical rods or horizontal conductors. Figure 4.2 (Step (a)) deals with the interception protection for the vulnerable corners and horizontal edges and Figure 4.3 (Step (b)) shows how Equations 4.4.2(2) and 4.4.2(3) are used for the flat roof. For more complex structures, the technique of rolling the sphere must be used directly to determine the required configuration of air terminals needed to achieve the interception protection efficiency that has been selected.

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Step (a): Protect corner and edge surfaces Either, Place vertical rods at 4 corners, try h = 1 m (shown). Using Equation 4.4.2(1) r =

(2ah − h 2 ) = 9.4 m

Max spacing along edge = 2 × 9.4 = 18.6 m Therefore, 3 additional rods are needed along each edge Check with rolling sphere of radius 45 m, okay (shown) or

Place metal railings h = 1 m along the 4 edges (not shown) This protects all 4 corners and 4 edges

FIGURE 4.2 APPLICATION OF STEP (a) OF RSM (WITH THE MODIFICATION FOR FLAT SURFACES) FOR PROTECTION LEVEL III FOR A RECTANGULAR STRUCTURE OF DIMENSIONS 70 X 50 X 20 m USING EITHER VERTICAL ROD OR RAISED HORIZONTAL CONDUCTOR AIR TERMINALS

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Step (b): Using increased sphere radius ai to determine if more terminals are required 14 × 1 m rods plus 4 × 1.25 m rods Using Equation 4.4.2(2), ri =

(2ai h − h 2 ) = 15 m

The building is protected

FIGURE 4.3(a) APPLICATION OF STEP (b) OF RSM (WITH THE MODIFICATION FOR FLAT SURFACES) FOR PROTECTION LEVEL III FOR A RECTANGULAR STRUCTURE OF DIMENSIONS 70 × 50 × 20 m USING 4 × 1.25 m VERTICAL RODS ON THE INTERIOR PLANE SURFACE

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Step (b): Using increased sphere radius ai to determine if more terminals are required 14 × 1 m rods plus 2 × 1.6 m rods Using Equation 4.4.2(2), ri =

(2ai h − h 2 ) = 16.9 m

The building is protected

FIGURE 4.3(b) AN ALTERNATIVE APPLICATION OF STEP (b) OF RSM (WITH THE MODIFICATION FOR FLAT SURFACES) FOR PROTECTION LEVEL III FOR A RECTANGULAR STRUCTURE OF DIMENSIONS 70 × 50 × 20 m USING 2 × 1.6 m VERTICAL RODS ON THE INTERIOR PLANE SURFACE

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Step (b): Using increased sphere radius ai to determine if more terminals are required Either, 1 (or more) additional raised horizontal conductors h = 1 m to protect the flat roof Using Equation 4.2.2(2), ri =

(2ai h − h 2 ) = 13.4 m

Using Equation 4.2.2(3), d hc ≤ 2ri = 26.8 m, but is < the 70 m between the edge railings, and so the 4 railings along the edges do not protect all the roof Add 2 additional raised horizontal conductors h = 1 m and all the roof is protected (shown) So need 4 × 1 m high railings plus 2 additional 1 m high raised horizontal conductors or

The 2 additional raised horizontal conductors could be replaced by 2 × 1.5 m vertical rods (r = 16.4 m) as shown

FIGURE 4.3(c) AN ALTERNATIVE APPLICATION OF RSM (WITH MODIFICATION FOR FLAT SURFACES) FOR PROTECTION LEVEL III FOR A RECTANGULAR STRUCTURE OF DIMENSIONS 70 × 50 × 20 m USING RAISED HORIZONTAL CONDUCTORS ONLY OR RAISED HORIZONTAL CONDUCTORS ON BUILDING CORNERS AND EDGES AND VERTICAL RODS ON THE INTERIOR PLANE SURFACES

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0.5 1 2

0.5 1 2

III (a = 45 m, a i = 90 m)

IV (a = 60 m, a i = 120 m)

2)

( =

(

)

7.7 10.9 15.4

6.7 9.4 13.3

5.5 7.7 10.8

4.4 6.2 8.7

2a h − h 2 )

r

Horizontal distance for which roof is protected

15.5 21.8 30.7

13.4 18.9 26.5

10.9 15.4 21.5

8.9 12.5 17.4

d (= r.2)

Maximum spacing for array

(=

10.9 15.5 21.8

9.5 13.4 18.9

7.7 10.9 15.4

7.7 10.9 15.4

(2ai h − h 2 ) )

ri

Horizontal distance for which roof is protected

For further information on raised horizontal conductors placed on the roof, see Clause 4.11.2.

For other air terminal heights (h) use equations 4.4.2(1), (2) and (3).

0.5 1 2

II (a = 30 m, a i = 60 m)

1)

0.5 1 2

h

Height of air terminal 1)

I (a = 20 m, a i = 60 m)

Protection level

Edges and corners of roof protected by vertical rod or raised horizontal conductor

15.5 21.9 30.9

13.4 18.9 26.7

10.9 15.4 21.7

10.9 15.4 21.7

( = ri

d vr 2)

Maximum spacing for an array of vertical rods

Middle of flat roof

HEIGHT AND SPACING OF AIR TERMINALS TO PROTECT ROOFS

TABLE 4.3

21.9 30.9 43.6

18.9 26.8 37.7

15.5 21.8 30.7

15.5 21.8 30.7

dhc (= ri.2)

Maximum spacing for raised horizontal conductor 2)

dimensions in metres

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4.5 METHODS OF PROTECTION 4.5.1 Structural steel-framed buildings Buildings with structural steel framing may be protected by the installation of metal air terminals at the high parts of the building, the air terminals being connected to the steel framing and the framing earthed in the vicinity of the foundation. A typical LPS is shown in Figure 4.4 (see also Clause 4.16.1). 4.5.2 Buildings without structural steel frames 4.5.2.1 General The required conditions of protection for non-metallic buildings are generally met by placing metal air terminals on the uppermost parts of the building or its projections, with conductors connecting the air terminals to each other and to earth. By this means a relatively small amount of metal properly positioned and distributed can afford a satisfactory degree of protection and, if desired, the material may be placed so as to give minimum interference to the appearance of the building. A typical LPS is shown in Figure 4.5. Additional methods utilizing the individual characteristics of particular types of building construction are given in Clauses 4.5.2.2 to 4.5.2.4, and in Figure 4.4. 4.5.2.2 Structures with continuous metal Structures containing continuous metal, e.g. metal within a roof, wall, floor or covering may, if the amount and arrangement of the metal is adequate in terms of the recommendations of Clauses 4.12 to 4.16, utilize such metal as part of the LPS. 4.5.2.3 Metal-roofed buildings Lightning tends to strike the ridges, corners, parapets and edges of building roofs. Generally these attachment points coincide with the metal roof sheeting, capping or guttering being fixed by multiple screws, rivets or clips directly to the supporting steel purlins, beams and trusses below. While a direct strike can puncture a hole in thin metal sheeting the instances of such damage are rare (refer to Paragraph B3, Appendix B for more information). For many buildings that are roofed, or roofed and clad, with metal, it may be possible to dispense with some or all air terminals provided the supporting roof steelwork is directly connected to a downconductor network or the earthing system. Any decision on dispensing with use of air terminals should consider the consequences of possible damage to the integrity of the roof and the contents within the building. 4.5.2.4 Reinforced concrete buildings The following recommendations apply to the use of steel reinforcement in reinforced concrete buildings as part of the LPS (see also Paragraph B5.5.2, Appendix B): (a)

General As far as possible, the steel reinforcement should be made electrically continuous in all concrete elements having a structural purpose, e.g. columns, beams and also in non-structural concrete elements, e.g. concrete wall panels, where the element, or a part of it, if dislodged, could endanger persons below. Where the steel reinforcement is used as the downconductor system, an effective electrical connection should be made from the air terminal network to the steel reinforcement at the top of the building. Such connections should be made, by means such as welding or clamping to a minimum of four vertical and/or horizontal bars, to ensure a multiplicity of conductive paths for the discharge of lightning current. NOTE: Steel reinforcement that is overlapped and tied by means of wire is not considered to provide an effective electrical connection for the purpose of air termination connection, however, such joints are acceptable elsewhere as part of the downconductor system where current sharing is assured. COPYRIGHT


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Modern reinforced concrete structures frequently involve several structural techniques including in situ reinforced concrete, prestressed reinforced concrete and precast concrete; recommendations for these are listed in Items (b), (c) and (d). (b)

In situ reinforced concrete Reinforcing steel rods are tied together by steel tie wire at splice points where reinforcing steel bars are overlapped for mechanical strength. Despite the fortuitous nature of the metallic connection, the very large number of rods and crossing points assures a subdivision of the total lightning current into a multiplicity of parallel discharge paths. Experience shows that with this splicing technique the rods can also be readily utilized as part of the LPS without thermal or mechanical damage to the structure. NOTE: Particular recommendations on the size, material or number of tie wires are not given in this Standard, normal building practice being relied upon to provide adequate continuity.

Normal building practice also ensures the multiple conducting paths continue into the building foundations (see Note). The foundations are deep in the mass of earth and the resistivity of concrete is generally comparable with that of clay or other moderately conductive ground. Hence, except in soils of low resistivity, the resistance to earth from the foundation reinforcement is often lower than can be obtained economically with driven rods, because of the much greater surface area. Concrete foundations themselves constitute a satisfactory earth termination network but their use, as such, precludes the inclusion of base conductors. It is desirable, however, that a metallic connection to the reinforcing be installed, in a position suitable for the bonding of metallic services associated with the building. NOTE: Conductive paths may not be ensured if special building techniques are used, e.g. grouting reinforcing bars into drilled holes in concrete after it has set, using an insulating epoxy-based material.

(c)

Prestressed reinforced concrete Prestressed reinforced concrete is used most commonly in the horizontal structural elements in a building, such as the beams and floor slabs, and only rarely in vertical elements such as columns. Consequently, the principal reason for avoiding insulating gaps in prestressed concrete relates to sideflashing rather than to the ability of the reinforcement to carry a lightning discharge to earth. See Clause 4.16.2 for details of the treatment of prestressed concrete in order to avoid side-flashing. These principles should be used in the rare instance where vertical prestressed elements, such as prestressed columns, occur in a building. Although prestressed concrete affords a large reduction in the cross-sectional area of steel reinforcement compared with conventionally reinforced concrete, calculations indicate that prestressed cables of 10 mm diameter or more, will not be damaged thermally by lightning and that thermal effects become negligible when several cables are connected in parallel.

(d)

Precast concrete Where electrical continuity is required through precast concrete elements, the structural connection details, e.g. attachment plates, threaded ferrules, bolt or dowel connections, should be carefully examined from an electrical continuity standpoint. In most cases, the attachment device will be a metallic one and continuity can be achieved by simply welding the attachment device to electrically continuous reinforcement within the precast concrete element.

4.5.3 Structures with flammable or explosive atmosphere Structures in which very small induced sparks present an appreciable element of danger, such as structures containing explosive atmospheres of flammable vapour or gas and structures in which easily ignitable fibres or materials producing combustible fines are stored, e.g. cotton, grain or explosives, usually require much more than the standard protection. Such structures can be protected by tall conducting masts earthed at the bottom, by bonding as detailed in Clause 4.16.2.2, or by overhead earthed wires (see Section 7). COPYRIGHT


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4.6 MATTERS TO BE CONSIDERED WHEN PLANNING PROTECTION 4.6.1 Structures to be erected For structures that are to be erected, the matter of lightning protection should be considered in the planning stage, as the necessary measures can often be effected in the architectural features without detracting from the appearance of the building. In addition to the aesthetic considerations, it is usually less expensive to install lightning protection during construction than afterwards. 4.6.2 Design considerations 4.6.2.1 General considerations The structure or, if the structure has not been built, the drawings, should be examined with due regard to all the relevant details of this Standard and in particular to the following: (a)

Metal used in the roof, walls, framework or reinforcement above or below ground, e.g. sheet piling, to determine the suitability of such metal in place of, or for use as, components of the LPS. NOTE: For a non-metallic roof, the position of any conduit, piping, water mains or other earthed metal immediately beneath the roof should be noted, as this may inadvertently attract a discharge if not shielded by an adjacent roof or structure, or downconductor on or above the roof.

(b)

Available positions for downconductors providing the required number of low impedance paths from the air terminal network to the earth termination; this is particularly important for internal downconductors.

(c)

The nature and resistivity of the soil as revealed by trial bore holes for foundation purposes or soil resistivity tests with, where economically practicable, the driving and testing of a trial earthing electrode with the object of designing a suitable earth termination.

(d)

Services entering the structure above and below ground.

(e)

Radio and television antennas and microwave communications antennae.

(f)

Flag masts, roof level plant rooms, e.g. lift motor rooms, ventilating plant and boiler rooms, water tanks and other salient features.

(g)

The construction of roofs to determine methods of fixing conductors with special regard to maintaining weatherproofing of the structure.

(h)

Possible penetration of waterproofing membrane where earth terminations are to be sited beneath the structure.

(i)

The provision of holes through, or fixing to, reinforced concrete.

(j)

The provision of bonding connections to steel frame, reinforcement rods or internal metalwork, and for any holes through the structure, parapets, cornices, and the like, to allow for the free passage of the lightning conductor.

(k)

The choice of metal most suitable for the conductor, e.g. aluminium conductors for structures where aluminium is employed externally.

(l)

Accessibility of test joints; protection by non-metallic casing from mechanical damage or pilferage and hazard to persons; lowering of flagmasts or other removable objects; facilities for periodic inspection, especially on tall chimneys.

(m)

The preparation of an outline drawing incorporating the foregoing details and showing the positions of the main components to form a basis for the record drawing recommended in Clause 8.4.

(n)

Requirements for the coordination of the structure’s lightning protection earthing and the earthing of electricity supply and telecommunications services. COPYRIGHT


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FIGURE 4.4 TYPICAL LPS USING METAL IN OR ON A STRUCTURE

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FIGURE 4.5 TYPICAL LPS EMPLOYING VERTICAL AIR TERMINALS, FOR PROTECTION LEVEL III

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4.6.2.2 Route for conductors Conductors should be installed with a view to offering the least impedance to the passage of discharge current between the air terminals and earth. The most direct path is the best (see Clause 4.12.2). The impedance to earth is approximately inversely proportional to the number of widely separated paths, so that from each air terminal there should be as many paths to earth as practicable. The number of paths is increased and the impedance decreased by connecting the conductors to form a cage enclosing the building. 4.6.2.3 Trouble-free installation Since an LPS, as a general rule, is expected to remain in working condition for long periods with little attention, the mechanical construction should be strong, and the materials used should offer resistance to corrosion. 4.6.2.4 Economy of installation Economy of installation can be effected by keeping the variety of equipment to a minimum, avoiding the use of unusual air terminal ornaments and similar features, and taking advantage of constructional features as far as practicable. 4.7 MATERIALS 4.7.1 General Copper is recommended for its conductivity and durability; however, alternative materials may be used if suitable for the environment in which they are installed and are otherwise satisfactory for the purpose (see Clause 4.8). Typical materials from which the currentcarrying component parts of LPSs may be chosen are given in Table 4.4 (see also Clause 4.7.2). Where insulating coatings are used, due regard should be given to their durability and nonflammability. For the protection of conductors at the tops of chimneys, see Clause 4.7.2.2(a). 4.7.2 Corrosion 4.7.2.1 Basic considerations The materials used in LPSs should be resistant to corrosion resulting from the environment in which they are installed. This includes the effects of atmospheric, soil or water-borne electrolytes or contaminants, and of contact with those metals or alloys that will lead to galvanic corrosion in the presence of moisture. Corrosion resulting from contact of dissimilar metals can exist where a conductor is held by fixing devices on or against external metal surfaces of a building or structure. Corrosion of this nature can also arise where water passes over a relatively cathodic metal such as copper carrying small amounts of copper corrosion product that is deposited as a fine film of metallic copper on relatively anodic metals such as aluminium, zinc or steel. This causes destructive galvanic corrosion of the latter metals, which are commonly used in building cladding or roofing. The metallic components of the LPS should therefore be compatible with the metals used externally on the structure over which these components pass or with which they may make contact. The components of LPSs may be constructed from a variety of materials as described in Clauses 4.7.2.2 and 4.7.2.3. 4.7.2.2 Air terminals and downconductors Specific recommendations for air terminals and downconductors are given in Clauses 4.11 and 4.12 respectively. Account should be taken of the principles outlined in Clause 4.7.2.1 in the selection of materials for those components. COPYRIGHT


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Where there is a risk of metallic building elements being contaminated by corrosion products, e.g. from copper conductors, the use of insulated conductors should be considered. Such insulation may need protection against ultraviolet radiation, e.g. by enclosure in conduit or by the application of appropriate paints or coatings. Where insulated cables are used as downconductors, bonding to the air terminal network should be effected at the specified intervals (see Clause 4.16.2.2) and bonding connections should be sealed against the ingress of moisture. Where structural steel or reinforcing bars form part of the downconductor system no further corrosion-protection will normally be required. With the common conductor materials, several specific precautions are necessary as follows: (a)

Bare copper Copper should be of the grade ordinarily used for commercial electrical work. NOTE: Where any part of a copper conductor used in an LPS is exposed to the direct action of chimney gases or other corrosive gases, it should be protected by a continuous coating of tin, lead or other material suitable for the environment to which it is exposed. Such a coating should extend not less than 500 mm below the top of a chimney or outside the area of exposure. The coating should not be removed at joints.

(b)

Bare alloys Galvanized iron or alloys of metals should be substantially as resistant to corrosion as copper under similar conditions. Galvanized iron may be used as part or the whole of the downconductor system provided it has adequate current-carrying capacity and is fastened with fittings having compatible corrosion characteristics. The galvanized iron may comprise the structural or decorative elements of the building subject to these requirements.

(c)

Bare aluminium or aluminium alloys Care should be taken not to use aluminium in contact with concrete, mortar, the ground, or in other situations where moisture may be retained causing the aluminium to deteriorate. Precautions should be observed at connections with dissimilar metals. In aluminium LPSs, copper, copper-covered and copper alloy fixtures and fittings should not be used. Aluminium or aluminium alloy fixtures and fittings or non-metallic components of adequate strength and durability are required. Special arrangements will be needed at any earth terminations for this class of LPS.

Other materials may be used to the extent recommended elsewhere in this Standard.

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TABLE 4.4 TYPICAL MATERIALS FOR CURRENT-CARRYING COMPONENTS Material

Standard

Grade or type

AS 1565 AS 1874

C92410 EA401 or AA607

Castings Leaded gunmetal Aluminium alloy Bars and rods Copper, hard-drawn or annealed Phosphor-bronze Naval brass Aluminium bronze Aluminium Aluminium alloy Galvanized steel Stainless steel

AS/NZS AS/NZS AS/NZS AS/NZS AS/NZS AS/NZS — —

1567 1567 1567 1567 1866 1866

110 518 464 627 1050 6063 or 6463A — —

Tubes Copper Galvanized steel

AS 1432 or NZS 3501 AS 1074

— —

Copper, annealed Aluminium Galvanized steel Stainless steel

AS 1566 AS/NZS 1866 AS 1397 ASTM A240M

110 1200 — —

AS 1746 AS/NZS 5000.1 AS 1531 AS/NZS 5000.1 AS 1222.1 —

— — — —

AS/NZS 1567 AS/NZS 1567 AS/NZS 1567 AS 2738 —

518 464 627 272 —

BS 1473 AS 1214

HB30 —

Strip

Stranded conductors Copper, hard-drawn Aluminium Galvanized steel Stainless steel Fixing bolts and screws for copper Phosphor-bronze Naval brass Aluminium bronze Common brass Stainless steel Fixing bolts and screws for aluminium and aluminium alloys Aluminium alloy Galvanized iron or steel

4.7.2.3 The earth termination network The design of the earth termination network should assume that each earthing electrode will be bonded, directly or fortuitously, to the following— (a)

the electrical installation earthing system and the MEN of the electricity supply service (see AS/NZS 3000);

(b)

the building structural steelwork or reinforcing material;

(c)

any incoming service earth(s);

(d)

any water, sewer and fire system supply pipes, if metallic; and

(e)

any pipelines for gaseous or liquid fuels, if metallic.

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Some supply authorities attempt to isolate services (d) and (e) from (a), for galvanic corrosion control reasons, by inserting insulating spacers at the pipe entry. Consideration should be given to the fitting of SPDs across the insulating spacers, in consultation with the supply authority, to prevent arc discharge without prejudicing the corrosion control measures. The earth termination network should be capable of satisfactory performance for the expected life of the LPS under the corrosion conditions existing at the site when bonded to— (i)

copper-based earthing systems (in most electrical installations);

(ii)

steel-based structural material;

(iii) incoming service earths; which may be stainless steel, galvanized iron, copper or lead; and (iv)

other metallic incoming services, e.g. steel or copper pipes for water or gas.

There are two hazards that arise from the bonding of other service earthing electrodes or service lines to the MEN of the electricity supply service. Firstly, if the earthing system of the electricity supply service is copper-based (as is mostly the case), it will cause progressive galvanic destruction of less cathodic metals, such as steel, to which it is connected. Secondly, the electricity supply service has many loads connected to it that generate a direct current component; this direct current is an electrolytic hazard to other earthing systems to which the electricity supply service earth is bonded. The amount of direct current that can be generated by each appliance is limited by AS/NZS 3100, but it is still sufficient to place at risk some types of earthing electrodes. In particular, steel rods clad with copper or stainless steel suffer premature failure when a small amount of direct current such as this perforates the cladding, initiating a process of self-destruction of the rod core. It will be clear that the selection of any common metal or alloy for the earth termination network places either itself or other systems or services at some risk from galvanic corrosion. For lower-cost installations the use of one of the common metals or alloys may be satisfactory. A list of these, with comments relating to their corrosion performance, is provided in Table 4.5. The extent to which the material combination ‘can be damaging’ is related to soil moisture, the type and nature of electrolytes present, and area and resistance relationships. Inherently, if such materials are used, a maintenance checking routine is essential. NOTE: For further information see Paragraph C9, Appendix C.

Where soil conditions are particularly aggressive from a corrosion viewpoint (soil resistivity typically below 30 Ω.m, especially if combined with a pH value of less than 5.5), such as may exist in reclaimed marine areas, the use of an inert anode material (see AS 2832.1) may be necessary. Expert advice on the selection of an appropriate earth termination network should normally be sought where such soil conditions exist.

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TABLE 4.5 CORROSION PERFORMANCE OF COMMON METALS AND ALLOYS USED AS EARTHING ELECTRODES Deleterious effect of this metal/alloy on other bonded underground ferrous metals

Deleterious effect on this metal/alloy from bonding to MEN (copper-based) systems

Nil

Damaging

Solid copper

Damaging

Nil

Copper-clad steel

Damaging

Can be damaging—may be acceptable

Solid stainless steel or nickel iron alloy

Generally acceptable

Can be damaging—may be acceptable

Stainless-steel-clad steel

Generally acceptable

Can be damaging

Bronze

Generally damaging

May be acceptable

Brass

Can be damaging

May be acceptable—can be dezincified

Zinc

Nil

Damaging

Aluminium

Nil

Extremely damaging

Magnesium

Nil

Extremely damaging

Metal/alloy Galvanized iron or steel

4.8 FORM AND SIZE OF CONDUCTORS 4.8.1 Factors influencing selection The form and size of the conductors of the LPS should be selected having regard to their— (a)

electrical and thermal characteristics (see Clause 4.8.2); and

(b)

mechanical strength, if required, and the likelihood of corrosion (see Clause 4.8.3).

The minimum cross-sectional area required of a main current-carrying component of a LPS is 35 mm 2 . The dimensions of typical conductors are given in Table 4.6. 4.8.2 Electrical and thermal considerations Air terminals, downconductors and other conductors of the LPS that may carry the full lightning current, should have a cross-sectional area and electrical conductivity such that they are able to carry the expected current without deterioration and without attaining temperatures that may give rise to risk of fire. Copper conductors having a cross-sectional area of not less than 35 mm2 will normally be necessary for this purpose. Conductors of other materials may be used provided they satisfy the above criteria for current-carrying capacity and temperature rise. (For further details, see Table 4.6). Conductors, which because of their arrangement in the LPS, will carry only a proportion of the lightning current, may have a cross-sectional area that is proportionately reduced but should be not less than one-fifth of the cross-sectional area needed to carry the full lightning current, or 6 mm2 , whichever is the greater. Conductors of larger cross-sectional area than recommended above may be needed as indicated in Clause 4.8.3.

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4.8.3 Mechanical strength and corrosion considerations Conductors of larger cross-section than those recommended in Clause 4.8.2 may be needed where— (a)

a significant reduction of cross-sectional area is likely to be experienced in service due to the effects of corrosion; or

(b)

an increase in cross-sectional area or section of different shape (e.g. tubular instead of solid) is required to provide adequate mechanical strength, e.g. for air terminals (see Clause 4.11.1).

Consideration should also be given to the use of a larger cross-sectional area than that recommended in Clause 4.8.2 in situations where inspection or repair of the conductor is unusually difficult. 4.9 JOINTS 4.9.1 Effectiveness of joints The LPS should have as few joints as possible. Joints and bonds should be mechanically and electrically effective, e.g. clamped, screwed, bolted, crimped, riveted or welded. Where overlapping joints are used, the length of the overlap should be not less than 20 mm for all types of conductor. Contact surfaces should first be cleaned then inhibited from oxidation with a suitable corrosion-inhibiting compound. All mechanical connections should be inspected on a regular basis in accordance with Section 8 to ensure the integrity of the connection over time. 4.9.2 Protective covering Joints and bonds may be protected with bitumen or embedded in a plastics compound according to the local conditions. Particular attention should be given to joints of dissimilar metals. 4.10 FASTENERS Conductors should be securely attached to the building or other object upon which they are placed. Fasteners should be substantial in construction and not subject to breakage, and should be, together with the nails, screws, or other means by which they are fixed, of the same material as the conductors, or of such nature that there will be no serious tendency towards galvanic corrosion in the presence of moisture because of contact between the different parts. Fasteners should be spaced so as to give adequate support to the conductor. Downconductors should be fastened at spacings not exceeding 1.0 m on horizontal runs and not exceeding 1.5 m on vertical runs. The method of fastening should not result in a reduction of the conductor cross-section below the minimum recommended in Clause 4.8. NOTE: Plastics materials may be used for the fixing of conductors provided such materials are suitable for long-term exposure to the outdoor environment (e.g. stabilized against the harmful effects of ultraviolet radiation) and otherwise satisfy the recommendations of this Clause.

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TABLE 4.6 TYPICAL SECTION DIMENSIONS OF MAIN CURRENT-CARRYING COMPONENTS Component

Typical section dimensions

Air terminals Strip Rods Stranded conductors

25 mm × 3 mm 10 mm dia. 35 mm 2

Downconductors Strip Rods Stranded conductors Galvanized materials

25 mm × 3 mm 10 mm dia. 35 mm 2 35 mm 2

Earthing electrodes and buried conductors Earth rods Galvanized star stakes (star pickets) Galvanized steel water pipe Galavanized steel strip Copper strip Stranded conductor

12 mm dia. Y – 25 mm × 19 mm × 19 mm 25 mm dia. 50 mm × 3 mm 25 mm × 3 mm 70 mm 2

Main current-carrying bonding conductors Strip Cable

25 mm × 3 mm 35 mm 2

4.11 AIR TERMINALS 4.11.1 General requirements An air terminal may consist of a vertical rod as for a spire, a single horizontal conductor as on the ridge of a small dwelling, or a network of horizontal conductors with vertical rods for the protection of roofs of large horizontal dimensions (see Figure 4.5). Protection may also be provided with a horizontal overhead wire supported, if necessary, independently of the building to be protected or by a vertical air terminal network (see Figures 4.2 and 4.3). Salient points of the structure should be incorporated in the air terminal network. The upper portions of the downconductors on tall buildings should be regarded as a continuation of the air terminal network and should be positioned so as to intercept side strikes to the building. Preference should be given to placing downconductors as near as possible to the exposed outer vertical corners of a building. All metallic projections, on or above the main surface of the roof, should be bonded to, and form part of, the air terminal network. In the case of telecommunications antennas, which have to be insulated from earth, a spark gap connection to earth or an SPD should be provided. Where roof construction consists of electrically continuous metallic materials, such metallic roofs may form part of an LPS, obviating the need for air terminals. If portions of a structure vary considerably in height, any necessary vertical air terminal or air terminal network of the lower portions should, in addition to their own downconductors, be bonded to the downconductors of the taller portions (see Figures 4. 4 and 4.5).

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Air terminals may be of any form provided the section used and the means of attaching it to the building structure have adequate mechanical strength to withstand the expected wind loading and natural harmonic resonances. Vertical air terminals of greater than three metres in height will generally need a form of additional fixing using brackets, bracing or guy wiring for suitable support. 4.11.2 Protection of roofs The parts of roofs most likely to be struck by lightning are parapets, the corner and edges of flat roofs, chimneys, and the ridges and eaves of sloping roofs. Preference should be given to positioning the air terminals so as to protect these highly exposed parts. The height of a vertical air terminal should be such that the tip will be not less than 500 mm above the object to be protected. On large flat and gently sloping roof areas a number of vertical rods of greater than 500 mm in height may be needed to establish a zone of protection over the whole roof area in accordance with Clause 4.4. Horizontal conductors such as strap or cable on parapets and metallic objects such as architectural features, flagpoles, metal railings, steel plant surrounds and roof access ladders may be used as air terminals to protect a planar roof surface. When positioned at a height of not less than 500 mm above the area to be protected and in accordance with the design rules (see Clause 4.3) and the RSM (see Clause 4.4.2), the conductors or objects will be at a suitable height to achieve the selected interception efficiency. In special circumstances, such as where it is desired to preserve the appearance of a historic building, horizontal air terminals may be installed immediately underneath the cladding (e.g. tiles) of a non-conductive roof. However, it should be noted that, in the event of a lightning strike to the roof, the cladding will be punctured and may suffer some damage. Horizontal and vertical air terminals and interconnecting conductors of the air terminal network should be located so as to constitute, as nearly as local conditions permit, an enclosing network that joins each air terminal to each other and to all downconductors. All metallic objects at roof level such as sheeting, plant, plant screens, tanks, gutters, walkways, ladders, antennas, masts, poles, vents, chimneys, conduits, piping, cable tray, enclosures, etc should be bonded to the air termination network. 4.11.3 Protection of the sides of tall buildings 4.11.3.1 Influence of forms of construction The consequences of a strike to the sides of a building may result in damage of a minor nature. Unless there are specific reasons for side protection, as would be the case with a structure containing explosives, it is considered that the cost of side protection will not normally be justified. Many buildings will have perimeter columns in which the reinforcement (or structural steel) is used as a part of the downconductor system. Where these columns on the external facade are no further than 20 m apart, no further protection will be required in respect of strikes to the side of the building. In the event of a strike to such a column or to isolated metal components such as small window frames, it is likely that a small section of masonry cladding material may be dislodged. Where the risk of this is unacceptable, conductors should be installed on the external faces of the columns to receive the strikes. These conductors will take the form of lightning air terminals/downconductors and should be bonded at the bottom into the LPS. Where side protection is a requirement of an LPS and suitable columns do not exist to receive strikes to the sides of buildings, vertical conductors should be installed for this purpose. These conductors should be spaced around the perimeter of the building at intervals not exceeding 20 m. COPYRIGHT


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4.11.3.2 Curtain wall construction It has become commonplace for tall buildings to have external glass curtain walls, with the curtain wall external to perimeter columns. The majority have major glass elements contained (and restrained) within a metallic framework. This framework is often inherently connected, electrically, to the metal in the building structure via the standard connection details used to mechanically fix the curtain wall structure to the structural frame of the building itself. Where this inherent connection occurs and where the frame of the building is incorporated into the LPS, no further bonding of the curtain wall to the LPS is necessary. In other cases, the curtain wall metal framing shall be bonded into the LPS at roof and ground level. It is essential that bonding be undertaken at the corners of the structure and around the perimeter of the building at intervals not exceeding 20 m. (See Clause 4.16.2.2 for further details). 4.12 DOWNCONDUCTORS 4.12.1 Structures—General Downconductors should be installed at each external corner of the building and additional downconductors installed, as necessary, at spacings not exceeding 20 m. 4.12.2 Route The route followed by downconductors should be in accordance with the following recommendations: (a)

Downconductors should be distributed around the outside walls of the structure. It is undesirable to locate downconductors in areas where persons are liable to congregate. The walls of light wells may be used for fixing downconductors, but lift shafts should not be used for this purpose.

(b)

Where the provision of suitable external routes for downconductors is impracticable or inadvisable, e.g. buildings of cantilever construction from the first floor upwards, downconductors may be housed in an air space provided by a non-metallic, non-combustible internal duct. Any covered recess or any vertical service duct running the full height of the building may be used for this purpose, provided that it does not contain any unarmoured or non-metal-sheathed service cable (see Clause 4.16.2.3).

(c)

Any extended metal running vertically through the structure should be bonded to the lightning downconductor at the top and bottom unless the clearances are in accordance with Clause 4.16.

(d)

A downconductor should follow the most direct path possible between the air terminal and the earth termination. Right angle bends may be used when necessary but deep re-entrant loops should be avoided.

(e)

A structure on bare rock, protected in accordance with Clause 4.14.3.1, should be provided with at least two downconductors equally spaced.

NOTE: The positioning and spacing of downconductors on large structures has often to be decided in practice by architectural considerations. However, their number should be governed by the recommendations above.

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It is now recognized that sharp bends in a downconductor, such as occur at the edge of a roof, do not significantly impede the discharge of a lightning current, nor are the mechanical forces produced by a lightning current likely to endanger the conductor or its fixings. In contrast, re-entrant loops in a conductor can produce high inductive voltage drops so that the lightning discharge may jump across the open side of the loop. As a rough guide it can be stated that this risk may arise when the length of the conductor forming the loop exceeds 8 times the width of the open side of the loop. It follows from the above that there is no need to round the path of the downconductors at the edge of a roof and that an upturn within the limits stated is acceptable. Where large re-entrant loops as defined cannot be avoided, e.g. for some cornices or parapets, the conductor should be arranged in such a way that the distance across the open sides of the loop complies with the principles given above. Alternatively, such cornices or parapets should be provided with holes through which the conductor can pass freely. (See Figures 4.6 and 4.7). An exception to the above practice is necessary for a building cantilevered out from the first storey upwards. The downconductors in this case should be taken straight down to the ground since, by following the contour of the building, a hazard could be created to persons standing under the overhang formed by the cantilever. In such a case, the use of internal ducts for downconductors is recommended (see Figure 4.8).

FIGURE 4.6 GENERAL PRINCIPLES OF A RE-ENTRANT LOOP IN A CONDUCTOR TAKEN OVER A PARAPET WALL

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FIGURE 4.7 ACCEPTABLE METHOD OF TAKING A CONDUCTOR THROUGH A PARAPET WALL

FIGURE 4.8 ROUTES FOR DOWNCONDUCTORS IN A BUILDING WITH CANTILEVERED UPPER FLOORS

4.12.3 Mechanical damage Where any part of an LPS is exposed to mechanical damage it should be protected by covering it with moulding or tubing preferably of non-conductive material. If metal is used, the conductor should be electrically connected to both ends of the protective covering.

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4.13 TEST LINKS Where practicable, test links should be provided to enable the continuity of each individual parallel path of the lightning conductor system to be measured. Where a driven or buried earthing electrode is provided as part of the LPS, test links should be provided to permit measurement of the resistance of the individual earth terminations, in such a position that, while not inviting unauthorized interference, is convenient for use when testing. Such resistance measurements are indicative only and provide the basis of comparison to determine whether any deterioration in the earth termination network has occurred in service (see also Appendix C). 4.14 EARTH TERMINATIONS 4.14.1 General principles Each downconductor should be connected to an earthing electrode or to the earth termination network. The design of earth terminations should be such that lightning currents are discharged into the earth in a manner that will minimize step and touch potentials and the risk of side-flashing to metal in or around a structure. This can be achieved by ensuring that the potential with respect to the general mass of the earth at each earth termination is limited by a sufficiently low resistance to earth, so that the discharged current flows in as close to uniform manner as possible in all directions away from the structure. Ionization of the soil near an earthing electrode carrying lightning current tends to reduce the potential of the earthing electrode relative to remote earth to a lower value than the potential that would be calculated using the earth resistance measured at low currents. Appendix C provides information on the effectiveness of various forms of earth termination networks for lightning protection purposes and on the associated calculation/measurement procedures. 4.14.2 Earthing resistance 4.14.2.1 Basis for measurements The term earthing resistance is used in this Clause and elsewhere in this Standard because the most common measuring instruments available are low frequency devices. A more appropriate measurement for lightning protection purposes is that of earth impedance and such measurements are preferred when suitable high frequency or impulse type instruments are available. 4.14.2.2 Recommended values In general, the whole of an interconnected LPS should have an earthing resistance not exceeding 10 Ω before any bonding is effected to services that are not part of the LPS. In addition, each earthing electrode of an interconnected LPS that is not interconnected at or below ground level should have an earthing resistance not exceeding the product obtained by multiplying 10 Ω by the number of downconductors. NOTE: Where the installation has two or more air terminal networks not directly interconnected, such as a twin-tower building, then for the purpose of determining the required earthing resistance, it should be considered as consisting of separate LPSs.

Where buildings are primarily used for telecommunications services or information technology installations, or installation of multiple items of other sensitive electronic equipment, an earthing resistance not exceeding 5 Ω should be required. A reduction of earthing resistance can be achieved by extending or adding to the earth termination network or by interconnecting the individual earth terminations of downconductors.

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Notwithstanding the above recommendations, earthing electrodes complying with either of the following, need not comply with the 10 Ω criterion: (a)

For a substantial structure effectively encircled by a buried earthing electrode, an earthing resistance not exceeding 30 Ω should be satisfactory. A buried earthing electrode covering at least three sides of the structure may be regarded as effectively encircling the structure.

(b)

For any system incorporating two or more downconductors, it should not be necessary to install a total length of more than 50 m of widely separated horizontal or vertical earthing electrodes per downconductor, regardless of the earthing resistance.

Where reinforced concrete footings are used as earthing electrodes for a building, compliance with the recommended maximum resistance values should be determined by the measurement of resistance of typical footings that support the building structure. The measurements should be made at the stage of building construction when the footings are structurally isolated and may be treated as independent earthing electrodes. 4.14.3 Common earthing electrode and potential equalization 4.14.3.1 Common earthing electrode Where conditions permit potential equalization techniques to be used, a common earthing electrode may be installed to serve the LPS and other appropriate services. The earthing electrode should comply with the recommendations in this Standard and with any regulations that may govern the appropriate services (for telecommunications services, see Clause 4.14.3.2). The earthing resistance should be the lowest required by any of the regulations for such services. Where isolation is required, a common earthing electrode should not be used, but the separate earthing electrodes should be bonded via an SPD to minimize potential differences between the LPS earth termination network and other earthing systems in the event of a lightning strike. 4.14.3.2 Communications earths Where a communications earth is installed at a dwelling or similar small building, that earth should be connected to other earths present (see Paragraph B5.6, Appendix B). However, where a communications earth, such as a Telecommunications Functional Earthing Electrode (TFEE), is required to be isolated from other earths, because of noise or direct current conduction considerations, this earth should be bonded through a normally nonconducting protector or SPD. 4.15 EARTHING ELECTRODES 4.15.1 General considerations An earthing electrode may be of any type provided— (a)

it achieves a low resistance to the general mass of earth, as recommended in this Standard;

(b)

it has adequate mechanical strength and corrosion resistance to ensure that the desired service life will be achieved when installed in the environment concerned; and

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it has adequate current-carrying capacity for the discharge of lightning currents without sustaining damage that might jeopardize its continued effectiveness. NOTES: 1

Electrode earthing resistance may be measured by standard methods (see Paragraph C10, Appendix C).

2

If the soil resistivity is known, the electrode earthing resistance may be calculated as shown in Paragraph C3, Appendix C. It should be noted, however, that such calculations are only approximate and it is important that the electrode earthing resistance should in fact be determined by field test.

3

It is fairly easy to determine soil resistivity by test as set out in Paragraph C10.1, Appendix C.

The selection and design of the earth termination network should therefore take account of the following: (i)

Soil resistivity.

(ii)

The corrosion aggressiveness of the soil.

(iii) The physical structure of the soil (rocks, obstructions and other services). (iv)

The corrosion compatibility of the earth termination network with other structures to which it will be, or may become, bonded.

(v)

The options available for installation at the site (trenching, driving, drilling, land excavation or use of structural metalwork).

(vi)

The effects that it may have on other systems (electrical or telecommunications).

If the structure has a reinforced concrete floor and/or footings (see Clause 4.5.2), it may not be necessary to install lightning protection earthing electrodes, (see Clauses 4.5.2.4 and Paragraph C8, Appendix C). Generally speaking, the impedance to earth of a reinforced concrete floor and footings will be lower than that of an earth termination system utilizing vertical earthing electrodes. 4.15.2 Connections to earthing electrodes 4.15.2.1 Mechanical protection Where conductors that are connected to earthing electrodes are accessible, such conductors and connections should be protected against mechanical damage and vandalism. Where conductors connecting driven earthing electrodes in parallel are not installed above the ground, they should be buried not less than 500 mm below the surface. 4.15.2.2 Selection of materials Care should be exercised in the selection and application of materials for connections to earthing electrodes to avoid the possibility of galvanic corrosion, e.g. because of differences between the materials of such connections and the earthing electrodes. 4.15.2.3 Joints Joints between earthing conductors and earthing electrodes should be of adequate strength and current-carrying capacity, and be arranged so as to ensure that there will be no failure of the connection under conditions of use or exposure that can reasonably be expected. For suitable methods, refer Clause 4.9. 4.15.2.4 Test links If test links are inserted in earthing conductors connected to earthing electrodes, they should be secured in the closed position and be arranged so that the opening of any one link does not interfere with earth connections other than the one under test.

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4.15.3 Inspection and testing of earthing electrodes The earthing resistance of electrodes should be determined by test both at the time of installation and regularly during the life of the installation. For details of inspection and testing, see Section 8. 4.16 METAL IN AND ON A STRUCTURE NOTE: The term ‘metal in or on a structure’ includes all metal such as reinforcement rods and bars, pipes, conduits, chimneys, corrugated iron, roof sheeting, rails, ladders, screens, sunshades, etc. Metal hidden from view should not be overlooked. Tubing, containing electrical conductors or metal-sheathed cables, is, for instance, often embedded in an external wall and may be quite close to the LPS.

4.16.1 Use of metal in or on a structure as a part of the lightning protection system Where a structure contains electrically continuous metal, e.g. continuous steel frame, or metal within a roof, wall, floor or covering, this metal suitably bonded in accordance with Clause 4.16.2.2 may be used as part of the LPS, provided that the amount and the arrangement of the metal render it suitable for use, as recommended in Clauses 4.11 to 4.15 inclusive. Where a structure is simply a continuous metal frame without external metal covering e.g. tower pole, it requires no air terminal or downconductor; it is sufficient to ensure that the conducting path is electrically continuous and that the base is adequately earthed. A steel frame structure or reinforced concrete structure may have foundations with sufficiently low inherent earthing resistance and, if connections are brought out from the reinforcement, a test may be made to verify its suitability for use as part of the LPS (see Clause 4.15.1). 4.16.2 Prevention of side-flashing 4.16.2.1 Methods of prevention When an LPS is struck, its electrical potential with respect to earth is raised and, unless suitable precautions are taken, the discharge may seek alternative paths to earth by sideflashing to other metal. Two methods exist to prevent side-flashing: bonding and isolation. Bonding is the procedure whereby metal parts are positively connected to one another so as to prevent inadvertent electrical connection occurring due to side-flash. Isolation is the separation or insulation of metal parts in such a way that electrical breakdown or side-flash to them is prevented. Isolation may be achieved by separation of the LPS from the structure protected or by separating metal parts and services in a non-conductive structure from the LPS. Bonding effectively eliminates any local potential difference between the metal parts that are bonded together. However, it is possible to obtain large potential differences for very short times between adjacent metallic objects that are connected together at a remote location. These potential differences could be hazardous if the bonding system is inadequate. Many structures can be effectively bonded so as to eliminate any hazard, however, care should be taken to prevent subsequent installation of a metallic service creating a hazard. It should be noted that any conductive element that is bonded into the LPS can be expected to carry a proportion of the lightning current. Therefore, the bond and bonded element should be capable of carrying such current. (Refer to Table 4.6 for typical dimensions of bonding conductors). With isolation, it is often difficult to obtain and to maintain the necessary safe clearances, and to prevent connection of an ‘isolated’ LPS back to the structure via earth and buried metallic services. To achieve isolation it may be necessary to utilize a protection system that is completely separate from the protected structure, and is remotely earthed. COPYRIGHT


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If the structure is constructed with conductive materials such as reinforced concrete or steel frames, isolation of a protection system mounted on the structure requires the use of high impulse strength, high voltage insulation. In general, where the LPS is mounted on the structure, isolation can only be achieved at low cost if the structure is small. 4.16.2.2 Bonding The conditions under which bonding should be effected are as follows: (a)

Where practicable, all structural steel and metallic reinforcement in a structure, if not used as a part of the LPS, should be bonded to that system (and hence become part of that system). As indicated in Clause 4.5.2.4(b), metal rods in in situ reinforced concrete may be considered to be electrically continuous. Consequently, bonding may be achieved with a reasonable number of connections to the rods, a bonding connection to each rod being unnecessary. Where prestressed or post-tensioned concrete elements are involved, it has been found that the stressing cables frequently remain electrically isolated from other structural metal at the completion of the stressing process. Such cables should be bonded at both ends to the LPS, particularly where the structural element is exposed to the weather. NOTE: This bonding is recommended, not out of concern for a side-flash causing immediate structural damage, but rather to avoid the chance of the side-flash causing cracking of the corrosion-protecting concrete grout used around the cable. Prestressed cables under stress are highly susceptible to corrosion.

Where metal exists in a structure, such as reinforcement in a precast concrete spandrel panel or post-tensioned concrete slab, which cannot be bonded into a continuous conducting network and which is not or cannot be equipped with external earthing connections, its presence should be disregarded. The hazard presented by the presence of such metal should be minimized by keeping it entirely isolated from the LPS, which includes consideration of the numbers of downconductors. (b)

Where the roof structure is wholly or partly covered by metal, care should be taken that such metal is provided with a continuous conducting path to earth.

(c)

Metal that is attached to the outer surface of a structure should preferably be bonded as directly as possible to the LPS. Where bonding is difficult and where the consequences of side-flashing to isolated metalwork is not considered serious, bonding may be omitted. Where such metal has considerable length, e.g. cables, pipes, gutters, stairways, and runs approximately parallel to a downconductor or column, it should be bonded to the LPS at each end and at vertical intervals of not more than 20 m.

(d)

In curtain wall construction, where the framework would otherwise be electrically isolated, the frame should be made electrically continuous and should be bonded to the LPS at intervals not exceeding 20 m around the perimeter of the building. This should occur at the top and bottom of each curtain wall and at levels separated by not more than 20 m vertically, including those sections that are less than 45 m above ground.

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(e)

Where there is insufficient clearance from the LPS, metal entering or leaving a structure in the form of sheathing, armouring or piping for electricity supply, gas, water, telephone, steam, compressed air or other services, should be bonded as directly as possible to the earth termination at the point-of-entry or exit outside the structure. In this operation, the appropriate Standards and any regulations that may apply to such services should be observed.

(f)

Masses of metal in a building, such as a bell-frame in a church tower, should be bonded to the nearest downconductors by the most direct route available.

4.16.2.3 Isolation The necessary separation distance from any point on the LPS depends on the electric potential, or voltage, generated at that point by the lightning discharge. To achieve a sufficiently low probability of side-flash, the responses of the protection system to a range of severe stroke current waveshapes have to be considered. Because the time for a lightning stroke current waveshape to significantly change its steepness is similar to the time taken by the incident wave to travel from the point of strike to the earth termination, travelling wave techniques are used to calculate the voltage waveforms generated. However, an approximate voltage waveform sufficient to estimate the required separation distance can generally be calculated from the resistive and inductive voltage drops in the system. Isolation may be provided by appropriate air clearance from bare metal downconductors or by appropriately insulated downconductors. The requirements for insulated downconductors are detailed in IEC 62305.3. NOTE: Appendix D gives examples of calculation procedures.

For conventional LPSs using typical bare metal downconductors, the separation distance in air at a given point on the protection system is required to be not less than D, where D, in metres, is the greater of D1 and D 2 as defined below and shown in Figure 4.9: (a)

D 1 —is the required clearance associated with the discharge voltage of the design first stroke of a severe lightning flash and takes account of the design maximum lightning current. D1 is defined only for H/n < 30.

(b)

D 2 —is the required clearance associated with the discharge voltage of the design subsequent stroke of a severe lightning flash and takes account of the design maximum steepness of the current wavefront.

To take account of systems with a common earthing electrode it is necessary to separate D1 into two components as follows: D1

=

Di + De

. . . 4.16.2.3

Di

=

H , for 12n

De

=

0.3R

H

=

length of downconductor from the point considered to earth, in metres

n

=

number of downconductors connected to a common air terminal

R

=

combined earthing resistance of the LPS, in ohms

where H n

< 30

D i is the component of the first stroke separation distance associated with potential difference generated within the structure.

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D e is the component of the first stroke separation distance associated with local EPR, and is independent of the point on the LPS considered. This term is applicable to any remotely earthed objects, such as services entering the building, that do not share a common earthing electrode with the LPS, and to any long unearthed objects within a relatively non-conductive structure. Where a common earthing electrode in accordance with Clause 4.14.3 is used, the term D e may be neglected (R = 0 in Figure 4.9). Where it is applicable, the clearance De should be maintained throughout the structure and thus determines the minimum separation distance at the base of the structure. The required clearance for steep-fronted surges, D 2 , may be read from the dotted curve given in Figure 4.9. As the separation distance D 2 varies with the length of downconductor from the point considered to earth, D 2 normally determines the required separation in the upper parts of tall structures. The shortest separation distance over the surface of non-conductive structural material should be 2D for protected dry surfaces and 3D for external surfaces. The separation distance through solid non-conductive structural material should exceed 0.5D. NOTES: 1

For a substantial reinforced or structural steel frame building that utilizes the structure as part of the LPS, the separation distance may be obtained from Figure 4.9 by taking n to be 1.5 times the number of reinforced or steel columns. The term D e may be neglected for these buildings by assessing D for R = 0, except when considering remotely earthed services entering the building.

2

The extent to which uninsulated services may be considered to be affected by local EPR can be determined by a test in which a known current is injected into the LPS and potential differences to the electrical installation earthing system are surveyed.

FIGURE 4.9 REQUIRED SEPARATION DISTANCES IN AIR

4.16.2.4 Effects of bonding on cathodically-protected metal In the bonding of adjacent metalwork to the LPS, careful consideration should be given to the possible effects such bonding would have upon metalwork that may be cathodicallyprotected (see AS 2832 series).

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4.16.2.5 Bonding of underground services In the ground, bonding between the LPS earth termination network of any structure and buried metallic service pipes is essential, unless the service can be effectively isolated. If this is not done, an electrical breakdown can occur through the soil between these systems and the resulting electric arc can cause structural damage or may puncture a service pipe (see also Clause 4.16.2.2 (e)) SPDs may usually be fitted where direct connection is unacceptable.

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SECT ION 5 PROTECT I ON OF PERSONS EQU IPME NT W ITH I N BU I L D I NGS

AND

5.1 SCOPE OF SECTION This Section sets out recommendations for the protection of persons and equipment within buildings from the effects of lightning. The provision of an LPS for the building structure will not automatically guarantee such protection. These recommendations may be applied irrespective of whether an LPS for the building structure is provided in accordance with other sections of this Standard. The recommendations principally consist of installation of equipotential bonding of services, conductive parts, earthing systems, and the provision of SPDs as appropriate on services. 5.2 NEED FOR PROTECTION Whilst persons and equipment within buildings may be protected from a direct lightning strike, many circumstances arise where the effects of lightning are transmitted within the building, by various means as described below, placing persons and equipment at risk. Communications and electronic equipment are particularly susceptible to damage from lightning impulses and such damage may occur at energy levels well below those needed to cause injury to persons. In addition, there is a significant fire risk associated with impulse failure of many types of electrical and electronic equipment. Installation of protective measures, including primary and/or secondary SPDs on services, depends on many factors which may be included in an appropriate risk assessment. From a telecommunications perspective, AS 4262 provides a method to determine when protection is required (AS 4262.1 for person protection and AS 4262.2 for equipment protection). Further information on protection of customer premises may be found in ITU-T Recommendation K.66. 5.3 MODES OF ENTRY OF LIGHTNING IMPULSES There are four principal modes of entry of lightning impulses into buildings, as described below and shown in Figure 5.1, and these may occur separately or in combination. The modes are: (a)

Directly by the interception of lightning on exterior metalwork Lightning impulses may be transmitted in the interior of the building as a consequence of a strike to exterior metal that has a direct conductive connection to the interior of the building, e.g. via telecommunications antennas, plumbing fittings and the like. This mode of entry is characterized by a series path for the full impulse energy and is capable of conveying the full destructive effect of the lightning discharge. The waveshape of the lightning impulse is usually not significantly modified.

(b)

Indirectly by the interception of lightning on other structures or services A lightning strike to other structures or services that have conductive connection to the building, e.g. the low-voltage electricity distribution system or other services, may result in an impulse being transmitted into the building. The impulse is characterized by a lower energy level compared to that involved in Item (a), being a parallel path to the interior of the building served by the low-voltage mains. It is an EPR effect caused by the lightning impulse passing to earth through the neutral/earth connection and resulting in a increase in potential because of the impedance of the earth termination network.

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The magnitude of the impulse at the structure is governed by the neutral/earth impedance at the interception point, the length of the service line, the number of earthing features per unit length on the line adjacent to the interception point and, lastly, the electrical characteristics of the lightning discharge. Where a combined MEN electricity supply service exists (high voltage earth bonded to low-voltage earth), the regulatory authorities require an earthing resistance of not more than 1 Ω. As might be expected this limits the EPR impulse voltage considerably. In addition, in urban areas the number of electricity supply services with an earth (i.e. neutral/earth) connection is considerable, perhaps 100 per km. Consequently, the EPR lightning impulse is rapidly reduced, perhaps to insignificant levels in about 60 m. On the other hand, sparsely settled areas with distribution systems other than the MEN type can give rise to high EPR values that may not reduce to safe levels for some hundreds of metres. The impulse wave is normally modified by the transmission path in the EPR mode by distributed electrostatic capacity and transmission line effects. This reduces the severity of the impulse but prolongs the time that the protection equipment must conduct the pulse to earth. Although the energy levels involved in an EPR impulse are substantially less than those that apply for Item (a), they may still be of a high order. Based on sparks or arcs observed in incidents involving personal injury or equipment damage under EPR conditions, voltages of the order of 100 000 V are not uncommon and cases involving voltages of about 1 000 000 V have been observed. Impulse currents in this mode can range from a few amperes to several thousand amperes. It should be noted that EPR conditions can arise singly or as a combination of occurrences. In addition to lightning intercepting the overhead low-voltage electricity distribution system, other lightning leaders may intercept trees, clothes lines, sheds or other nearby structures, giving rise to a quite complex overall EPR condition. NOTE: Of the lightning incidents involving electrical equipment that have been investigated by Telstra, some 80 percent can be attributed to the entry mode described in the preceding paragraphs.

(c)

Inductively by electric and magnetic field coupling In general, this mode of entry happens more frequently but involves lower energy levels in comparison with the mode of entry in Item (b). Induction occurs when a lightning strike to ground gives rise to electromagnetic and electrostatic fields. These fields induce an impulse in conductors that intercept them. The conductors that are most affected are electricity reticulation and telecommunications lines. Commonly, the former are not damaged but the impulse may be transmitted to customer terminals and appear as a lower level lightning-induced impulse. This may damage or disable some forms of telecommunications equipment.

(d)

Direct induction into internal building wiring This mechanism is similar to Item (c), but the induction occurs directly into the building wiring itself. For this to be significant, the lightning strike needs to be very near by, typically to the LPS itself. Measures to reduce this effect, including reduction of installed conductor loop areas, are discussed in Clause 5.6.4.

Protection systems designed to counteract EPR and other impulses will normally provide adequate protection against impulses arising from the entry modes described in Items (b), (c) and (d).

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FIGURE 5.1 MODES OF ENTRY OF LIGHTNING IMPULSES

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5.4 GENERAL CONSIDERATIONS FOR PROTECTION Because of the many variables involved, each building will require specific consideration of the protective measures that should be applied. Particular attention should be given to possible entry and exit points for lightning current, which may include one or more of the following: (a)

Rooftop or external structures (e.g. TV antennas, communications hardware such as microwave dishes, metallic flues and ventilation outlets) or other exposed metal work not protected by the LPS for the building structure (e.g. metallic guttering and downpipes, metallic fences) These features will invariably be possible entry points for a lightning discharge.

(b)

The electricity supply service entry This will normally be an entry point for lightning if the service is aerial or overhead. It may be either an entry point or an exit point if the service is underground but it is more likely to be an exit point in such cases.

(c)

The telecommunications services entry This may be an entry point if the service is overhead using a dropwire or aerial cable. The service is more commonly underground and in such cases could be either an entry point or exit point.

(d)

Gas supply systems These are usually exit points for lightning but may occasionally present an EPR entry condition.

(e)

Metallic water supply and sewerage systems These are usually exit points for lightning but may occasionally present an EPR entry condition.

(f)

Other conductive services These are usually exit points for lightning but may occasionally present an EPR entry condition.

(g)

Building earthing systems (often there are several) These are usually exit points for lightning but may occasionally present an EPR entry condition.

(h)

The LPS for the building (if provided) By design these systems provide both an entry and exit point for a lightning discharge but, because of bonding, will present an EPR condition to other services.

An illustration of the possible entry and exit points for a lightning discharge is provided in Figure 5.2.

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FIGURE 5.2 POSSIBLE ENTRY AND EXIT POINTS FOR A LIGHTNING DISCHARGE

5.5 PROTECTION OF PERSONS WITHIN BUILDINGS 5.5.1 Objectives of protection The principal objective of measures for the protection of persons within buildings is to prevent hazardous potential differences between conductive parts with which the person(s) may be in contact. This is normally achieved by applying equipotential bonding between any conductive path into and out of the building, i.e. the entry points and exit points referred to in Clause 5.4. If such bonding has been installed it does not matter if a person is subject to an EPR with respect to distant earth as all conductive materials in the vicinity will be at approximately the same potential. An important consideration in the installation of equipotential bonding is how to install such bonding without adversely affecting the operation of the various services involved, particularly the protection systems associated with the respective systems. This is explained further in Clause 5.5.2. AS 4262.1 provides further information on the protection of users of telecommunications equipment from overvoltages that may exist between a user environment and telecommunications facilities in that environment.

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5.5.2 Installation of equipotential bonding In general, if made of copper, bonding conductors shall have a cross-sectional area as given below, unless they are main current-carrying conductors of the LPS. The typical dimensions of the main current-carrying conductors of a LPS are given in Table 4.6. All possible entry and exit points for the lightning discharge should be electrically bonded together in as direct a manner as practicable. The route taken by the bonding conductors is important. If incorrectly routed the bonding conductors themselves may damage other circuits or equipment by induction or side-flashing as currents of the order of tens of kiloamperes and voltages of the order of several thousand volts with respect to remote earth may be involved. Consequently, bonding conductors should not be grouped with other cables that are sensitive to induction unless the other cables are also bonded to the LPS. If the bonding conductor is long (some tens of metres) it shall be considered as an impulse transmission line, in which mode the protection afforded by the bonding will be limited. Some specific recommendations applicable to bonding of the entry and exit points referred to in Clause 5.4 are given below: NOTE: Appendix E provides additional information on earthing and bonding.

(a)

Rooftop antennae and communications hardware The bonding conductor should be attached to the most substantial part of the structural metal supporting the equipment consistent with it fulfilling the requirements of an air terminal for the LPS of a building. The bonding conductor to the antenna or communications hardware should be insulated to at least the level required in AS/NZS 3191, if run within the building, but may be uninsulated if run externally. The cross-sectional area of the bonding conductor should be not less than 16 mm 2 if made of copper.

(b)

The electricity supply service entry There are two distinct considerations that apply. Firstly, the electrical installation earth should be bonded to the LPS earth termination network with a copper conductor of not less than 6 mm2 cross-sectional area. Secondly, SPDs should be installed for each active conductor of the electricity supply service. Where an SPD is mounted on, or in, the building, its earthing system should be bonded to the LPS by a conductor having a cross-sectional area of not less than that utilized for its own earthing conductor. Where SPD equipment is separated from the building (e.g. mounted on a customer’s electricity supply service pole), the SPD earth should not be used as the earthing termination for the building LPS, however, the LPS earth termination network and the SPD earth may be bonded together, if desired.

(c)

The telecommunications service entry This may be either aerial (overhead) or underground. If aerial, the service should be regarded as a potential entry point for lightning and an SPD should be fitted, subject to the requirements of the telecommunications regulatory authority*. The telecommunications service earthing system shall be bonded to the LPS earth termination network. If the telecommunications service is underground, the service will act essentially as an exit point for lightning. In this mode it may be necessary to fit an SPD to the service to provide a bonding point for potential equalization. The bonding conductor should have a cross-sectional area of not less than 6 mm2 if made of copper.

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Metallic water supply and sewerage systems Metallic water supply and drainage systems should be bonded to the LPS and connected to the electricity supply service earth. However, some water supply authorities fit insulating spacers or ferrules for galvanic corrosion control at customers’ installations. These may require bridging by an SPD as determined in consultation with the water supply authority. Bonding conductors to these services should have a cross-sectional area of not less than 4 mm2 if made of copper. Metallic piping systems associated with fire sprinklers, water, hot water or flammable liquid, that are unavoidably in contact with the exposed conductive parts of wiring enclosures, cable components or other electrical equipment shall be connected to such equipment by means of an equipotential bonding conductor. If calculation or local experience indicates that the water supply system is of very low resistance to earth (e.g. less than 0.5 Ω), it may well be the principal exit point for the lightning impulse. In such circumstances, consideration should be given to upgrading the current-carrying capacity of the bonding conductor between the LPS earth and the water supply system to a cross-sectional area of not less than 35 mm2 if made of copper.

(e)

Other service lines Specific considerations may apply for some structures. For example, a radio telephone tower should be bonded to its associated equipment building; similarly, a pump station should be bonded to an elevated water tower. For both examples given, the bonding conductor is likely to carry the full lightning current and should therefore have a cross-sectional area of not less than 35 mm2 if made of copper.

(f)

Building earthing systems Buildings frequently have several earthing systems that may be installed independently at different times. These include the electricity supply service earthing system, the telecommunications earthing system (sometimes more than one), the LPS earth termination network and other special purpose earthing systems. It is generally desirable to bond all such earthing systems but there may be specific reasons for not doing so. Direct-current-carrying earths, e.g. older telex systems, should usually be isolated to prevent corrosion damage to other services and earths. In such cases consideration should be given to bonding these earths through a galvanic isolator, to facilitate the protection of persons from lightning surges. This type of isolator can be used where there is a corrosion-based objection to bonding, e.g. copper-based earths to galvanized iron earths or structures, of which the latter would suffer galvanic corrosion. If 50 Hz or audio frequency bonding is not needed, a gas discharge arrestor may serve the purpose. Bonding conductors between earthing systems should have a cross-sectional area of not less than 4 mm2 if made of copper.

(g)

The LPS earth termination network Where an LPS is in place all of the services described in Items (a) to (f) should be bonded to the LPS earth termination network. Caution should be applied when bonding the LPS earth termination network to cathodically-protected earthing systems, such as cathodically-protected concrete reinforcing of fuel tanks. Bonding to such systems will require galvanic isolation as in (f) above.

(h)

Bonding of conductive parts Where a reinforced concrete pad, reinforced concrete walls or metallic building components, e.g. a metal roof, are used as part of an LPS, these should be interconnected with each other and bonded to the LPS. This bonding should be performed regardless of whether the building is in a high risk lightning area.

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5.6 PROTECTION OF EQUIPMENT 5.6.1 General Lightning induces overvoltages in electrical service lines, including electricity supply, telecommunications service lines, signalling, data, and coaxial lines. Cables carrying these services usually have different points of entry to a structure and may have protective devices connected to different earthing systems. Equipment overvoltages may be experienced in the following ways: (a)

By direct conduction of lightning current on the conductors feeding into the building. An example would be lightning striking overhead power lines. This mechanism is of lower probability, but involves higher surge currents.

(b)

Indirectly, (through magnetic induction, or electrostatic coupling) where lightning strikes nearby, and surges are induced on the conductors feeding into the building. This mechanism has a higher probability, but typically results in lower surge currents.

(c)

Lightning striking the LPS or other nearby objects, resulting in an EPR. This can cause potential differences in earthing systems, causing flashover and equipment damage.

(d)

Temporary overvoltages at mains a.c. system frequency that can occur for a number of reasons.

The strategies to deal with this problem involve equipotential bonding of the earthing systems, the provision of SPDs, and consideration of wiring practices and shielding techniques, as appropriate. AS 4262.2 provides further information on the protection of telecommunications equipment from overvoltages. 5.6.2 Equipotential bonding for equipment protection Equipotential bonding is dealt with in other sections of this Standard (Clause 5.5.2 and Appendix E). The important aspect to note is that for effective protection of equipment, very good attention needs to be paid to the implementation of the equipotential bonding scheme. Voltage differences that are insufficient to cause injury to persons can be extremely damaging to equipment. It is possible to have voltage drops in bonding conductors that are carrying lightning surges in excess of 1000 V per metre. It is important that bonding conductors be kept short to reduce this voltage difference, and to achieve this, all services should enter in close proximity. For protection of equipment, this concept can be extended to particular areas within the building. For example, consider the case of a multistorey building with incoming underground services, and a telephone system installed on an upper floor. On the lower level, the required equipotential bonding will be performed, and primary surge protection to both power and telecommunications lines can be fitted at that location, and will connect to the same equipotential earth system. On this lower level, it may well be that the bonding conductor lengths are not ideal, depending on where the services enter, and thus the protection provided is compromised. However, on the upper floor where the telephone system is installed, the same concept can be repeated, but at this point more control is possible over wiring and equipment locations. That is, at this location all the services should enter the room at the same point, and secondary surge protection to both power and telecommunications lines can be provided at this location with short, direct connections to the common earthing point.

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5.6.3 Surge protective devices (SPDs) 5.6.3.1 Introduction to SPDs An SPD is a device intended to mitigate surge overvoltages and overcurrents. To perform this function, it generally has at least one non-linear component. That is, it behaves in a relatively benign state until a voltage or current exceeds a predetermined value, then the SPD begins to act to reduce the voltage or current, in order to prevent damage to equipment being protected. Although single components can be used as SPDs, often greater functionality is achieved by combining components in various arrangements, and SPDs often have other features, such as mechanisms to indicate their operational status. 5.6.3.2 Components Overvoltage protective components usually fall into one of the following categories: (a)

Gas discharge devices These devices usually consist of glass or ceramic tubes filled with an inert gas sealed at each end with a metal electrode. They have breakdown voltages in the range 70 V to 1 kV with surge current ratings up to 100 kA. The strike time and firing voltage of these devices are dependent on the rate of increase of voltage. Unlike clamping devices, gas discharge devices conduct at a much lower voltage than their firing voltage. This conduction voltage is typically below 30 V. This fold-back behaviour generally precludes their use by themselves on a.c power circuits. Gas discharge devices are available in both two electrode and three electrode configurations. The latter provide a means of clamping a pair of wires to earth if either or both conductors are subjected to an overvoltage.

(b)

Spark gaps These devices are similar in principle to a gas discharge device, but they use air, rather than an inert gas, between the electrodes. The devices are inherently rugged and can handle high surge energies, but typically suffer from relatively high firing voltages. Devices are available that have been designed to operate on a.c. power circuits, and their design incorporates mechanisms to extinguish the arc, and prevent excessive follow-on current. Care needs to be exercised in their installation to ensure that adequate clearances are provided, and that fusing is chosen that reduces the incidence of nuisance fuse operation that may occur especially on low current circuits. The manufacturer’s advice should be followed on these matters.

(c)

Varistors Most modern varistors are made from metal oxide and are known as metal oxide varistors (MOVs). The resistance of varistors drops significantly when the voltage exceeds a limit thus clamping the voltage near the limit. The MOV is widely used in SPD construction and offers a good balance between surge rating and clamping voltage. Varistors are used on circuits operating at voltages between 10 V and in excess of 1 kV. They can handle surges in the range of 3 to 100 kA and respond in tens of nanoseconds. Because MOVs deteriorate with repeated operation, it is usual to allow a high safety margin in the selection of the device rating in lightning prone areas. Alternatively, facilities should be provided to give an indication of device failure.

(d)

Solid state devices One form consists of special zener diodes that exhibit voltagelimiting characteristics and are optimized to handle surge currents. The breakdown voltages of such devices are typically in the range 5 V to 200 V. They have current ratings up to several hundred amperes and response times of the order of 10 picoseconds. Another form consists of thyristors, that switch when their operation voltage is exceeded, and act to clamp overvoltages. Their reduced voltage during conduction means they can handle higher surge currents, compared with the zener diode type. COPYRIGHT


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The main overvoltage protective components discussed in Items (a) to (d) belong to two categories: (i)

Voltage-limiting components Sometimes referred to as clamping components, these include varistors, avalanche or suppressor diodes.

(ii)

Voltage-switching components Sometimes referred to as crowbar components, these include air gaps, gas discharge tubes, thyristors (silicon-controlled rectifiers), triacs.

Care needs to be exercised when using voltage-switching components to make sure that the devices will cease conduction once the surge has dissipated. The benefit of a voltageswitching type component is their ability to handle relatively high surge currents. Overcurrent protective components act to limit excessive currents. These are often used in telecommunications and signalling circuits, and include fuses, and solid state components providing the fusing function, but in a resettable manner. 5.6.3.3 SPD configuration SPDs are configured as being either shunt or series protectors, defined as follows: (a)

Shunt protector Sometimes known as a one-port SPD, it is an SPD connected in shunt with the circuit to be protected, as shown in Figure 5.3(a). A one-port device may have separate input and output terminals without a specific series impedance between these terminals, as shown in Figure 5.3(b). This latter arrangement is sometimes known as a ‘series connected, shunt protector’. A shunt protector with just two terminals (i.e. does not have separate input and output terminals) has no inherent limitation with regard to the load current of the circuit to which it is applied. However, its ability to clamp overvoltages is reduced by the additional voltage drop that occurs across its connecting leads. For this reason, some shunt protectors are manufactured with separate input and output terminals. This arrangement substantially reduces the connecting lead voltage drop problem, but does mean that the full load current passes through the device, which needs to be designed to handle this current.

FIGURE 5.3 EXAMPLES OF SHUNT (ONE-PORT) PROTECTORS

(b)

Series protector Sometimes known as a two-port SPD, it is an SPD with two sets of terminals, input and output. A specific series impedance is inserted between these terminals. Typical examples are shown in Figure 5.4.

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A series protector also has virtually no connecting lead voltage drop problem, and the inclusion of the series component does allow coordination between different overvoltage components, and also enables the creation of a true low pass filter (generally by the addition of shunt capacitance after the series impedance) that allows further surge attenuation. The series current limitation of the series protector will typically be determined by the series impedance. Sometimes a protector is referred to as an n-stage protector, and although this term is not consistently applied, the ‘n’ should refer to the number of shunt overvoltage stages that are separated by series elements.

FIGURE 5.4 EXAMPLES OF SERIES (TWO-PORT) PROTECTORS

5.6.3.4 Multi-service surge protection device (MSPD) An MSPD is a combination protector that combines both power protection and signalling/telecommunications protection in the one device. It is an effective way of protecting IT and associated equipment that has more than one connected service. By including all the protection in the one device, the distance between the SPD earth connections is very short, which dramatically reduces the potential difference between these services under incident surge conditions. A general diagram of an MSPD is shown in Figure 5.5.

FIGURE 5.5 MSPD CONFIGURATION

An example of an MSPD, and its installation, is given in Clause 5.6.5.3, Item (d).

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5.6.3.5 Parameters of an SPD There are many parameters that could be specified and measured or tested for an SPD, but the primary parameters relate to how well an SPD limits overvoltages, how much surge current it handles, and what voltage system is it designed for. The following parameters are listed together with the IEC symbols for these parameters. It is suggested that manufacturers and users adopt these parameters and symbols. NOTES: 1

A product marked using these IEC symbols does not mean that it has been tested to the relevant IEC Standards.

2

See Appendix F for definitions of the waveshapes.

Maximum continuous operating voltage (Uc)—This is the maximum voltage that can be continuously applied to the protector. For the Australian and New Zealand power system, this should be at least 275 V for SPDs connected between the phase and neutral conductors. Rated load current (I L )—Maximum continuous rated r.m.s. or d.c. current that can be supplied to a load connected to the protected output of an SPD. Maximum surge current (I max )—This is the peak value of the 8/20 µs waveshape current impulse that the protector can handle. The protector only has to be able to withstand this surge current once, and so this is known as the single shot rating. Nominal surge current (I n )—This is the peak value of the 8/20 µs waveshape current impulse that the protector can handle many times. A protector must be able to withstand at least 15 impulses at I n . Voltage protection level (U p)—This is the peak voltage that the protector protects to (limits the voltage to). It is sometimes referred to as the let-through, or residual, voltage. Under IEC 61643-1, it is measured at the value of I n . However, there are practical issues with gaining consistent voltage measurements at such high currents, and so it is recommended that manufacturers quote the value of U p for a.c. power systems protectors at least for the value of surge current equal to 3 kA, so that comparisons between protectors can be made. In any case, the value stated for U p must be clearly coupled with the value of surge current for which it applies. NOTE: Where the SPD protects multiple modes (e.g. A-N, A-E and N-E) the surge ratings above apply to each mode.

There are many other parameters that could be listed, for example, signalling protectors are often tested with a 10/700 µs voltage waveshape, and have parameters such as insertion loss, return loss, longitudinal balance, bit error rate, near-end crosstalk, etc, but such detail is beyond the scope of this Standard. The performance specification for a.c power systems SPDs incorporating filters is not well specified in other Standards. The primary advantages of incorporating filters into SPD designs is that they reduce the maximum dv/dt that occurs on the output of the filter, and this can reduce equipment damage and upset. A suggested performance measure for filters is— SPD filter dv/dt—The average dv/dt occurring on the output of an SPD measured between the 20% and 80% of the maximum voltage output, when the SPD is tested with a 6 kV 1.2/50 µs, 3 kA 8/20 µs waveform as shown in Figure F1. The filter should be loaded with a resistive load representing 50% of the full load current. Some SPDs include shunt capacitance that provides a filtering effect at EMI/RFI frequencies, but provides little benefit at typical lightning surge frequencies. Similarly, surge current levels may cause inductor saturation in standard EMI/RFI filters, which will degrade the filter action.

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The use of an effective filter also typically reduces the let-through voltage of the SPD as well, and this effect is captured by the U p parameter defined above. 5.6.3.6 Temporary overvoltage (TOV) A TOV is the situation whereby the power frequency voltage on an a.c power system rises above its normal value. This can be caused by many factors including poor regulation, faults on the LV or HV distribution system (including phase shorts to neutral or earth, and loss of neutral conductor), capacitor switching, sudden load rejection, and HV contact on LV circuits. These events typically last from 0.2 s up to 5 s. The voltages that can occur during these events are worse for TT and IT power systems, but even on the TN-C-S system (the MEN system as used in Australia and New Zealand) may be up to 1.7 times the nominal system voltage. The effect of these TOVs on SPDs can be catastrophic. Manufacturers of SPDs should consider this aspect in their product design, and ensure that if the product fails under these conditions, that it does so safely, which generally means failing in a manner that cannot cause a fire. The Standards IEC 61643-1 and UL 1449 have specific TOV tests to verify safe failure mechanisms. 5.6.3.7 Application of SPDs The following aspects should be considered in the application of SPDs: (a)

Modes of protection With any signal or electrical transmission system employing two lines and a separate protection earth, two types of transients can occur. The first type appears as a difference between the two lines, independent of their potential differences to earth; this is known as a differential mode transient (also called transverse mode or normal mode). It is illustrated in Figure 5.6 where the transient voltage source is superimposed onto the normal signal carried by the lines. The second type appears as a transient between each line and the earth, and is known as a common mode transient (sometimes called a longitudinal transient). It is illustrated in Figure 5.7 where the transient voltage sources are superimposed onto the normal potentials between the lines and earth. This mode is that commonly experienced by twisted pair circuits as each wire is equally exposed to the transient voltage source.

FIGURE 5.6 DIFFERENTIAL MODE TRANSIENT

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FIGURE 5.7 COMMON MODE TRANSIENT

The use of two non-earthed lines is common. The a.c. mains use the active and neutral lines to supply electricity, with an accompanying earth line for protection. Telephone lines use two wires over which the signal is transmitted, with neither line tied to earth. RS-422 signalling for computer data uses two lines for each data channel, which is known as balanced-pair signalling. When protective equipment is connected to such lines, both differential and common mode transients must be suppressed. Placing a protective device across the two signalling lines alone is not sufficient. The high potentials to earth created by common mode transients can cause insulation breakdown and arc-over, and can damage electronic components. Protection against transients can be achieved by the provision of voltage clamping or diversion devices between the lines, and between the lines and earth. These will shunt common mode transients to earth before they are allowed to reach breakdown potentials. Equipment to be protected is typically more robust to transients from line to earth (L–E) than for transients from line to line (L–L). This aspect can be capitalized on in the design and application of SPDs. Telephone line and signalling line SPDs often use a three terminal gas arrester as the primary energy handling component, and this component provides both common mode and differential mode transients. Although a gas arrester has a relatively high striking voltage, it typically provides sufficient protection against the L–E transient, and if required, a varistor or solid state device can provide additional L–L protection. Similarly, experience has shown that a.c. equipment is more easily damaged from line to neutral (L–N) transients, and although protection could be provided in all modes (L–N, L–E, N–E), good protection is usually obtained by providing L–N and N–E protection modes only. Indeed, at the building point-of-entry in a TN-C-S system, the neutral conductor is connected to earth, and SPDs applied as recommended adjacent to this MEN link, need only comprise of the L–N protection mode. (b)

SPD location There are many possible locations where SPDs could be installed within a facility. The aim is to install SPDs in locations that provide effective protection to the equipment nominated for protection, and to do so economically. The most effective method of providing effective surge protection for a facility is to provide SPDs at the building service point-of-entry (known as primary protection), and then, if necessary, to provide additional surge protection within the building closer to the equipment to be protected (known as secondary protection). Primary protection is important because the main function of such an SPD is to keep most of the surge current from entering the building, by diverting it directly to earth.

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When considering a.c. power circuit protection, the concept of location categories can be used (see Figure F3). Location category C is the building point-of-entry and is where primary protection would be installed. Location categories A and B are further within the building and are potential locations for secondary protection, to be fitted as required, especially in the following situations: (i)

Sensitive equipment is present. An example would be a computer room.

(ii)

The distance between the SPD located at the entrance and the equipment to be protected is too long (see Coordination in Item (d)).

(iii) There is internal equipment generating switching surges, or other internal interference sources, inside the building. (c)

Surge ratings Having defined location categories, appropriate surge ratings for these categories can be recommended. Before considering these ratings several aspects need to be discussed— (i)

a lightning surge travelling within a building is attenuated by the SPDs it encounters and the impedance from the building wiring itself. Thus higher levels of surge current are likely to be encountered at the building point-ofentry, compared to the distant end of a branch circuit;

(ii)

in choosing an appropriate surge rating, it needs to be understood that the surge rating of a SPD is a major factor in determining its useful lifetime. An SPD with a particular I max rating is not chosen on the basis of handling a single event of that magnitude, but rather it will be expected to handle many events of a lower amplitude; NOTE: For example an SPD with an I max rating of 40 kA might typically handle 15 surges of 15 kA.

(iii) the lightning surge current to be handled by a point-of-entry SPD has traditionally been considered to come into the building via the service conductors. However, another mechanism is now understood to exist. If lightning strikes the building LPS, or even the ground or an object nearby, a local EPR occurs. The incoming service conductors are typically referenced to a distant earth (such as the neutral conductor grounded at the secondary transformer some distance down the street, with the phase conductor also being referenced to that distant earth by virtue of the transformer winding). The effect of the local EPR is that a proportion of the lightning current flows out through the point-of-entry SPDs on its way to reaching the distant earth. The surge current in the SPDs in this case is very large, being a significant proportion of the lightning current itself. While Table 5.1 gives a surge rating for SPDs in this case (Category C3) using the 8/20 µs waveshape, it should be acknowledged that the IEC standards make reference to a 10/350 µs waveshape for use in this case, and the symbol given to the current rating using this waveshape is I imp . It has been found that a factor of 10 may loosely be used to provide an indication of the equivalence between these two waveshapes for typical SPD ratings. For example, an SPD withstanding a 100 kA 8/20 µs impulse might be expected to withstand a 10 kA 10/350 µs impulse. The actual factor can only be determined for a particular SPD by testing. Given this discussion, for a.c. power system SPDs, the following surge current ratings are recommended, where the surge rating is the I max , or single shot, 8/20 µs value, and apply for each SPD from the phase to neutral conductors.

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TABLE 5.1 RECOMMENDED SURGE RATINGS FOR A.C. POWER SYSTEM SPDs PER PHASE Category

(d)

SPD location

I max rating

A

Long final subcircuits and electricity supply outlets

3 – 10 kA

B

Major submains, short final subcircuits and load centres

10 – 40 kA

C1

Service entrance, other than below

C2

Service entrance, building fed by long overhead service lines, or is a large industrial or commercial premises

40 – 100 kA

C3

Service entrance, building in a high lightning area, or fitted with a LPS

100 kA

40 kA

Coordination Often the approach taken is to have the primary SPD handle the bulk energy (surge current) and not be too concerned about the U p value for that protector. A secondary protector that will not need to handle such a high value of surge current, can be installed close to the equipment and can be chosen to have an acceptable U p value. However, to achieve this result, careful coordination between the two devices needs to be undertaken. This is quite a complex matter, and a total examination of the issues is beyond the scope of this Standard. However, a simple rule to apply is to ensure that there is at least 10 to 20 m of electrical cabling between the two SPDs. If this cannot be achieved, purpose built inductors are available that can be placed in the circuit to achieve this effective separation. A more complex primary SPD may incorporate multiple stages with the decoupling impedance within the device itself, and such an approach ensures the stages are properly coordinated and typically results in an SPD with good energy handling capability and relatively low U p values.

(e)

Fusing and wiring considerations It is essential to provide a fuse or circuit breaker ahead of the SPD to provide for the safe disconnection of a failed SPD. Manufacturers usually recommend an appropriate rating for such a fuse, and often the value specified has been used in approval testing of the SPD. Safe disconnection of a failed SPD would point toward a low amperage fuse, but too low a value will cause nuisance operation as the SPD diverts surge currents. In the absence of any specific guidance from the SPD manufacturer, a high rupturing capacity (HRC) 32 A or 63 A fuse or circuit breaker with a fault rating compatible with the switchboard could be used. SPDs should be installed after the main switch but prior to any residual current device (RCD). The connection wiring from the main switch via the protective circuit breaker and the SPD back to the neutral near the MEN point should be at least 6 mm 2 and as short and direct as possible. This total connection lead length must be less than 1 m in length (ideally 300–600 mm). When carrying a lightning surge, the voltage dropped across a conductor can be 1000 V/m, and is caused by the inductance, and not the resistance, of the conductor. This is shown in Figure 5.8(a), and the voltage drop across the connecting conductor adds to the U p of the protector. For example, if the U p of the SPD was 600 V, and the interconnecting cable had a voltage drop of 1000 V, then the effective protection provided by the installed protector would be the sum of these, at 1600 V. If the terminals of the SPD are rated for the required load current level, a configuration as shown in Figure 5.8(b) is preferred. This wiring method is sometimes known as a Kelvin connection, and eliminates the interconnecting lead problem. Failing this, twisting the connecting conductors together as shown in Figure 5.8(c) can have a substantial impact on reducing the voltage drop.

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FIGURE 5.8 WIRING OF SPDs

(f)

Failure modes of SPDs Typically an SPD initially fails to a low impedance state, and the resulting a.c. current that then flows into the SPD either causes the SPDs internal fusing to operate, or causes external fusing to operate. Consideration needs to be given to the most desirable location for the external fuse, and whether it is desirable for power to be disconnected from the load when the SPD fails. If the external fuse is in series with the load current, power will be disconnected from the load when the SPD fails. In some applications this is considered beneficial, since the SPD is no longer protecting the load. However, locating the fuse in the non-load carrying SPD connection wiring, means that a ruptured fuse will isolate the SPD, but allow power to continue to the load. SPDs are often fitted with some form of visual status indication to show their operational status, and may additionally be fitted with contacts to allow for remote monitoring.

(g)

Withstand voltage of equipment The equipment to be protected may have a level of resistability to surges designed into it. In particular, a resistance level to electromagnetic interference (EMI) disturbances is mandated. Ideally, the U p of the externally provided SPDs should be lower than the U p of any equipment internal surge protection components, otherwise the internal components may be damaged instead of being protected by the external SPDs. At some point in the future, equipment to be protected may be marked with a surge voltage level that it will withstand, and a symbol of Uw has been proposed for this withstand voltage. Selection of SPDs to protect such equipment would be straightforward. Lightning surges need not physically damage equipment for it to experience problems. Erratic operation, that may or may not require manual resetting, can occur and lead to data loss at surge levels which are lower than those required to cause hardware failure. Rapid changes in the voltage supply, even those for which the amplitude does not exceed the normal a.c. power peak, can cause problems. This dv/dt problem can be reduced by utilizing SPDs with filters. Figure F2 contains a curve that illustrates the typical voltage/time tolerance of computing equipment.

5.6.4 Magnetic shielding and line routing Magnetic shielding reduces incident electromagnetic fields, and can also provide a reduction in the emissions from electrically noisy equipment being operated within the building. The complexity of shielding can range from the use of metallic conduits, to simple metal enclosures or cabinets, and up to whole rooms being comprised of shielding materials. Such shields need to be earthed to be effective, and any SPDs provided to conductors entering the shielded area need to be effectively connected to the shield.

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The amount of surge energy directly induced into building conductors from nearby lightning strikes depends on the closeness of the current source, and the loop area formed by the conductors. Conductors of the same service should be run together, along with an earthing conductor, or otherwise run in close proximity to other earthed components, such as earthed cable trays. To reduce the inductive loop area, such cables should be neatly tied together and not allowed to splay out over the whole tray width, for example. 5.6.5 Practical installation examples 5.6.5.1 Determine protection needs There are many aspects to providing effective lightning protection to equipment as has been detailed in preceding clauses. However, it is useful to consider some examples that show a simple methodology for determining where SPDs are required, and how to correctly apply them. The method consists of firstly determining what it is that requires protection, and then drawing an imaginary box around it. Each location where electrical lines cross the box is a potential location for SPDs, and they should be provided where the particular electrical line is long or otherwise prone to having surges on it. Then the earths of the SPDs are connected together as well as to the earths of the equipment, and taken to earth. If the initial box is very large and services physically enter the box at widely spaced distances, it may be prudent to draw other boxes inside (or alongside) the first, around particular items of sensitive equipment to be protected, and repeat the process. Typically it will be necessary to consider the following points for SPDs: (a)

At the point-of-entry telecommunications.

of

external

services

e.g.

electricity

supply

and

(b)

At the connection of the external services to the equipment.

(c)

At the connection of long internal cabling to the equipment e.g. communications and LAN.

The following two mechanisms can damage equipment: (i)

An excessive voltage/current enters the building via a service due to either a lack of protection or incorrectly installed protection. NOTE: When protection is correctly installed both the mains and telecommunications pointof-entry SPDs are bonded to the main earth bar by conductors of 1.5 m or less.

(ii)

An excessive voltage/current is induced into the internal wiring loop.

5.6.5.2 Protection procedure The procedure for protecting equipment is as follows: (a)

Install secondary protection at the equipment when the risk of damage due to induction into the external service conductors (electricity supply and telecommunications) and the building conductors exceeds an acceptable level.

(b)

Install point-of-entry protection when the risk of damage due to a direct strike to the structure or the service conductors exceeds an acceptable level. NOTES: 1

A prime role of the point-of-entry protection, apart from preventing dangerous discharges and step and touch potentials, is to protect the secondary protection from damage.

2

Telecommunications SPDs on customer cabling should be installed in accordance with AS/ACIF S009 (in Australia only). In Australia, a licensed cabler must do this installation. The installation of SPDs in the main switchboard (MSB) and in electrical distribution boards (DBs) should be in accordance with AS/NZS 3000 and AS 4070. A licensed electrician must do this installation. COPYRIGHT


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5.6.5.3 Protection examples The following examples show these principles being implemented: (a)

A central PLC and remote sensor In this example it is determined that a particular industrial process must be protected. It consists of a central controller (PLC), and various sensors and controls. For simplicity, the example will show two sensors, one at a considerable distance from the PLC, and another close to the PLC. The arrangement is as shown in Figure 5.9.

LEGEND: BB = Bonding bar MDF = Main distribution frame MEB = Main earth bar MEB = Main earth bar MSB = Main switchboard PLC = Programmable logic controller

FIGURE 5.9 EXAMPLE OF CENTRAL PLC AND REMOTE SENSOR

The PLC has a operator console and a modem to allow remote communication, and consequently has electrical and telephone services. The signal line is over 1 km in length. The first box is drawn around the PLC and associated close by equipment. Other close by sensors or controls would be included within this box. Three electrical services are identified that cross this box – the a.c. power, the telephone line, and the signal line to the remote sensor. Since all three are long or otherwise prone to having surges on them, they need SPDs. Since the operator console is close to the PLC there is no reasonable likelihood of surges impinging on that connection. Likewise, any sensors or controls close to the PLC would not normally require protection.

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Having identified the SPDs required close to the PLC, their installation is important. They should be installed close to the equipment they are to protect, and must have their earths connected together, and connected to the PLC earth, and taken to earth. In practice, this might be laid out as follows. The PLC and modem are mounted next to each other on a rack. Immediately below them, the required SPDs are mounted in a row, with an earthing busbar immediately below them, connected at each end to the rack frame. Each SPD will have a short direct connection to the busbar not exceeding 100 mm, and the PLC will also have a direct connection to the busbar. In this manner, excellent equipotential bonding will be achieved for this equipment and associated SPDs. The SPD on the signal line at the PLC end does not provide protection for the remote sensor. Again, a box is drawn around the sensor. The only line here is the signal line, and so an SPD is provided on it. This SPD is earthed with a direct short connection to the sensor earth (and associated pipe or vessel), and taken to a local earth. There is no point in trying to connect this very distant earth to the PLC earth via a bonding conductor. In addition to the secondary SPDs installed closed to the equipment, the prudent provision of primary point-of-entry SPDs has also been shown. Depending on the assessed exposure of the signal line, which would include such considerations as whether it is run in metallic conduit or not, it may be prudent to include primary point-of-entry protection on the PLC signal lines where they enter the building, in addition to the secondary protection shown at the PLC. (b)

A video surveillance system This example is very similar to the case just described. The central monitoring and recording equipment will have surge protection applied to the long video camera feeds and to the a.c. power line. The video feeds that are selected for protection will need all the associated signal lines protected, and this may include the video feed, the power supply leads, and any pan and tilt control signals. The earths of these SPDs will connect to each other and to the equipment earth. Video cameras that are located nearby in the same building may not need surge protection fitted. Those video cameras that are located a long distance from the central monitoring and recording equipment will need the same type of protection fitted at the remote end as was fitted at the central end. The earths of these SPDs must be connected to the video camera earth, and then be taken to a local earth. Although all earth connections should be as short as possible, it is particularly important to keep the length of the SPD earths to video camera earth short.

(c)

A multistorey building with PABX on upper floor In this example illustrated in Figure 5.10 it has been determined that protection is required. This is a multistorey building with services entering on ground floor, and PABX on an upper floor. A box has been drawn around the PABX. Regardless of whether the point-of-entry location of the telecoms and power services are co-located, there can be induction into the internal wiring between the ground floor and the PABX. Therefore the electricity supply, the exchange line and the outdoor extensions will require protection at the PABX. The local handsets do not need protection (inside the inner box). To ensure adequate protection of the PABX it may be necessary to have a DB and a telecoms distributor (IDF) co-located with the PABX. SPDs are installed in the DB and the IDF. In areas where direct strike protection is required SPDs need to be installed at the point-of-entry (see AS 4262.1).

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LEGEND: BB = Bonding bar DB = Distribution board IDF = Intermediate distribution frame MDF = Main distribution frame MEB = Main earth bar MSB = Main switchboard PABX = Private automatic branch exchange

FIGURE 5.10 EXAMPLE OF MULTISTOREY BUILDING WITH PABX

(d)

A domestic computer and ADSL modem In this example it has been determined that protection is required. Figure 5.11 shows how this has been implemented. A box has been drawn around the ADSL modem and all interconnected equipment. A box has also been drawn around the fax machine. The simplest way to provide SPDs with short bonding conductors to a common earth point is to use an MSPD so this has been used, along with a power board to provide additional protected outlets. A Fax machine at the same physical location has also been protected using a second MSPD.

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LEGEND: ADSL = Asynchronous digital subscriber line MEB = Main earth bar MSPD = Multi-service surge protection device SB = Switchboard

FIGURE 5.11 EXAMPLE OF DOMESTIC COMPUTER AND ADSL MODEM

Although shown in Figure 5.11, point-of-entry protection would normally only be installed in areas where direct strikes are likely. (e)

A rooftop cellular base station In this example a cellular base station is located on the roof of a multistorey building. Figure 5.12 shows an effective means of providing lightning, earthing and surge protection.

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LEGEND: BB = Bonding bar DB = Distribution board MEB = Main earth bar MSB = Main switchboard

FIGURE 5.12 EXAMPLE OF ROOFTOP CELLULAR BASE STATION

The tower, being all steel, is self protecting. It should be bonded to the building lightning protection system. If there is no building LPS the tower and cabin should be earthed by a minimum of two downconductors. Should lightning strike the tower or the building LPS the potential of the tower and cabin will rise. Therefore the tower and cabin together must be treated as a separate entity.

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The a.c. power supply is fed to the base station distribution board from a main switchboard located in the basement of the building. An SPD, normally a surge diverter, would be installed at the MSB. A second SPD, a surge filter, is installed on the a.c. power feed to the rooftop distribution board. All other metallic services, for example antenna feeders should be bonded and fitted with SPDs. All equipment should be referenced to a common earth bonding bar in the cabin and this in turn bonded to the tower and building LPS. There will be a connection to the main a.c. earth via the earth conductor in the power cable to the roof. Good practice would dictate that all metallic services should enter the cabin on the same side and the common earth to the tower and LPS should exit the same side. It should be noted that in the event of a lightning strike to the tower, conducted currents will flow through the power earth conductor, and care should be taken to segregate cables if possible. Also, this description only considers the equipment inside the box (i.e. inside the cabin). Antennas installed on the tower that contain electronics may require additional protection measures.

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S E C T I ON 6 PRO T E CT I O N O F M ISCE L L AN EOUS STRUCTURES AND PRO PE RT Y 6.1 SCOPE OF SECTION This Section provides recommendations for the protection of a variety of structures and property against lightning where such protection is deemed necessary (see Section 2). The recommendations of Sections 4 and 5 should be observed except where otherwise indicated. 6.2 STRUCTURES WITH ANTENNAS 6.2.1 Indoor antenna system Structures protected against lightning in accordance with the recommendations of this Standard may be equipped with indoor radio and television receiving antennas without further precautions, provided that the separation between the antenna system, including its down leads or feeders, and the external LPS or any of its internal sections is in accordance with the values in Clause 4.16. 6.2.2 Outdoor antennas on protected structures Structures protected against lightning in accordance with the recommendations of this Standard may be equipped with outdoor radio and television receiving antennas without further precautions, provided that every part of the antenna system, including any supporting metalwork, is within the zone of protection of the LPS (see Clause 4.4). Where these conditions cannot be fulfilled, precautions should be taken to ensure that the lightning current can be discharged to earth without damage to the structure or injury to its occupants with an antenna system fitted— (a)

directly onto a protected structure. This can be accomplished by connecting the antenna bracket structure to the LPS at the nearest point accessible below the antenna installation; or

(b)

on a metallic support structure that projects above the LPS. This can be accomplished by connecting the antenna support structure to the LPS at the nearest point accessible below the antenna installation.

NOTE: Consideration should be given to the fitting of SPDs in the conductors connected to the antenna system.

6.2.3 Antennas on unprotected structures Before installing an antenna on an unprotected structure, the need to provide an LPS should be assessed as described in Section 2. 6.2.4 Earthing of radio systems The earthing electrode of the LPS may also be used for the purpose of earthing a radio system. 6.3 STRUCTURES NEAR TREES When a tree is struck by lightning, a voltage drop develops along its branches, trunk and roots. The side-flash clearances between the tree and adjacent structures are set by taking 100 kV/m as the flashover strength of unseasoned wet timber and 500 kV/m as the breakdown strength of air. If the tree does not exceed the height of the structure its presence can be disregarded. If the tree is taller than the structure, the following clearances between the structure and the tree may be considered as safe: (a)

For normal structures; one-third of the height of the structure. COPYRIGHT


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For structures with explosive or highly-flammable contents; the height of the structure.

If the clearances cannot be met then the structure should be fitted with lightning protection in such a manner that the side-flash always terminates on the protection system. If the tree is fitted with an LPS, no further protection will be necessary for the structure provided that the conditions for the zone of protection and separation are fulfilled. 6.4 PROTECTION OF TREES The protection of trees against the effects of lightning needs to be considered only where the preservation of the tree is desired for historical or other reasons. For such cases the following recommendations are made: (a)

A main downconductor should be run from the topmost part of the main stem to the earth termination and should be protected from mechanical damage near ground level.

(b)

Large upper branches should be provided with branch conductors bonded to the main conductor.

(c)

In the fixing of the conductors, allowance should be made for swaying in the wind and the natural growth of the tree.

(d)

Test joints may be waived.

(e)

The earth termination should consist of two rods driven into the ground on opposite sides of, and close to, the trunk of the tree. A strip conductor should be buried to a depth of 300 mm to encircle the roots of the tree at a minimum distance of 8 m radius from the centre of the tree or at a distance equal to 1 m beyond the spread of the foliage, whichever is the greater. This conductor should also be bonded to the rods by two radial conductors. The earth terminations and resistance should comply with Clause 4.15.

(f)

Where two or more trees are so close together that their encircling earth conductors would intersect, one conductor adequately connected to the earth rods should be buried so as to surround the roots of all the trees.

NOTE: The recommended earth termination network is designed to protect the roots of the tree and to reduce the potential gradient, in the event of a lightning discharge to the tree, to a safe value within the area bounded by the outer buried strip conductor.

6.5 CHIMNEYS, METAL GUY-WIRES OR WIRE ROPES 6.5.1 General Metal guy-wires or wire ropes attached to steel anchor rods set in earth or buried reinforced concrete anchor blocks may be considered as sufficiently earthed. Other guy-wires or wire ropes should be earthed e.g. where fixed above ground to structures. For means of securing conductors to structures, see Clause 4.10. Metal chimneys or flues need no protection against lightning other than that afforded by their construction, except that they should be properly earthed. If the construction of the foundation does not provide ample electrical connection with the earth, earth connections should be provided similar to those recommended for chimneys made of materials other than metal (see Clause 6.5.3). 6.5.2 Metal ladders and metal linings Where chimneys have a metal ladder or lining they should be connected to the LPS at their upper and lower ends.

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6.5.3 Chimneys Chimneys consisting partly or wholly of reinforced concrete should comply with the recommendations of Clauses 4.5, 4.10, 4.12 and 4.16, and, in addition, the reinforcing metal should be electrically connected together and electrically connected to the downconductors at the top and bottom of the concrete. Chimneys, other than those of reinforced concrete or metal construction, shall be protected in accordance with the requirements of Section 4, when required by the risk assessment of Section 2. NOTE: In existing chimneys, the reinforcement of which may be electrically continuous, it is recommended that additional connections be made at points where the reinforcing rods are accessible.

6.6 PROTECTION OF MINES 6.6.1 Factors influencing need for protection In mining operations, electric shocks, possible premature detonation of explosives, and ignition of flammable gases from the effects of lightning are recognized additional hazards. Because these hazards are associated with the effects of lightning at or below the ground surface, factors additional to the risk assessment approach of Section 2 influence the need to provide lightning protection. These additional factors are associated with earth resistivity, depth of the mining operation, presence of persons and the presence of flammable gas or explosives. The degree of hazard is regarded as greater the shallower the depth of the operation and the higher the resistivity of the earth involved. Generally, these additional factors will indicate that lightning protection should be provided or precautionary work procedures adopted. 6.6.2 Object of recommendations The following recommendations for lightning protection for mining operations are aimed at reducing the risk of electric shock and premature detonation of explosives. While the recommendations will also reduce the risk of ignition of flammable gases from the effects of lightning, flammable gas ignition is best prevented by ensuring that flammable concentrations of gases do not occur. The intent of the recommended LPS is to reduce the possibility of substantial voltages appearing between conducting structures and between conducting structures and earth in their immediate vicinity. Absolute protection against the effects of lightning cannot, however, be guaranteed with the recommended protection system alone. Consequently, recommendations are also given for operational procedures for the use of explosives when lightning occurs close to the mine site. In surface workings, the premature detonation of explosives, both directly and through electric detonators, are considered possible, while in underground operations, the premature detonation of explosives is considered possible only through electric detonators. 6.6.3 Underground workings 6.6.3.1 General The following recommendations apply particularly to underground workings where electric detonators are used as the means of initiating explosives. 6.6.3.2 Electric detonators Detonators specially designed to reduce the risk of ignition by electrical discharge across the fuse head should be used.

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6.6.3.3 Shot firing circuit Requirements for circuit equipment and procedures to be adopted for firing explosives electrically are set out in AS 2187.2 or NZS 4403. Additional to those requirements, where fixed wiring is used as part of the firing circuit, the conductors should be enclosed in metal screening, armouring or conduit. This metal screening, armouring or conduit should be connected to the electrical system earthing and bonded to other metallic structures as described in Clause 6.6.3.7. 6.6.3.4 Overhead electricity supply service lines To minimize the magnitude of incoming lightning surges on overhead electricity supply service lines, overhead earth wires should be provided on all overhead electricity supply service lines within 1.5 km of the mine. Additionally, SPDs should be installed at the termination of the overhead electricity supply service line for protection of connected cables or equipment. 6.6.3.5 Surface structures Lightning protection should be provided on all structures above underground openings, such as winder head frames. With other structures and buildings, the need to protect or not should be determined from Section 2. It must be remembered that a surface LPS will still discharge current to underground workings. Lightning protection of surface structures should be carried out in accordance with Section 4 and, where these buildings have explosive or highly-flammable contents, the additional recommendations of Section 7. Where various conductive structures, such as metallic enclosures of air, water and electricity services, or reinforcing steel in concrete foundations, are laid in or on the ground, advantage should be taken of these to reduce the earthing resistance of the LPS by interconnecting and bonding these structures together and to the LPS earth termination network. Typical sizes of bonding conductors are given in Table 4.6. 6.6.3.6 Bonding of surface metalwork All metal structures entering openings to underground workings of a mine should be bonded together at the point-of-entry to the opening and connected to the LPS earth termination network of structures above the opening. This includes any reinforcing steel in the shaft, concrete lining, shaft steel work, guides and ladders, armouring and sheathing of electrical cables, air, water and ventilating pipes, rails and bell rope attachments. Typical sizes of bonding conductors are given in Table 4.6. 6.6.3.7 Bonding of underground metalwork In addition to the bonding recommended in Clause 6.6.3.6, metal structures and services in underground access shafts should also be bonded together at intervals of not more than 75 m. Rock-bolted support structures are deemed to provide an adequate earth for this purpose. Winding ropes, guide ropes and balance ropes cannot be bonded to other structures except at fixing points and, possibly ineffectively, through conveyances. High voltages relative to their surrounds could occur during lightning activity. 6.6.3.8 Further precautions The degree of hazard in any mine, both from electric shock and initiation of electric detonators, is related to the depth of the operations. This relationship is inadequately defined at present.

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Shaft sinking and drifting are particular operations where lightning is a recognized hazard. With these operations all work associated with electrical blasting should be suspended and persons withdrawn to a safe distance when an electrical storm is approaching. A conservative approach would require that the precautions applied to shaft sinking and drifting be applied to all underground operations. 6.6.4 Surface workings 6.6.4.1 General The following recommendations apply to surface mining operations where any type of explosive is used in the mining operation. 6.6.4.2 Equipment For many surface workings involving blasting operations, action need only be taken in the immediate vicinity of the area where blasting takes place. This is because no interconnection by metallic structures, such as air/water/electricity services, exists with distant structures or earth. Where these services exist the recommendations for underground working, of Clauses 6.6.3.3 to 6.6.3.5 apply and, where practicable, the bonding recommendations of Clause 6.6.3.7 should also apply. Where electric detonators are used, electric detonators of the type described in Clause 6.6.3.2 should be used. 6.6.4.3 On-site precautions All work associated with blasting operations should be suspended and persons should be withdrawn to a safe distance from explosives when an electric storm is approaching. High equipment, such as drilling rigs, shovels and draglines, that may increase lightning locality concentration, should be moved to a safe distance from the area where blasting is to take place prior to explosives being brought to the site. 6.6.5 Lightning detector Specially designed lightning detectors should be provided to warn of approaching electrical storms so that the precautions set out in Clauses 6.6.3.8 and 6.6.4.3 may be taken. 6.7 PROTECTION OF BOATS 6.7.1 General A boat should be considered to be at risk both because of its method of construction (except for metal-hulled boats) and because it forms a marked protrusion above the surrounding water surfaces. Overseas statistics show that in excess of 10 percent of fatalities occurring on cruising sailing boats are due to lightning. While the principles to be applied will not differ from those for land-based structures, the methods employed will depend on the form of construction and the type of boat to be protected. NOTE: For further detail on the protection of boats against lightning refer to ISO 10134 and Paragraph G2, Ref 7.

6.7.2 Elements of the protection system 6.7.2.1 Air terminal A metal mast or the metal fitting on a timber mast will act as an adequate air terminal. 6.7.2.2 Downconductors The mast, if metallic or if provided with a metal track, and stays will both act as downconductors and each should be connected to an earth termination. Stays as small as 3 mm diameter steel wire will serve as effective downconductors, but may be damaged under severe lightning discharges. COPYRIGHT


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6.7.2.3 Earthing Any metal surface that is normally submerged in the water will provide adequate earthing. Propellers, metal rudder surfaces and metal keels may be used. The earth plate for the radio transmitter may also be used, providing that it is constructed of solid material and not of the porous type. A metal or a ferro-cement hull also constitutes an adequate earth. 6.7.2.4 Metallic objects Metallic objects that are permanent parts of the boat and whose function would not seriously be affected by earthing should be made part of the LPS by interconnection with it. NOTE: The purpose of interconnecting the metal parts of a boat with a downconductor is to prevent side-flashes to metal objects that could form part of an alternative path to earth or which could bridge out a substantial length of the downconductor.

A general rule is, that if the non-conducting part of the alternative path through such object is less than one-eighth of the length of downconductor bridged out, then that object should be electrically interconnected with the downconductor. 6.7.2.5 Radio transceivers A whip antenna consisting of a fine wire embedded in a glass fibre tube cannot be considered a satisfactory lightning conductor and should be folded down during a lightning storm. All radio equipment or other navigational equipment with exposed transducers such as radar, wind speed/direction indicators, and the like, should be fitted with effectively-earthed spark gaps or SPDs. Alternatively, input cabling should be disconnected from the equipment if there appears to be imminent danger of the boat being struck by lightning. 6.7.2.6 Corrosion Care should be taken that the design of the LPS does not promote the occurrence of electrolytic or galvanic corrosion. Bonding of dissimilar metals and interconnection of the earth terminals of different pieces of electrical equipment should not be undertaken without expert knowledge of the possible problems involved (see also Clause 4.7.2). 6.7.3 Installation recommendations 6.7.3.1 Protection of boats with masts Sailing or power boats that have a mast or masts of sufficient height to give an adequate zone of protection in accordance with Clause 4.4 may be protected by earthing the lower ends of the standing rigging and the base of a metallic mast, or the lower end of a continuous metal sail track on a timber mast. Where the mast of a boat is stepped on deck, particular care should be taken to ensure that the conductor from the base of the mast follows a direct route if it passes through the accommodation section of the boat, otherwise a situation analogous to that shown in Figure 4.8 may occur. A typical small sailing boat with aluminium mast stepped on deck, glass fibre hull with the metal ballast encapsulated in the glass fibre (or unballasted and with a non-metallic centreboard) and with chainplates moulded into the hull provides something of a problem. In such cases, it is suggested that some protection be sought when necessary by temporarily connecting the mast and stays together at deck level by a length of chain or other flexible conductor and allowing a short length of chain or the conductor to hang in the water at each chainplate.

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6.7.3.2 Protection of boats without masts Boats without masts do not constitute as high a risk as boats with masts. However, where the size of the boat is such as to cause a marked protrusion above the surrounding water surfaces, such boats should be fitted with air terminals that will give at least the protection recommended in Section 4 for land-based structures. 6.7.4 Precautions for persons and maintenance suggestions To the extent consistent with safe handling and navigation of the boat during a lightning storm, persons should remain inside a closed boat and avoid contact with metallic items such as gear levers or spotlight control handles. Persons should stay as far as practicable from any parts of the standing rigging or other items forming part of a downconductor. No person should be in the water or dangle arms or legs in the water. If a boat has been struck by lightning, compasses and navigation instruments should be checked for calibration. Protective coatings on steel hulls and glass fibre sheathing over ballast keels should also be checked for damage. All standing and running rigging and associated fittings should be checked in detail. 6.7.5 Bonding the lightning protection system to the vessel’s electrical wiring system earth The interworking and bonding of the LPS on a boat should recognize that the electrical wiring system on a boat is commonly only a final subcircuit. As such, the wiring will be very light, and neither the live conductors (whether or not energized) nor the earthing arrangements, are capable of carrying lightning discharge current. Even with a larger vessel, where the wiring is for a submain or a complete installation with a generator, this will often still be the case, though larger wire sizes would be in use. As a consequence, the LPS should be designed initially as a self-contained unit, even as far as selecting and arranging the most appropriate below water level earthing electrode, in accordance with Clause 6.7.2.3. The wiring systems likely to be encountered (see AS/NZS 3004) are— (a)

a conventional AS/NZS 3000 arrangement with an inlet socket; or

(b)

a conventional arrangement, but with the shore earth broken (for voltages less than about 2 V) by a galvanic isolator. The galvanic isolator may be located either on board, or on shore; or

(c)

a system where the need for an onboard earthing system is removed by either an onboard or a shore-mounted isolation transformer.

Items (b) and (c) above are to avoid galvanic or electrolytic corrosion of metallic skin fittings below the water line from interaction with shore earthing. It should be noted that Item (c) is not intended to give effect to the usual function of isolation transformers, which is to avoid earth path electric shock. The ship installation may have common fed accessories off the isolation transformer secondary, and all systems may incorporate RCD protection. One side of the isolation transformer secondary may be a pseudo ‘neutral’ with ship earthing. In such cases, a ship earth or ship bonding of one side of the isolation transformer secondary would be expected to be provided. Ships wired to earlier standards or overseas standards may vary from the above. When the LPS is completed, the earthing conductor at its final point connection to its chosen earth termination network should be bonded to the ship earth or ship bonding point at its termination. Where the bonding conductor on the electrical wiring is not actually terminated on ‘earth’, the bond from the LPS should be connected to its lowest (height) control point by a conductor of not less than twice the cross-sectional area of the electrical wiring system. COPYRIGHT


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6.8 FENCES If an extended length of metal fence is struck it is raised momentarily to a high potential relative to earth. Persons or livestock in close proximity to, or in contact with, such fencing at the time of a lightning discharge to the fencing may therefore be exposed to danger. Fences that give rise to the most risk are those constructed with posts of poor conducting material, such as wood or concrete. Fences built with metal posts set in earth are less hazardous, especially if the electrical continuity is broken. Breaking the electrical continuity prevents a lightning stroke from affecting the entire length of a fence, as it can if the stroke is direct and the fence continuous, even though earthed. Thus it is desirable to limit the length of fencing so affected by the provision of gaps, and also to provide several earthing electrodes in each section so as to facilitate the discharge to earth of the lightning current. In addition, persons or livestock can be endangered by potential differences in the ground in the proximity of fences (see Figure 6.1). The risk is greatest on rocky ground. No value can be given for the earth termination resistance, since this must be largely governed by the physical conditions encountered, but the lower the resistance to earth the less risk will result to persons and livestock. In this connection, it should be borne in mind that because of large body spans and bare contact areas many types of livestock are more susceptible to electric shock than humans.

FIGURE 6.1 EQUIPOTENTIAL LINES NEAR METAL FENCE CAUSED BY LIGHTNING DISCHARGE TO FENCE

6.9 MISCELLANEOUS STRUCTURES 6.9.1 Shelters, shade structures and rotundas in the public domain Structures of this nature, which have metallic roofs supported on wood or other electrically non-conductive materials, shall have the metallic roof earthed by a minimum of two standard downconductors at intervals not exceeding 20 m of the roof perimeter. These downconductors should be arranged outside the structure away from exits and entrances and to reduce the hazard of touch potential, should be installed in a suitable heavy duty electrical conduit to the AS/NZS 2053 series. Where the metallic roof is in contact with, and supported by, metallic supports, no additional downconductors are required. In the case of structures of this nature with a non-conductive roof, air terminations and downconductors shall be installed as per Section 4, with earthing as described below.

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Each specially provided downconductor shall be suitably earthed. Where doubt exists as to the effective earthing of metallic supports being used as downconductors, they should also be suitably earthed. In addition to these minimum requirements, further protection may be afforded to the occupants of the shelter by installation of a ring earth. 6.9.2 Large tents and marquees Where large temporary structures of this type are used for such purposes as exhibitions and entertainments involving large numbers of people, consideration should be given to their protection against lightning. In general such structures are manufactured from non-metallic materials and the simplest form of protection will usually consist of one or more horizontal air terminals suspended above them and connected solidly to earth. A non-metallic extension of the vertical supports provided for such structures may, if convenient and practicable, be used for supporting a network of horizontal air terminals but a clearance of not less than 1.5 m should be maintained between the conductor and the fabric of the enclosure. Downconductors should be arranged outside the structure away from exits and entrances and be connected to earthing rods that, in turn, should be connected to a ring conductor in such a manner as to be inaccessible to the general public. Those types of tented structure that have metal frameworks should have these efficiently bonded to earth at intervals of not more than 20 m of perimeter. 6.9.3 Small tents For small tents, no specific recommendations can be given. 6.9.4 Metal scaffolding and similar structures, including overbridges Where metal scaffolding is readily accessible to the general public, particularly when it is erected over and on part of the common highway or may be used in the construction of public seating accommodation, it should be efficiently bonded to earth. A simple method of bonding such structures consists of running a strip of metal other than aluminium, 20 mm × 3 mm size, underneath and in contact with the base plates carrying the vertical members of the scaffolding and earthing it at intervals not exceeding 20 m. With public seating accommodation only the peripheral members of the structure need bonding to earth. Other steel structures, such as those used for pedestrian bridges over main trunk roads, are frequently sited in isolated situations where they may be prone to lightning strikes and should therefore be bonded to earth, particularly at the approach points. 6.9.5 Tall metal masts, towers, cranes and revolving and travelling structures Masts and their guy-wires, floodlighting towers and other similar structures of metallic construction, particularly those to which the general public have access, should be earthed in accordance with this Standard. Cranes and other tall lifting appliances used for building construction purposes, shipyards and port installations should also be bonded to earth. For cranes or revolving structures mounted on rails, efficient earthing of the rails, preferably at more than one point, will usually provide adequate lightning protection. In special cases, where concern is felt regarding possible damage by lightning to bearings, additional measures may be justified. Mobile towers, portable cranes and similar structures mounted on vehicles with pneumatic tyres can be given a limited degree of protection against lightning damage by drag chains or tyres of conducting rubber such as are provided for dissipating static electricity.

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6.10 PROTECTION OF HOUSES AND SMALL BUILDINGS 6.10.1 General considerations The application of this Clause is intended to be restricted to relatively small buildings, such as houses or similar buildings, of a smaller size than those envisaged in Section 4 of this Standard. Lightning protection for a house or small building in complete accordance with the recommendations of Section 4 may be difficult to justify on economic grounds. However, there may be a need to provide some degree of protection against lightning damage. Houses and small buildings vary greatly in the degree to which their construction provides inherent lightning protection. Small buildings with mainly non-metallic materials offer little or no inherent protection against lightning, whereas a building with a metallic roof, metallic gutters, and metallic downpipes leading into the ground has a high degree of inherent protection, since the main elements of an LPS are already present. If lightning strikes a house with little or no inherent lightning protection, the lightning is likely to penetrate the roof and attach to electrical wiring in the roof area. This will usually result in damage to electrical equipment in the house, and in some cases, may result in a fire, or in hazard to persons within the house. The objective in protecting small buildings should be to provide conductors to intercept the lightning, to provide a low-resistance path to earth, and to provide at least two earth stakes or equivalent earthing electrodes for conveying the lightning current into the earth. 6.10.2 Air terminal network for the building If the building roof consists mainly of metallic materials, then it will serve as the air terminal network. It is necessary to ensure that there is electrical continuity between the various parts of the roof. Adequate continuity will often be provided by the way in which the metallic parts are overlapped and fastened. If the building roof consists mainly of non-metallic materials, then an air terminal network should be provided. Suitable materials are listed in Clause 4.7. Copper wire and copper strip are recommended for their durability. At least one conductor should be run along the highest parts of the roof, for example, the highest ridge of the building. If the roof has a complicated shape, it may be necessary to run additional conductors along the highest parts of each section of the roof. All conductors should be joined together. To be in accordance with this Standard, the cross-sectional area of the conductors should be not less than 35 mm 2 , achieved, for example, by copper strip 25 mm × 1.5 mm. However, it should be realized that much thinner conductors are able to carry most lightning currents without damage. Even if the conductor were to melt, it would have carried out its function for that one strike, as the lightning current would flow through the path of the molten metal, rather than penetrate below the roof of the house. For a large, more-or-less flat roof of non-conducting material, the simplest form of air terminal network may be a series of vertical metallic rods above the roof level, all connected together. The zone of protection provided by a vertical rod may be estimated using the information in Clause 4.4. Metallic gutters may become a strike attachment point. If there are metallic gutters around the roof, these should be connected to the air terminal network. With metallic roofs, these connections may already exist in the fastenings of the guttering to the roof. With non-metallic roofs, the guttering should be connected to the air terminal network at no less than two points.

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6.10.3 Provision of downconductors for the building There should be at least two low-resistance paths to convey the current from any lightning strike to the roof to earth. Metallic downpipes from metallic gutters may be used for this purpose, provided that they afford a direct electrically continuous path for the lightning current. In the absence of any low-resistance path from roof to earth, at least two conductors should be provided to serve as downconductors. These may be continuations of the conductors forming part of the air terminal network, and the same recommendations apply as in Clause 6.10.2. 6.10.4 Provision of earthing electrodes A path to earth for the lightning current should be provided at no less than two well separated points, for example, at opposite ends of the house. Preference should be given to areas that are usually damp, such as gardens. A metallic water pipe buried in the ground would be a satisfactory earthing electrode provided that the water pipe is also connected to the electricity supply service earth. Each downconductor should be connected to an earthing electrode by the shortest possible route, with the proviso that downconductors and earthing electrodes should not be placed close to entry doors, or places where persons are likely to stay for long periods. For example, earthing electrodes should not be placed close to swimming pools. Earthing electrodes and their connected conductors should be examined periodically to ensure that they are intact, and not suffering corrosion or mechanical damage. 6.11 PROTECTION OF METALLIC PIPELINES Recommendations for the protection of metallic pipelines are given in AS/NZS 4853.

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S E C T I ON 7 PRO T E CT I O N O F S T R U C T UR ES W IT H E X P L O S I V E O R H I GH L Y -F L A M M A B L E CO N T E N T S 7.1 SCOPE OF SECTION This Section provides a guide to the protection of structures containing explosives, or highly-flammable solids, liquids, gases, vapours or dusts, from lightning or induced discharges, and indicates ways of protecting those structures that are not inherently self-protecting. Reference should be made to the AS/NZS 2430 series for information on areas that are likely to have an explosive atmosphere. Reference should also be made to AS/NZS 1020 for information on the control of static discharges. 7.2 GENERAL CONSIDERATIONS 7.2.1 Risk assessment The approach for the assessment and management of risk due to lightning detailed in Section 2 may be applied to structures with explosive or highly-flammable contents. In some cases, the risk to life and property may be so obvious that the provision of every means possible for protection from the consequences of a lightning discharge is essential. Similarly, the owner of such a facility may decide that there should be no avoidable risk and specify that every means possible for protection from the consequences of a lightning discharge be installed. Alternatively, the risk may be assessed as acceptable where the quantity of dangerous material is strictly limited, as in a laboratory or small store, or where the structure is specifically designed and situated to restrict the effects of a catastrophe. Also, lightning protection may not be necessary in some circumstances where the dangerous materials are not exposed but are completely encased in metal of an adequate thickness. 7.2.2 Protection required Unless the risk assessment considerations in Clause 7.2.1 indicate that protection is not required, the recommendations in this section should be followed for structures in which explosives or highly-flammable solids, liquids, gases, vapours or dusts may accumulate, i.e. in those areas that may be classified as hazardous. Due to the increased risk, protection level I as defined in Section 4 should be applied to these structures (e.g. a rolling sphere of 20 m radius when using the RSM). 7.2.3 Electrostatic shielding The electrostatic induced voltage on isolated objects in the field of a storm cloud may cause sparks to earth when a lightning discharge occurs to some adjacent object. Isolated objects within a structure that is adequately shielded will themselves be electrostatically shielded. If the structure is not shielded or is only partly shielded, then the isolated objects should be earthed to prevent electrostatic sparks. For further discussion on the earthing of isolated internal objects, see Section 5.

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7.3 AREAS OF APPLICATION Protection should, in all cases, be provided for the following structures: (a)

Tanks and vessels containing flammable solids, liquids, vapours or gases, or highly-flammable or explosive dusts.

(b)

All metallic pipes and electricity supply and telecommunications service lines at the point where they enter or leave a hazardous area. Piping that is not in electrical contact with its associated tank or vessel, such as an open discharge line into a water tank, should be bonded to the tank or vessel by a flexible conductor, and earthed. Cathodic protection may justify the insertion of an insulating flange that will interrupt the electrical continuity of the total length of line. Cathodic protection usually has its converter and monitoring equipment outside the hazardous area, with ELV d.c. leads feeding into the latter. This may require separate protection to each part of the circuit. Where flexible connections between pipelines and tanks do not incorporate an earth-continuity conductor, a separate conductor for earthing should be provided. No pipeline should be used for earth-continuity purposes as a substitute for the recommended earthing conductor.

(c)

Buildings that may contain explosive or large quantities of highly-flammable materials, or nominated buildings that may, in an emergency, be used for the storage of explosives.

(d)

Buildings that may contain small quantities of highly-flammable material or a large quantity of combustible material if sited within 50 m of a building specified in Item (c).

(e)

Any structure sited within 30 m of a building containing explosives, which thus constitutes a projectile hazard to this building in the event of dislodgment of masonry and the like by lightning.

(f)

Any structure sited within 30 m of a building containing explosives that, if struck by lightning, might constitute a subsequent fire hazard.

7.4 EQUIPMENT APPLICATION 7.4.1 Earth bonding points Earth bonding points should be designed and installed to provide permanent, electrically sound connections between the tank, plant or structure and the earthing system. The bonding points can be bosses tapped to receive a bolt up to 50 mm long, a tag (minimum 50 × 50 × 10 mm) or a 50 mm threaded stud. The points should be fabricated to provide or accept a minimum of M10 (10 mm diameter) bolt, lug or fixing. Earth bonding points should be fabricated from the same (or compatible) metal as the structure it is being welded to. Pressure vessels should be provided by the manufacturer with a suitable bonding point to take the earth connection. In order to avoid corrosion, earth bonding points should be installed not less than 500 mm above ground level. In addition, all earth connections to the points should be protected with a suitable corrosion inhibiting compound or paint. 7.4.2 Bonding conductors Where various items of process plant or a number of vessels are mounted on an extensive concrete plinth that elevates the equipment above ground level, bonding conductors should be provided to form a common earth connection for all the downconductors from the plant.

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Copper strip should be installed along two opposite sides of the plinth, fastened to the walls not less than 500 mm above ground level to avoid corrosion. Tee-joints may be used between down and bonding conductor. Diagonally opposite ends of the base conductor should be provided with a test link from which connection is made to the earth termination network, preferably to earth busbars that provide alternative earth connections. Where one bonding conductor only is installed, test links and earth connections should be provided at each end. 7.4.3 Sizes of copper strips Sizes of copper strips should be in accordance with Table 4.6. For common earthing systems, larger sizes may be needed depending on the fault current. These should be selected in accordance with AS/NZS 3000. 7.4.4 Downconductors (see Clause 4.12) All high salient structures within a process area should be provided with at least two downconductors unless they are of welded construction or electrically continuous down to base level. Wherever possible, downconductors should be installed remote from stairs and operational walkways and ladders. Downconductors should preferably be installed at diagonally opposite corners of the structure in positions that provide the shortest possible path for connection to the earth termination network. They should be installed on the outside of the structure and should not pass through it. Copper strip should be used for downconductors and while, wherever possible, it should be in a continuous length, test links may be attached for connection of down or base conductors at various levels. Where structural steelwork or columns do not require the installation of an air terminal, the downconductor should extend from above the highest point of the structure. Provision should be made for thermal expansion of the earthing conductor and associated structure. A test link should be installed in the downconductor in accordance with Clause 4.13, not less than 500 mm above ground level. Each downconductor from the highest point or points within the process area should take the shortest possible path direct to earth and should be equipped with its own set of earthing electrodes to provide a path of minimum impedance for a lightning discharge. The earthing electrodes should be interconnected below ground level with the bonding conductor(s) belonging to other earthing systems. 7.4.5 Air terminals (see Clause 4.11) All high salient structures that are not electrically continuous and that are not within the zone of protection of an adjacent protected structure should be equipped with air terminals in accordance with the recommendations of this Standard. Where two or more air terminals are employed they should be interconnected by roof conductors for connection to at least two downconductors as follows: (a)

Roof conductors Copper strips should take the shortest salient route between the various air terminals, with fasteners spaced as for downconductors.

(b)

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7.5 SPECIFIC OCCUPANCIES 7.5.1 Protection of steel tanks 7.5.1.1 General precautions The following precautions should be taken to minimize the effects of lightning discharge on tanks containing petroleum products, including tanks with fixed roofs and tanks with floating roofs: (a)

The shells of all tanks intended for the storage of highly-flammable liquids that can produce an explosive gas atmosphere should be permanently and effectively earthed. Other tanks, such as water tanks, if located in a hazardous area should also be permanently and effectively earthed. The combined earth resistance of permanent earth connections to the tank should not exceed 10 Ω. The recommended method of earthing is by means of earthing electrodes as detailed in Clause 4.15, but in some installations soil conditions and the earth resistance of the tank when isolated from associated pipelines may in themselves constitute permanent and effective earthing. In such cases, the necessity for tank earthing electrodes should be considered with particular reference to site measurements of earth resistance.

(b)

The minimum number of individual earthing electrodes on storage tanks will depend upon the diameter and soil condition, and should be in accordance with the following schedule for single tanks: Diameter of tank m

Minimum number of independent earthing electrodes

≤ 30

2

> 30

3

For a group of small tanks, earthing electrodes common to the group may be installed, provided that each tank has two independent paths to earth. One of these paths may be through the pipeline earthing system. NOTE: The reason for the minimum of two earthing electrodes is that during testing of one electrode the tank will remain earthed by the other electrode.

Earthing electrodes for a tank may be interconnected around the periphery of the tank, and where two or more connections are used they should be spaced symmetrically round the tank. (c)

Each earthing conductor should be terminated and attached by means of a bolted connection to a steel boss welded to the tank body. The steel boss should be tapped to receive a bolt or stud, preferably 10 mm diameter. Lock washers should be used on the connecting assembly. Soldered connections should be avoided. It is suggested that the boss be welded on the tank at a minimum height of 500 mm above the bottom of the tank.

(d)

When a pipe or rod earthing electrode is driven into the ground, mechanical protection should be given to the head of the electrode. NOTE: It is the practice of some organizations to enclose all earth stake heads in a pit, where they are associated with ‘special’ earthing, such as lightning protection or static earthing.

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Steel tanks with floating roofs should be protected by one of the measures described in Items (i) and (ii): (i)

Multiple parallel connections between the floating roof and the tank shell, in particular those designs incorporating mechanical linkage in the seal assembly. This is the most effective method of discharging induced static charges caused by atmospheric conditions; under this arrangement it is not necessary to bond across internal drainpipe joints or external moving stairway joints.

(ii)

Overhead earth wires or other suitable forms of interception protection in accordance with Clause 4.4 (see also Clause 7.2.2). This may be appropriate in areas where there is a known high ground flash density.

7.5.1.2 Above-ground steel tanks containing flammable liquids at atmospheric pressure The contents of steel tanks with steel roofs of riveted, bolted, or welded construction, with or without supporting members, used for the storage of flammable liquids, are considered to be reasonably well protected against lightning if the tanks comply with the following recommendations: (a)

All joints between steel plates should be riveted, bolted, or welded.

(b)

All pipes entering the tank should be metallically bonded to the tank at the point of entrance.

(c)

All vapour or gas openings should be closed.

(d)

The metal tank and roof should have adequate thickness so that holes will not be burned through by lightning discharges (5 mm sheet steel roofs on tanks are considered adequate for this purpose*).

(e)

The roof should be continuously welded to the shell, or bolted, or riveted and caulked, to provide a gastight seam and electrical continuity.

7.5.1.3 Steel tanks with non-metallic roofs Steel tanks with wooden or other non-metallic roofs are not considered to be self-protecting, even if the roof is essentially gastight and sheathed with thin metal and with all gas openings closed or flameproofed. Such tanks should be provided with air terminals of sufficient height and number to receive all discharges and keep them away from the roof. The air terminals should be thoroughly bonded to each other, to the metallic sheathing, if any, and to the tank. Isolated metal parts should be avoided, or else bonded to the tank. In lieu of air terminals any of the following may be used: (a)

Conducting masts suitably spaced around the tank.

(b)

Overhead earth wires.

(c)

A combination of masts and overhead earth wires.

7.5.2 Installations handling crude oil and products—Jetties for marine tankers and barges The following recommendations should be observed as applicable: (a)

General All pipelines to jetties and any structural steelwork, plant and bollards on jetties together with associated dolphins, walkways, and shore bollards should be bonded to the electrical installation earthing system.

* This value is based on a recommendation in ANSI/NFPA 780. COPYRIGHT


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Electrical equipment on a jetty should be connected to an earthing system as specified in AS/NZS 3004. Dependent upon site and operating conditions, it may be possible to obtain overall protection by using one earthing system. Where it is considered that one common earthing system may be adapted to comply with all the requirements, it is necessary to ensure that the value of earth resistance does not exceed 4 Ω. Where steel or steel box piles are not employed, an earthing conductor should be installed to enter the water below low water mark to provide a direct path for lightning discharge. (b)

Jetties with cathodic protection It is recommended that the following precautions be taken where jetties are protected by either sacrificial anodes or power-impressed systems to prevent sparking at the tanker manifold when loading lines are being connected or disconnected: (i)

Install an insulating flange at the jetty end of each loading line between jetty and vessel whereby all flanges shore-side of the insulating flange are earthed to the jetty earthing system and all flanges to the seaward side are earthed via the vessel.

(ii)

Ensure that the insulating flange cannot inadvertently be short-circuited by the electrical connection of exposed metallic flanges on the seaward side of the insulating flange to the jetty structure either by direct contact or by hosehandling equipment.

(iii) Where sacrificial anodes are installed, it may be necessary to use manila mooring ropes or straps to extend the life of the anodes and minimize current flow between jetty and vessel. (c)

Ship/shore bonding cables An independent cable bonding connection between ship and jetty, with or without cathodic protection, is not considered as serving any useful purpose in— (i)

the dispersal of static electricity; or

(ii)

minimizing possible current flow in conductive type loading hoses.

7.5.3 Aircraft fuelling and de-fuelling Aircraft fuelling and de-fuelling should be suspended when electrical storms are in the vicinity. 7.5.4 Structures with explosive or highly-flammable contents 7.5.4.1 Methods of protection Structures with explosive or highly-flammable contents should be protected in one or more of the ways detailed in Clauses 7.5.4.2 to 7.5.4.6 and in accordance with the recommendations of Clauses 7.5.4.7 to 7.5.4.16, as appropriate. 7.5.4.2 Air terminal network An air terminal network should be suspended at an adequate height above the area to be protected (see Clause 4.4). Where a suspended conductor crosses chimneys or vents that emit explosive dusts or gases under forced draught, the suspended conductors should be not less than 5 m above the chimney or vent.

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7.5.4.3 Network of horizontal conductors Where the expense of the method described in Clause 7.5.4.2 cannot be justified, and where no risk is involved in discharging the lightning current over the surface of the structure to be protected, a network of horizontal conductors with a mesh between 3 m and 8 m, according to the risk, should be fixed to the roof of the structure. Each separate structure protected as above should be equipped with twice the number of downconductors recommended in Clause 4.12. 7.5.4.4 Strikes to the sides of buildings Consideration should be given to providing an array of vertical conductors to protect the sides of buildings. These conductors should be spaced every 10 m and be connected to the air terminal network (top) and earth termination network (bottom) of the LPS. 7.5.4.5 Vertical conductors A structure or a group of structures of small horizontal dimensions may be protected by one or more vertical lightning conductors (see Clause 4.4). 7.5.4.6 Below-ground structures A structure that is wholly below ground and not connected to any services above ground can be protected by an air terminal network as described in Clause 7.5.4.2 by virtue of the fact that soil has an impulse breakdown strength that can be taken into account when the risk of flashover from the protection system to the structure to be protected, including its services, is being determined. Where the depth of burying is adequate, the air terminal network may be replaced by a network of earthing strips arranged on the surface. Where the underground structure has a reinforced concrete roof at or immediately below soil level, the reinforcement may be used as a protection system provided that the reinforcement is welded so that rectangular electrical conducting paths are formed with sides not exceeding 2 m in length. Where the underground structure has a roof that is not reinforced or where the reinforcement is not electrically continuous, a buried conductor network located above the structure and buried not less than 500 mm below the soil level may be used. Where the structure is such that protection cannot be provided by use of the reinforcement and the depth of soil above the roof is less than 500 mm, air terminals may be mounted on suitable bases above the structure at soil level and interconnected by a roof conduction network of closed mesh of between 3 m and 8 m. 7.5.4.7 Interconnection of earth terminations The earth terminations of the earth protective system should be interconnected by a ring conductor. This ring conductor should preferably be buried to a depth of not less than 500 mm and be not less than 2 m from the walls of the structure unless other considerations, such as the need for bonding other objects to it, testing or risks of corrosion, make it desirable to leave it exposed. The resistance value of the earth termination network should be maintained permanently at 10 Ω or less. If this value proves to be unobtainable, the methods recommended in Clause 4.14 should be adopted, or the ring conductor should be connected to the ring conductor of one or more neighbouring structures until the above value is obtained. 7.5.4.8 Bonding of structural metal All major metal forming part of the structure, including continuous metal reinforcement and services should be bonded together and connected to the LPS. Such connections should be made in at least two places and should, as far as is possible, be equally spaced round the perimeter of the structure at intervals not exceeding 15 m. COPYRIGHT


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7.5.4.9 Bonding of internal metal Major metalwork inside the structure should be bonded to the LPS. 7.5.4.10 Electrical conductors entering structure Electrical conductors entering the structure should be enclosed in metal. The metal enclosure should be electrically continuous within the structure; it should be earthed at the point-of-entry outside the structure on the supply side of the service and bonded directly to the LPS. 7.5.4.11 Electrical conductors connected to overhead electricity supply service line Where the electrical conductors are connected to an overhead electricity supply service line, a length of buried cable with metal sheath or armouring should be inserted between the overhead line and the point-of-entry to the structure, and an SPD, e.g. of the type containing voltage-dependent resistors, shall be provided at the termination of the overhead line. The earth terminal of this SPD should be bonded directly to the cable sheath or armouring. The spark overvoltage of the SPD should not exceed half the breakdown withstand voltage of the electrical equipment in the structure. In this operation, the appropriate Standard and any regulations that may apply should be observed. 7.5.4.12 Metal not continuously earthed Metallic pipes, metallic electrical cable sheaths or metallic armouring, steel ropes, rails or guides that enter the structure and are not in continuous electrical contact with the earth should be bonded to the LPS. They should be earthed at the point-of-entry outside the structure and at two points, one about 75 m away and one a further 75 m away. 7.5.4.13 Adit or shaft For a buried structure or underground excavation to which access is obtained by an adit or shaft, the recommendations in Clause 7.5.4.12 as regards extra earthing should be followed for the adit or shaft at intervals not exceeding 75 m as well as outside the structure. 7.5.4.14 Fences and retaining walls The metal uprights, components and wires of all fences, and of retaining walls in close proximity to the structure, should be connected in such a way as to provide continuous metallic connection between themselves and the LPS. Discontinuous metal wire fencing on non-conducting supports or wire coated with insulating material should not be employed. 7.5.4.15 Avoidance of tall components Structures with explosive or highly-flammable contents should not be equipped with tall components such as spires and flagstaffs or radio antennas on the structure or within 15 m of the structure. This clearance applies also to the planting of new trees, but structures near existing trees should be treated in accordance with Clause 6.3. 7.5.4.16 Tests of system Tests should be carried out in accordance with Clause 8.3 at intervals of not more than two years. The test equipment used should be certified for use in the particular hazardous area. In some cases, non-certified testing equipment may be used provided that the location where the tests are to be conducted has been proven to be free of combustible gases or vapours.

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SECT ION 8 I NSTA L L A T I ON AN D MA I N T E N A N CE PRA CT IC E 8.1 WORK ON SITE Throughout the period of erection of a structure, all large and prominent masses of metalwork, such as steel frameworks, scaffolding and cranes, should be effectively connected to earth. Once work has commenced on the installation of an LPS, an earth connection should be maintained at all times. During the construction of overhead electricity supply service lines, overhead equipment for railway electrification and the like, the danger to persons can be minimized by ensuring that an earth termination network is installed and properly connected before any conductors other than earth wires are run out. Once the conductors are run out and insulation installed, they should not be left ‘floating’ while persons are working on them, but should be connected to earth in the same manner as when maintenance is being carried out after the line is commissioned. 8.2 INSPECTION All LPSs should be inspected after completion, alteration or extension, in order to verify that they are in accordance with this Standard. A routine inspection should be made at least every two years. More frequent inspections may be warranted in some circumstances. Such circumstances include, but are not limited to— (a)

areas subject to severe weather and lightning activity;

(b)

structures located in areas where aggressive soil or other conditions may accelerate corrosion or other aspects of system degradation;

(c)

changes in technology use within the structure that may necessitate a review of the protection means and their continued effectiveness; and

(d)

any other time where it is deemed necessary to update the original risk assessment for lightning damage.

8.3 TESTING On the completion of the installation or of any modification to it, and at the time of any maintenance inspection, the resistance to earth of the whole installation and of each earth termination should be measured, and the electrical continuity of all conductors, bonds and joints and their mechanical condition verified. The testing should be carried out in accordance with Appendix B. Where regular testing during maintenance reveals that the earthing resistance is substantially unchanged, the frequency of maintenance testing may be reduced to each alternate inspection. If the resistance to earth of an LPS, when so determined, exceeds the specified value for the particular applications the value should be reduced to be in accordance with the recommendations of this Standard. If the resistance is less than the recommended value but significantly higher than the previous reading, the cause should be investigated in accordance with Appendix C. The condition of the soil, the procedure adopted, details of salting or other soil treatment, and the results obtained should all be recorded as listed in Clause 8.4.

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8.4 RECORDS The following records should be kept on site, or by the persons responsible for the upkeep of the installation: (a)

Scale drawings showing the nature, dimensions and position of all component parts of the LPS.

(b)

The nature of the soil and any special earthing arrangements.

(c)

Date and particulars of salting, if used.

(d)

Test conditions, date and results in accordance with Clause 8.3.

(e)

Alterations, additions or repairs to the system.

(f)

The name and contact details of the persons responsible for the installation or for its upkeep.

NOTE: Detection of the occurrence of lightning flashes to the structure and the magnitude of the discharge current may be estimated by magnetic links, magnetic tape strips or other current monitoring devices. While the use of instruments to count the number of strikes intercepted by the protection system is highly recommended, in practice, this may be impractical to achieve on multiple downconductor LPSs.

8.5 MAINTENANCE The general recommendations of this Standard provide, as one of their objectives, LPSs that do not require a lot of maintenance. Nevertheless, some system components will lose their effectiveness over time because of weathering, corrosion, and stroke damage. Both physical and electrical characteristics of the LPS must be maintained to ensure compliance with the design requirements. The periodic inspection and tests described in Clauses 8.2 and 8.3 will indicate what maintenance, if any, should be undertaken. Particular attention should be paid to earthing, to any evidence of corrosion and to any alterations or extensions to the structure that may affect the LPS. Examples of such alterations or extensions are as follows: (a)

Changes in the use of a building.

(b)

Installation of fuel oil storage tanks.

(c)

The erection of radio and television antennas.

(d)

Installation or alteration to the electrical, telecommunications or computing facilities within, or closely connected to, the building.

A good maintenance program should also contain provision for the following: (i)

Inspection of all system components.

(ii)

Tightening of all accessible clamps and splices.

(iii) Measurement of system resistance, including earth resistance of terminals. (iv)

Inspection or testing of SPDs.

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APPENDIX A

EXAMPLES OF LIGHTNING RISK CALCULATIONS (Informative) A1 INTRODUCTION A number of lightning risk calculations have been included in this Appendix to illustrate different structures and risk categories. The risk calculation examples are: (a)

Example 1—40 m office block.

(b)

Example 2—Darwin hotel.

(c)

Example 3—Historic church.

(d)

Example 4—Remote pump station.

(e)

Example 5—Two storey house.

The front page of the Microsoft  Excel spreadsheet file LIGHTNING RISK.XLS has been reproduced in Paragraphs A2.1 to A2.5 for each of these examples. In addition to this, all the workings of the 40 m office block example have been included in Paragraph A3.

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20

40

Width (m)

Height (m)

Low

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For DOD Internal Use ONLY 1.0E-05 1.0E-03 1.0E-03 1.0E-03

0.00E+00 3.16E-05

Loss of Cultural Heritage

Economic Loss

(Ra)

(R)

None

0.8

Date: 28/1/2006

Fire Damage Factor Overvoltage Damage Factor

Fire Damage Factor

Special Hazard

1.07E-05

0.00E+00

0.00E+00

4.81E-06

Direct Strike Risk (Rd)

Surge Protection on All Equipment

Surge Protection at Point of Entry

Yes

0.05

Overvoltage Damage Factor


2.09E-05

0.00E+00

0.00E+00

0.00E+00

0

Acceptable Risk of Economic Losses Step & Touch Potential Damage Factors

Category 4 - Economic Loss

Fire Damage Factor

Category 3 - Loss of Cultural Heritage

Indirect Strike Risk (Ri)

No

Fire Damage Factor

Category 1 - Loss of Human Life

Loss Categories

Category 2 - Loss of Essential Services

1.E-03

0.3

0

10

40m Office Block

Version 3.0

Protection Measures

Acceptable Risk

4.81E-06

0 Unscreened

Overall Risk

Fire Protection

0.00E+00

Urban

Service Line Density

Efficiency of Building Protection

Cable Type

Number

Loss of Essential Services

Isolated

Environmental Factor

0 Unscreened

Other Underground Services

Cable Type

Number

Loss of Human Life

3

Ground flash density

No Transformer

Unscreened

Underground

Other Overhead Services

Transformer at Structure

Cable Type

Service

Power Line

Service Lines

Calculated Risk

Unscreened

Medium

Environment

Internal Wiring Type

Risk of Fire or Physical Damage Risk of Dangerous Discharge

Structure Attributes

20

Length (m)

Structure Dimensions

Standards Australia

Risk Assessment for Lightning Protection

Structure Identification

A2.1 Example 1—40 metre high office block

A2 SAMPLE SPREADSHEET CALCULATIONS

0

1.E-03

0

0

AS/NZS 1768:2007 112

15

30

Width (m)

Height (m)

Low

Unscreened

Medium

No Transformer

Unscreened

Underground

Unscreened

0

Fire Protection

Similar Height

Urban



0

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3.26E-04

Economic Loss

1.70E-05

0.00E+00

1.0E-03 1.0E-03

0.00E+00


3.36E-06 0.00E+00

1.0E-03

0.00E+00


(Ra) 1.0E-05

(R)




3.36E-06

None

0.8

Loss of Human Life

Acceptable Risk

Calculated Risk


Fire Damage Factor

Yes

0.1



3.20E-10

3.09E-04

0.00E+00

0.00E+00

0



Fire Damage Factor



No

Fire Damage Factor


Loss Categories


5.E-03

0.5

0

10

Hotel in Darwin

Date: 28/1/2006

Special Hazard

Version 3.0

Protection Measures

Unscreened

Overall Risk


8

Cable Type

Number


Cable Type

Number



Cable Type

Service

Power Line

Service Lines


Standards Australia



Environment




15

Length (m)


A2.2 Example 2—Darwin hotel

0

1.E-03

0

0

113 AS/NZS 1768:2007

40

130

Width (m)

Height (m)

High

Low

Unscreened

Similar Height

Urban



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For DOD Internal Use ONLY Unscreened

0

Cable Type

Number

1.0E-05 1.0E-03 1.0E-03 1.0E-03

4.13E-06

0.00E+00

1.42E-05

3.13E-06



Economic Loss

(Ra)

Acceptable Risk

Overall Risk

Fire Protection


Fire Damage Factor

None

0.8

3.13E-06

1.42E-05

0.00E+00

4.13E-06




Yes

0.005



0.00E+00

5.00E-09

0.00E+00

0.00E+00

0.5



Fire Damage Factor



No

Fire Damage Factor


Loss Categories


1.E-05

0.1

0

10

Historic Church

Date: 28/1/2006

Special Hazard

Version 3.0

Protection Measures

Unscreened

0


Cable Type

Number

Loss of Human Life

(R)

No Transformer

Unscreened

Underground



Cable Type

Service

Power Line

Service Lines


Calculated Risk

1


Environment




40

Length (m)


Standards Australia



A2.3 Example 3—Historic church

0

1.E-03

0

0

AS/NZS 1768:2007 114

3

3

Width (m)

Height (m)

Low

Low

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For DOD Internal Use ONLY 0.00E+00

1.0E-03

0.00E+00

3.74E-07


Economic Loss

2.99E-09

2.97E-09

1.0E-03

7.43E-10

1.0E-03




1.0E-05

(Ra)

(R)

None

0

2.06E-05

Acceptable Risk

Overall Risk

Fire Protection


Protection Measures

Unscreened

0

7.44E-10

Rural



Cable Type

Number

Fire Damage Factor

Special Hazard


Isolated


Unscreened

0


Cable Type

Number

Date: 28/1/2006

Fire Damage Factor 0


3.71E-07

0.00E+00

2.06E-05

8.32E-13


Yes

Yes

0



Fire Damage Factor




1.E-04

0.2

0.1

1

Loss Categories Category 1 - Loss of Human Life

Remote Pump Station

Version 3.0

Loss of Human Life

2


No Transformer

Unscreened

Overhead



Cable Type

Service

Power Line

Service Lines

Calculated Risk

Unscreened

Environment




3

Length (m)


Standards Australia



A2.4 Example 4—Remote pump station

0

1.E-03

0.01

0

115 AS/NZS 1768:2007

30

12

Width (m)

Height (m)

High

Low

Unscreened

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0.00E+00

0.00E+00

8.07E-04



Economic Loss


Fire Damage Factor

Special Hazard

1.0E-03

8.01E-04

0.00E+00 0.00E+00

1.0E-03

5.79E-06

1.07E-06

1.0E-03



0



Fire Damage Factor


No

0.05


0.00E+00



No

Fire Damage Factor Category 2 - Loss of Essential Services

1.E-04

0.3

0

1

Loss Categories Category 1 - Loss of Human Life

1.65E-06

None


Protection Measures 0

Date: 28/1/2006

Two Storey House

Version 3.0

0.00E+00

1.0E-05

(Ra)

2.72E-06

Loss of Human Life

1 Unscreened

Overall Risk

Fire Protection


Cable Type

Number

(R)

Suburban


Unscreened

0


Cable Type

Number

Acceptable Risk

Lower Height


No Transformer

Unscreened

Overhead



Cable Type

Service

Power Line

Service Lines

Calculated Risk

2


Environment




20

Length (m)


Standards Australia



A2.5 Example 5—Two storey house

0

1.E-03

0

0

AS/NZS 1768:2007 116

117

AS/NZS 1768:2007

A3 SAMPLE BY-HAND CALCULATION—40 METRE HIGH OFFICE BLOCK The following calculates the risk of lightning damage to a structure and its equipment outlined in initial work by IEC Committee TC 81. One power line and any number of overhead and underground service lines are considered. Selectable parameters have been tailored for Australian and New Zealand conditions. Parameter

Example value

Lightning activity

Ng

Ng

=

3

Ground strike density in ground strikes per km 2 per year

STRUCTURE L

L

=

20 m

Structure length in metres

W

W

=

20 m

Structure width in metres

H

H

=

40 m

Structure height in metres

Cd

Cd

=

1.0

Environmental factor for surrounding object height (direct strikes to structure)

Dm

Dm

=

250 m

Structure in large area of structures or trees of the same height or greater e.g. typical building in CBD, or shed in an industrial area

0.25

Structure surrounded by smaller structures. e.g. tall building in urban area

0.5

Isolated structure with no other structures or objects within a distance of 3 × height from the structure. e.g. structure in a rural area

1

Isolated structure on hilltop or knoll. e.g. communications site

2

Distance from structure that a lightning strike to ground creates a magnetic field which may be great enough to induce an overvoltage exceeding the impulse level of internal equipment. Fixed factor

Cs

Cs

=

0.01

Correction factor for surrounding area. (Density factor relating to service drops) CBD

0

Urban

0.1

Suburban

0.5

Rural Ph

Ph

=

0.01

Pg

=

10 −5

1

Probability that lightning will cause a shock to animals or human beings outside the structure due to dangerous step and touch potentials. Fixed factor

Pg

250 m

10 −2

Probability that lightning will cause a shock to animals or human beings inside the structure due to dangerous step and touch potentials. Fixed factor

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10 −5

AS/NZS 1768:2007

Parameter

118

Example value

Pf

Pf

=

0.001

Probability that a dangerous discharge will initiate a fire, explosion, mechanical destruction or chemical release. Explosive high risk of explosion (petrochemical plant) High risk of mechanical and thermal effects (High or significant risk of fire or mechanical damage, roof of combustible material, e.g. thatched roof)

10 −1

Medium risk of mechanical and thermal effects (Significant use of combustible building material, e.g. timber frame; OR risk of mechanical damage, e.g. significant masonry dislodged)

10 −2

Low risk of mechanical and thermal effects (eg. modern reinforced concrete building)

10 −3

Negligible risk of mechanical and thermal effects (all metal structure) Ps

Ps

=

0.2

1

0

Probability of a dangerous discharge based on structure type High (Brick, masonry, flammable material, timber or non-conducting material, unprotected roof installations with electrical lines to inside, e.g. antennae)

1

Medium (Continuous reinforced concrete or steel columns or downconductors. Maximum spacing 20 m)

0.2

Low (all metal construction) Pi

Pi

=

1

0.01

Probability of a dangerous discharge based on internal wiring type Unscreened wiring Continuously screened wiring

1 0.1

EQUIPMENT kw

kw

=

1

Correction factor for impulse level of equipment. Fixed factor

1 (applies to impulse level of 1.5 kV)

CONDUCTIVE SERVICE LINES Assumes one or no power line and it is either overhead or underground. The quantity of overhead and underground service lines in separate routes must be selected. The length of service lines is determined based on ‘rural’, ‘urban’, ‘suburban’ or ‘CBD’ situation. Assumes there is no adjacent structure connected by a service line. Power line Pl

Pl

=

2

Power line type Overhead

1

Underground

2

None

0

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Parameter P e0

AS/NZS 1768:2007

Example value P e0

=

1

Probability of dangerous discharge based on external wiring type Unscreened wiring Screened cable with screen earthed or wiring in continuous metal conduit that is earthed

C t0

C t0

=

1

1 0.4

Correction factor for transformer LV line, no transformer MV line with HV/LV transformer or isolation transformer

1 0.25

Overhead service line n oh

n oh

=

0

Number of overhead service lines in separate routes

P e1

P e1

=

1

Probability of dangerous discharge based on external wiring type Unscreened wiring Screened cable with screen earthed or wiring in continuous metal conduit that is earthed Fibre optic lines without metal conductors

ρ1

ρ1

=

100 Ω m

H cl

=

7m

L1

=

100 m

0 100 Ω × m

Height of conductors above ground Fixed value

L1

0.4

Soil resistivity Fixed factor

H cl

1

7m

Length of line to the structure from the last distribution node CBD

0m

Urban

100 m

Suburban

500 m

Rural

1000 m

Assume there is no adjacent structure 1 a1

1 a1

=

0m

Length of adjacent structure

w a1

w a1

=

0m

Width of adjacent structure

h a1

h a1

=

0m

Height of adjacent structure

C t1

C t1

=

1

Correction factor for transformer Fixed factor

1 (no isolation transformer)

Conductive underground services—Electrical services e.g. Communication lines n ug

n ug

=

0

Number of underground service lines in separate routes

P e2

P e2

=

1

Probability of dangerous discharge based on external service type Unscreened wiring Screened cable with screen earthed or wiring in continuous metal conduit that is earthed

ρ2

ρ2

=

100 Ω m

1 0.4

Soil resistivity Fixed factor COPYRIGHT


100 Ω × m

AS/NZS 1768:2007

Parameter L2

120

Example value L2

=

100 m

Length of line to the structure from the last distribution node CBD

0m

Urban

100 m

Suburban

500 m

Rural

1000 m

Assume there is no adjacent structure 1 a2

1 a2

=

0m

Length of adjacent structure

w a2

w a2

=

0m

Width of adjacent structure

h a2

h a2

=

0m

Height of adjacent structure

C t2

C t2

=

1

Correction factor for transformer Fixed factor

1 (no isolation transformer)

ACCEPTABLE RISK AND DAMAGE FACTORS Damage category 1—Loss of human life R a1

R a1

=

10 −5

Acceptable risk. Probability of loss of human life per year Fixed value for loss of human life

k h1

δ h1

k h1

δ h1

=

=

10

0.01

Increasing factor applied to damage factor for fire and overvoltage when risk of loss of human life is aggravated by special hazards Low level of panic (building with less than three floors and less than 100 people)

1

Difficulty of evacuation, immobilized people

5

Average level of panic (sport or cultural structure with between 100 and 1000 people)

10

High level of panic (theatres, concert halls, cultural and sport events with more than 1000 people)

100

Hazards for surroundings or environment

200

Contamination of surroundings or environment

500

Damage factor for step and touch potential outside structure Fixed value

δ g1

δ g1

=

0.0001

δ f1

=

0.05

0.01

Damage factor for step and touch potential inside structure Fixed value

δ f1

10 −5

0.0001

Damage factor for fire Loss of human life Hospitals, hotels, public buildings

0.1

Industrial properties, properties for commercial activities, schools, offices

0.05

Public entertainment buildings, churches, museums, temporary structure

0.005

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Parameter δ o1

AS/NZS 1768:2007

Example value δ o1

=

0

Damage factor for overvoltages Loss of human life Hospitals Industrial properties with risk of explosion Other structures

0.0005 0.05 0

Damage category 2—Loss of essential service to the public R a2

R a2

=

10 −3

Acceptable risk. Probability of loss of essential service to the public per year Fixed value for loss of service to the public

δ f2

δ f2

=

0

10 −3

Damage factor for fire Unacceptable loss of service to the public Gas supply

0.2

Water supply

0.1

Radio and television

0.08

Telecommunications

0.06

Power supply, railway

0.04

No essential service function associated with the structure δ o2

δ o2

=

0

0

Damage factor for overvoltages Unacceptable loss of service to the public Gas supply

0.02

Water supply

0.01

Radio and television

0.005

Telecommunications

0.003

Power supply, railway

0.001

No essential service function associated with the structure

0

Damage category 3—Loss of cultural heritage (it is assumed there are no electronic devices inside) R a3

R a3

=

10 −3

Acceptable risk. Probability of loss of cultural heritage per year Fixed value for loss of cultural heritage

δ f3

δ f3

=

0

10 −3

Damage factor for fire Loss of irreplaceable cultural heritage Typical value No cultural heritage value

0.5 0

Damage category 4—Economic loss R a4

R a4

=

10 −3

Acceptable risk for economic loss. Probability of economic loss per year Depends on structure owners requirement Range available is 0.1, 0.01, 0.001, 0.0001, 0.00001

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Suggest 0.001

AS/NZS 1768:2007

Parameter k h4

122

Example value k h4

=

1

Increasing factor applied to damage factor for fire and overvoltage when risk of loss of human life is aggravated by special hazards Fixed value

δ h4

δ h4

=

0

Damage factor for step and touch potential outside structure Fixed value for agricultural properties with animals around the structure Otherwise

δ g4

δ g4

=

0

Otherwise δ f4

=

0.3

0.01 0

Damage factor for step and touch potential inside structure Fixed value for agricultural properties with animals inside the structure

δ f4

1 (no special hazard factor)

0.0001 0

Damage factor for fire Economic loss δ f = average value of possible loss/total value structure, contents and activities Typical values for economic loss:

δ o4

δ o4

=

0.001

Hospitals, hotels, industrial properties, museums

0.5

Properties for public use, agricultural properties

0.4

Offices, schools

0.3

Commercial activities, public entertainment

0.2

Prisons, churches

0.1

No economic loss

0

Damage factor for overvoltages Economic loss δ o = average value of possible loss/total value structure, contents and activities Typical values for economic loss: Hospitals, hotels, industrial properties, museums

0.005

Properties for public use, agricultural properties, offices, schools

0.001

Commercial activities, public entertainment

0.0001

Prisons, churches

0.00001

No economic loss

0

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Parameter

AS/NZS 1768:2007

Example value

LIGHTNING PROTECTION MEASURES E

E

=

0.8

Efficiency of lightning protection system on the structure. Takes into account interception and sizing efficiencies Level I

98%

Level II

95%

Level III

90%

Level IV

80%

No protection

0%

Assumes surge protection is applied to all OR none of the internal equipment of the structure k2

k2

=

1

Reduction factor for isolation equipment on internal equipment Fixed factor

k3

kf

k3

kf

=

=

1

1 (no isolation transformer on internal equipment)

Reduction factor for surge protection device on input of equipment

1

Surge protection device on input of equipment

0.01

No surge protection device on input of equipment

1.0

Reduction factor for fire protection measures No protection measures

1.0

Extinguishers, hydrants, manual alarm installations, fixed manually operated extinguishing installations

0.8

Protection escape routes, fireproof compartments, automatic alarms protected from overvoltage, automatically operated extinguishers, operating time of escape routes less than 10 minutes

0.6

Conductive service lines—assumes surge protection must be applied to all OR none of the service lines k4

k4

=

1

Reduction factor for isolation equipment at entry point of service line Fixed factor

k5

k5

=

0.01

1 (no isolation transformer on line entry point)

Reduction factor for surge protection device on entry point of service line Surge protection device on entry point of line

0.01

No surge protection device on entry point of line

1.0

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For DOD Internal Use ONLY A c1 = 0 km 2

A c1 = 2 × D c1 × Lc1

A a1 = 0 km

L c1 = 0 m

L c1 = if (Lc1 ≥ 0 m, Lc1, 0 m)

A a1 = 1a1 × w al + 6 × hal × (1 a1 + wa1) + 9 × π ×hal

L c1 = −20 m

L c1 = L 1 – 3 × H – 3 × h a1

2

D c1 = 21 m

n ohp = 0

D c1 = 3 × Hc1

Direct strikes to overhead line

n ohp = if (P l = 1, 1, 0)

OVERHEAD SERVICE LINE

N m = 0.4845

Collection area for direct strikes to adjacent structure

Collection area for overhead line

Effective length for overhead line

Lateral distance for overhead line

Calculate number of overhead power service lines

Average number of strikes direct to ground or to grounded objects near the structure per year causing overvoltages

Limit minimum value to zero if height of structure is too large

A m = if (A m < 0, 0 km 2 , A m )

Nm = Ng × Am

Area of influence for generation of overvoltages

Average number of direct strikes to the structure per year

Collection area in square metres for direct strikes to the structure. Based on intersection of ground surface and a line with slope of 1/3 that touches the top of the structure

A m = L × W + 2 × D m × (L + W) + π ×D m 2 – A d A m = 0.162 km 2

N d = 0.1657

N d = N g × Ad × C d

Indirect strikes to structure

A d = 0.055 km 2

Example value

A d = L × W + 6 × H × (L + W) + 9 × π × H 2

Direct strikes to structure

Parameter

CALCULATIONS

AS/NZS 1768:2007 124

2

N c1 = 0

N c1 = N g × (Ac1 + A a1 ) × Ct1 × Cs

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A a2 = 1a2 × w a2 + 6 × ha2 × (1a2 + w a2) + 9 × π × h a2

A a2 = 0 km

A c2 = 0 km

A c2 = 2 × D c2 × L c2

2

2

L c2 = 0 m

L c2 = if (Lc2 ≥ 0 × m, L c2, 0 × m)

D c2 = 2 m L c2 = −20 m

ρ2 × m 0.5 × ohm −0.5

n ugp = 1

L c2 = L 2 – 3 × H – 3 × h a2

D c2 = 0.2 ×

Direct strikes to underground service line

n ugp = if (Pl = 2, 1, 0)

UNDERGROUND SERVICE LINE

N 11 = 0.30

N 11p = 0.30

N 11p = N g × A 11 × C t0 × C s

N 11 = N g × A 11 × Ct1 × Cs

A 11 = 1 km 2

ρ1 × m 0.5 × ohm −0.5

A 11 = 2 × D11 × L 1

D 11 = 500 ×

D 11 = 5000 m

N c1p = 0

N c1p = N g × (A c1 + Aa1) × C t0 × Cs

Indirect strikes to overhead line

Example value

Parameter

CALCULATIONS

Collection area for direct strikes to adjacent structure

Collection area for underground line

Effective length for underground line

Lateral distance for underground line

Calculate total number of underground power service lines

Average number of strikes to ground near other conductive overhead line per year that cause potentially dangerous induced voltages

Average number of strikes to ground near the overhead power line per year that cause potentially dangerous induced voltages

Collection area for induced overvoltages due to strikes to ground near the overhead line

Lateral distance for induced overvoltages due to strikes to ground near the line

Average number of strikes direct to other conductive overhead line per year that are potentially dangerous

Average number of strikes direct to the overhead power line per year that are potentially dangerous

125 AS/NZS 1768:2007

N c2 = 0

N c2 = N g × (Ac2 + A a2 ) × Ct2 × Cs

N 12p = 0.15

N 12 = 0.15

N 12p = N g × A 12 × C t0 × C s

N 12 = N g × A 12 × Ct2 × Cs

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Probability of dangerous step and touch voltages outside structure due to direct strikes to unprotected structure

P h1 = 4 × 10-4

R h1 = 6.628 × 10 -7

P h1 = k 1 × P h × P s

R h1 = Nd × Ph1× δ h1

Risk of dangerous step and touch potential to humans outside the structure due to direct strikes to the structure

Reduction factor for structure lightning protection system

Average number of strikes to ground near other underground cable per year that cause potentially dangerous induced voltages

Average number of strikes to ground near the underground power cable per year that cause potentially dangerous induced voltages

Collection area for induced overvoltages due to strikes to ground near the underground cable

Lateral distance for induced overvoltages due to strikes to ground near the underground cable

Average number of strikes direct to other conductive underground line per year that are potentially dangerous

Average number of strikes direct to the underground power line per year that are potentially dangerous

k1 = 1 – E

Risk of dangerous step and touch potential outside structure due to direct strikes to structure

RISK CALCULATIONS FOR DAMAGE CATEGORY 1 – Loss of human life

A 12 = 0.5 km 2

ρ2 × m 0.5 × ohm −0.5

A 12 = 2 × D12 × L 2

D 12 = 250 ×

D 12 = 2500 m

N c2p = 0

N c2p = N g × (A c2 + Aa2) × C t0 × Cs

Indirect strikes to underground service line

Example value

Parameter

CALCULATIONS

AS/NZS 1768:2007 126

Example value

Probability that external wiring carries a surge from structure that causes physical damage Probability a direct strike to structure will cause physical damage Risk of physical damage due to direct strikes to the structure

P etc = 1 P ewd = 0.01

P s = 5 × 10-5 R s1 = 4.143 × 10-6

P etc = if (P et > 1,1, P et)

P ewd = k5 × Petc

P s = kf × Pf × (k1 × Ps + Pewd)

R s1 = Nd × Ps × δf1 × k h1

P w = 0.05

R w1 = 0

P w = 1 – (1 – k 1 × Ps × Pi × k 2 × k 3 × k w) × (1 – P ewdo )

R w1 = Nd × Pw × δ o1 × kh1

Risk due to failure or malfunction of electrical and electronic equipment due to overvoltages from direct strikes to the structure

Probability of electronic failure due to overvoltages from direct strike to the structure

Probability that external wiring carries a surge from structure that causes a damaging overvoltage to internal equipment

P m = 0.04

R m1 = 0

P m = k1 × k2 × k3 × kw × Ps × Pi

R m1 = N m × P m × δ o1 × kh1

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Risk of internal equipment failure due to a direct strike to ground near the structure causes a damaging overvoltage

Probability a direct strike to ground near the structure will cause a damaging overvoltage to internal equipment

Risk of electrical/electronic equipment malfunction or failure due to overvoltages from indirect strikes to the structure

P ewdo = 0.01

P ewdo = k w × k 2× k 3 × k 4 × k 5 × P etc

Risk of electrical/electronic equipment malfunction or failure due to overvoltages from direct strikes to the structure

Limit partial probability to a maximum of 1

Total partial probability that external wiring carries a surge

P et = 1

P et = P e0 + noh × P e1 + nug × P e2

Check if there is no power line. If there is no power line then set to zero

P e0 = 1

P e0 = if (P1 = 0, 0, P e0)

Risk of physical destruction due to fire, explosion, mechanical damage and chemical discharge due to direct strikes to structure

Parameter

CALCULATIONS

127 AS/NZS 1768:2007

Example value

P c1 = 0

P c2p = 0.01

P c2 = 0

R g1 = 0

P c1 = noh × k 5 × P e1

P c2p = n ugp × k5 × Pe0

P c2 = nug × k 5 × P e2

R g1 = P g × (N c1 × P c1 + Nc2 × Pc2 + N c1p × P clp + N c2p × P c2p) × δ g1

Risk of dangerous step and touch potential to humans inside the structure due to direct strikes to the structure

Probability a direct strike to other underground line will cause a damaging overvoltage to internal equipment

Probability a direct strike to the underground power line will cause a damaging overvoltage to internal equipment

Probability a direct strike to other overhead line will cause a damaging overvoltage to internal equipment

Probability a direct strike to the overhead power line will cause a damaging overvoltage to internal equipment

R c1 = k f × P f × (Nc1 × P c1 + N c2 × Pc2 + Nc1p × P c1p + N c2p × Pc2p) × δ f1 × kh1

R c1 = 0

Risk of fire, explosion, mechanical damage and chemical release caused by internal equipment failure due to a direct strike to service lines which causes a damaging overvoltage

Risk of fire, explosion, mechanical damage and chemical discharge caused by electrical/electronic equipment malfunction or failure due to overvoltages from direct strikes to service lines

P clp = 0

P c1p = n ohp × k5 × Pe0

Risk of dangerous touch potential inside structure due to direct strikes to service lines

Parameter

CALCULATIONS

AS/NZS 1768:2007 128

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Example value R e1 = 0

Risk of internal equipment failure due to a direct strike to the service lines that causes a damaging overvoltage

R 1 = 4.806 × 10

R 1 = Ri1 + R d1

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Compare total risk to acceptable risk. If total risk exceeds acceptable risk, increase the level of protection measures.

R a1 = 1 × 10 -5

R i1 = 0

R i1 = R g1 + Rc1 + R m1 + R e1 + R11

-6

R d1 = 4.806 × 10 -6

R d1 = R h1 + Rs1 + Rw1

Total risk (R 1)

R 11 = 0

R 11 = (N 11 × Pi1 + N 12 × Pi2 + N 11p × P i1p + N 12p × P i2p ) × δ o1 × kh1

P i2p = 0.01

P i2p = n ugp × kw × k2 × k3 × k4 × k 5 × Pe0

P i2 = 0

P i1 = 0

P il = noh × k w × k 2 × k 3 × k 4 × k 5 × P el

P i2 = nug × k w × k2 × k3 × k4 × k 5 × P e2

P i1p = 0

P i1p = n ohp × kw × k2 × k3 × k4 × k 5 × Peo

Acceptable risk

Total risk for a structure due to lightning strikes

Risk due to indirect strikes to the structure


Risk of internal equipment failure due to an indirect strike near the service lines that causes a damaging overvoltage

Probability an indirect strike near other underground cable will cause a damaging overvoltage to internal equipment

Probability an indirect strike near the underground power cable line will cause a damaging overvoltage to internal equipment

Probability an indirect strike near other overhead line will cause a damaging overvoltage to internal equipment

Probability an indirect strike near the overhead power line will cause a damaging overvoltage to internal equipment

Risk of electrical/electronic equipment malfunction or failure due to overvoltages from indirect strikes to service lines

R e1 = k w × k 2 × k 3 × k 4 × (N c1 × P c1 + Nc2 × P c2 + N c2p × Pc2p) × δo1 × k h1

Risk of electrical/electronic equipment malfunction or failure due to overvoltages from direct strikes to service lines

Parameter

CALCULATIONS

129 AS/NZS 1768:2007

R f1 = 4.143 × 10 R o1 = 0

R f1 = R s1 + Rc1

R o1 = R w1 + R m1 + Re1 + R 11

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For DOD Internal Use ONLY Acceptable risk

R 12 = 0 R d2 = 0 R i2 = 0 R2 = 0 R a2 = 1 × 10 -3

R 12 = (N 11 × Pi1 + N 12 × Pi2 + N 11p × P i1p + N 12p × P i2p) × δ o2

R d2 = 0 + Rs2 + R w2

R i2 = R c2 + R m2 + Re2 + R 12

R 2 = Ri2 + R d2



R e2 = 0

R e2 = k w × k 2 × k 3 × k 4 × (N c1 × P c1 + Nc2 × P c2 + N c1p × Pc1p + N c2p × P c2p) × δo2



Risk of internal equipment failure due to a indirect strike near the service lines that causes a damaging overvoltage


Risk of internal equipment failure due to a direct strike to ground near the structure that causes a damaging overvoltage

R m2 = 0

R m2 = N m × P m × δ o2


Risk of fire, explosion, mechanical damage and chemical release caused by internal equipment failure due to a direct strike to service lines which causes a damaging overvoltage

Risk of physical damage due to direct strikes to the structure

Risk due to failure of internal electrical and electronic equipment due to overvoltage

Risk due to fire, explosion, mechanical damage and chemical release

Risk of step and touch voltage

R w2 = 0

R c2 = 0

R c2 = k f × P f × (Nc1 × P c1 + N c2 × Pc2 + Nc1p × P c1p + N c2p × Pc2p) × δ f2

-6

R w2 = Nd × Pw × δ o2

R s2 = 0

R s2 = Nd × Ps × δf2

RISK CALCULATIONS FOR DAMAGE CATEGORY 2 – Loss of essential service to the public

R t1 = 6.63 × 10-7

Example value

R t1 = R h1 + R g1

Risk factors presented in different format

Parameter

CALCULATIONS

AS/NZS 1768:2007 130

Example value

Acceptable risk

R i3 = 0 R3 = 0 R a3 = 1 × 10 -3

R i3 = 0 + R c3 + 0

R 3 = Ri3 + R d3

R h4 = 0

R s4 = 2.486 × 10 -6 R w4 = 8.22× 10-6

R m4 = 1.938 × 10 -5

R g4 = 0

R h4 = Nd × Ph × δ h4

R s4 = Nd × Ps × δf4 × k h4

R w4 = Nd × Pw × δ o4 × kh4

R m4 = N m × P m × δ o4 × kh4

R g4 = P g × (N c1 × P c1 + Nc2 × Pc2 + N c1p × P c1p + N c2p × P c2p) × δ g4

RISK CALCULATIONS FOR DAMAGE CATEGORY 4 – Economic loss



R d3 = 0

R d3 = 0 + Rs3 + 0

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Risk of dangerous step and touch potential to animals outside the structure due to direct strikes to the structure

Risk of internal equipment failure due to a direct strike to ground near the structure that causes a damaging overvoltage



Risk of dangerous step and touch potential to animals outside the structure due to direct strikes to the structure



Risk of fire, explosion, mechanical damage and chemical release caused by internal equipment failure due to a direct strike to service lines that causes a damaging overvoltage

R c3 = 0

R c3 = k f × P f × (Nc1 × P c1 + N c2 × Pc2 + Nc1p × P c1p + N c2p × Pc2p) × δ f3


R s3 = 0

R s3 = Nd × Ps × δf3

RISK CALCULATIONS FOR DAMAGE CATEGORY 3 – Loss of cultural heritage

Parameter

CALCULATIONS

131 AS/NZS 1768:2007

R e4 = 0

R 14 = 1.50 × 10-6

R d4 = 1.070 × 10 -5 R i4 = 2.088 × 10 R 4 = 3.158 × 10

R e4 = k w × k 2 × k 3 × k 4 × (N c1 × P c1 + Nc2 × P c2 + N c1p × Pc1p + N c2p × P c2p) × δo4 × k h4

R 14 = (N 11 × Pi1 + N 12 × Pi2 + N 11p × P i1p + N 12p × Pi2p ) × δ o4 × k h4

R d4 = R h4 + Rs4 + Rw4

R i4 = R g4 + R m4 + R c4 + R e4 + R14

R 4 = Ri4 + R d4 -5

R d = 1.551 × 10 -5 R i = 2.088 × 10 -5

R d = Rd1 + R d2 + R d3 + Rd4

R i = Ri1 + R i2 + R i3 + Ri4


R a4 = 1 × 10 -3

R c4 = 0

R c4 = k f × P f × (Nc1 × P c1 + N c2 × Pc2 + Nc1p × P c1p + N c2p × Pc2p) × δ f4 × k h4

-5

Example value

Parameter

CALCULATIONS

Acceptable risk




Risk of internal equipment failure due to an indirect strike near the service lines that causes a damaging overvoltage


Risk of fire, explosion, mechanical damage and chemical release caused by internal equipment failure due to a direct strike to service lines that causes a damaging overvoltage

AS/NZS 1768:2007 132

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APPENDIX B

THE NATURE OF LIGHTNING AND THE PRINCIPLES OF LIGHTNING PROTECTION (Informative) B1 SCOPE OF APPENDIX This Appendix deals with the nature of the phenomena involved in a study of lightning protection and the basic principles of designing such protection. A brief description of various elements of an LPS and their function is also provided. Recommendations for systems to protect against the direct or indirect effect of lightning are given in the body of this Standard. B2 THE NATURE OF LIGHTNING B2.1 Nature of lightning Thunderstorms occur under particular meteorological conditions, and partial separation of electrical charges within the thundercloud usually results in regions with net negative charge mainly in the lower parts of the thundercloud, and regions with net positive charge mainly in the upper part. Lightning is an electrical discharge between differently charged regions within the cloud (cloud flash) or between a charged region, nearly always the lower negatively charged region, and earth (ground flash). A complete ground flash consists of a sequence of one or more high amplitude short duration current impulses, or strokes. Significant numbers (about 40%) of ground flashes have more than one ground termination, usually separated by distances up to a few kilometres (Paragraph G2, Ref. 8). In some ground flashes low amplitude long duration currents (sometimes termed continuing currents) flow between the strokes or after a sequence of strokes. The currents are unidirectional and usually negative, i.e. a negative charge is injected into the object struck. For all practical purposes the stroke can be considered to be generated by a current source whose waveshape and magnitude are unaffected by the characteristics of the ground termination. B2.2 The lightning attachment process The first stroke of a ground flash is normally preceded by a downward-progressing low-current leader discharge that commences in the negatively charged region of the cloud and progresses towards the earth, depositing negative charges in the air surrounding the leader discharge channel. When the lower end of the leader is roughly 100 m from the earth, electrical discharges (streamers) are likely to be initiated at protruding earthed objects, and to propagate upwards towards the leader discharge channel. Several streamers may start, but usually only one is successful in reaching the downcoming leader. The high current phase (return stroke) commences at the moment the upward moving streamer meets the downcoming leader. The position in space of the lower portion of the lightning discharge channel is therefore determined by the path of the successful streamer, i.e. the one that succeeded in reaching the downcoming leader. The primary task in protecting a structure is therefore to ensure a high probability that the successful streamer originates from the lightning protection conductors, and not from a part of the structure that would be adversely affected by the lightning current that subsequently flows.

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AS/NZS 1768:2007

134

As the path of the successful streamer may have a large horizontal component, e.g. many tens of metres, as well as a vertical component, an elevated earthed conductor will provide protection for objects spread out below it. It is therefore possible to provide protection for a large volume with a relatively small number of correctly positioned conductors. This is the basis for the concept of a zone of protection provided by an elevated earthed conductor, and provides the basic principle underlying interception lightning protection. Thus the basic protection system consists of air terminals to provide launching points for streamers, and downconductors and earthing electrodes to deliver the lightning current into the earth. B2.3 Thunderstorm and lightning occurrence Thunderstorm occurrence at a particular location is usually expressed in terms of the number of calendar days in a year when thunder was heard at the location, averaged over several years. The resulting information is usually given as an average annual thunderday map. The frequency of occurrence of thunderstorms varies significantly depending on location. Moreover, the severity of lightning storms, as distinct from their frequency of occurrence, is known to be much greater in some areas than in others. Hence, the need for protection varies across the country, although not necessarily in direct proportion to thunderstorm frequency. A few severe thunderstorms in a season may make the need for protection greater than a relatively large number of storms of lesser activity. Therefore, lightning ground flash density is a better measure of lightning activity in a particular location. Data on the geographical distributions of average ground flash density are given in Section 2— (a)

in Figure 2.3 for Australia; and

(b)

in Figure 2.4 for New Zealand.

Earlier editions of AS/NZS 1768 provided Thunderday maps, and ground flash density (N g) could be estimated from thunderdays (Td) using an equation of the form— N g = a Td b

. . .B2.3

in which a and b are empirically determined constants, with values of a = 0.012 and b = 1.4 appropriate for Australia (Paragraph G2, Ref. 9); the so called CIGRE formula adopted by IEC has a = 0.04 and b = 1.25, and generally gives higher values for N g . Local topographical features may cause variations in the occurrence of ground flashes. The occurrence will be higher than the average on high ground, e.g. ridges, and lower than average on nearby low ground. In some cases, a large topographical feature such as a high mountain may interact with prevailing meteorological conditions to cause a concentration of thunderstorms and ground flashes. Such effects may be identified by enquiry of local telecommunications and electricity supply engineers or meteorological stations, and of local residents. On a smaller scale, tall objects, e.g. roof of a building, tree top or overhead conductor, tend to divert lightning flashes to themselves, as explained in Paragraph B2.2, thus shielding a certain surrounding area from direct strikes. There is a network of lightning flash counters (LFC) in Australia. The available count data enable estimates to be made of ground flash density at the LFC sites, but these require corrections for site and calibration errors and for response to nearby cloud flashes. Recently lightning positioning systems have been installed in Australia and New Zealand and data are available from the commercial providers. Earlier systems have been in operation in central-east New South Wales and south-east Queensland for about twenty years, but their data are not extensive and may not be reliable. Optical observations of lightning flashes are made from satellites fitted with appropriate sensors. Considerable data on worldwide lightning activity are now available from U.S. satellites which usually survey the earth from near-polar orbits. Worldwide data and maps since April 1995 of recorded lightning flashes are readily accessible on a NASA website (Paragraph G2, Ref. 10). COPYRIGHT


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In Australia, the geographical distribution of lightning incidence is described by a map of average annual lightning ground flash density, Ng (Paragraph G2, Ref. 11). The map has been derived using lightning data obtained by ground-based lightning detection instruments such as CIGRE-500 and CGR3 and by NASA satellite-based instruments such as Optical Transient Detector and Lighting Imaging Sensor. This map is contoured in units of flash density (km −2 yr −1). N g varies from over 6 km −2 yr −1 in the northern parts of Australia to about 1 km −2 yr−1 and below in the southern parts. The map is recommended for estimating N g values for lightning protection system design. The overall uncertainty in the flash density values indicated on the Australian flash density map is estimated to be ±30%. The New Zealand N g map has been derived from ground flash density data obtained from the Lightning Detection Network of New Zealand for the period 1 January 2001 through to 9 February 2006. B3 EFFECTS OF LIGHTNING The principal effects of a lightning discharge to an object are electrical, thermal and mechanical. These effects are determined by the magnitude and waveshape of the current discharged into the object and the nature of the object itself. Statistical distributions of some characteristics of ground flashes are given in Table B1. When the lightning current flows through the building or LPS, the electrical potential of the building may rise to a high, usually negative, value with respect to remote earth. The lightning current produces a high potential gradient around the earthing electrode that can be dangerous to persons and to livestock. The rate of rise of current in conjunction with inductance of the discharge path produces a voltage drop that will vary in time depending upon the current waveshape; because of the fast rate of rise of the lightning discharge current, the inductance of conductors assumes a far greater importance in determining potential rise than does the resistance of conductors. The voltage across an inductor is given by the expression V = L × di/dt. Assuming an approximate inductance of 1µH/m for a typical conductor and a typical rate of rise of current of 25 kA/µs, the voltage developed across a 40 m length of 25 × 3 mm copper strap is 1 MV, compared to just 770 V due to its resistance. As the point of strike on the LPS may be raised to a high potential, there is also the risk of a flashover from the LPS to nearby objects. This is called a side-flash. The risk of side-flash is increased at any deeply re-entrant bend or loop in a downconductor due to the local increase in inductance. If such a flashover occurred, part of the lightning current would be discharged through internal installations with consequent risk to the occupants and the fabric of the building. The amount of energy deposited in any object carrying lightning current may be calculated by multiplying the action integral by the electrical resistance of the object. From this, the temperature rise may be calculated. It should be noted however, that the resistance of most objects other than metallic conductors, e.g. wood, masonry or earth, is very non-linear for the large currents associated with lightning. It should also be noted that the passage of lightning current through moist resistive materials such as masonry or wood can convert the moisture to high-pressure steam, causing the material to explode or shatter. The thermal effect of a lightning discharge is confined to the temperature rise of the conductor through which the lightning current is discharged. Although the amplitude of a lightning current may be high, its duration is so short that the thermal effect on an LPS, or on the metallic parts of a structure where this is included in the LPS, is usually negligible. This ignores the fusing or welding effects that occur locally consequent upon the rupture of a conductor that was previously damaged or was of inadequate cross-section. In practice the cross-sectional area of a normal lightning conductor is determined primarily by mechanical and secondarily by thermal considerations.

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At the point of attachment of a lightning discharge channel to a thin metal surface, a hole may be melted in the surface. In the rare case where this occurs, some thermal energy will be deposited directly in the metal from the hot plasma of the lightning discharge channel, as well as the thermal energy caused by the passage of current through the metal. The size of the hole melted in the sheet depends on the material, the thickness of the sheet, and the charge delivered. For example, a moderately severe lightning flash delivering a charge of 70 C would melt a hole about 12 mm in diameter in a sheet of roofing iron.

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TABLE B1 SUMMARY OF THE FREQUENCY DISTRIBUTIONS OF THE MAIN CHARACTERISTICS OF THE LIGHTNING FLASH TO GROUND Item No

Lightning characteristic

Percentage of events having value of characteristic greater than value shown below (see Note 1) 99

90

75

50

25

10

1

Unit

1

Number of common strokes

1

1

2

3

5

7

12

—

2

Time interval between strokes

10

25

35

55

90

150

400

ms

3

First stroke peak current I max.

5

12

20

30

50

80

130

kA

4

Subsequent stroke peak current I max.

3

6

10

15

20

30

40

kA

5

First stroke (di/dt) max.

6

10

15

25

30

40

70

kA/µs

6

Subsequent stroke (di/dt) max.

6

15

25

45

80

100

200

kA/µs

7

Total charge

1

3

6

15

40

70

200

C

8

Continuing current charge

6

10

20

30

40

70

100

C

9

Continuing current I max.

30

50

80

100

150

200

400

A

10

Overall duration of flash

50

100

250

400

900

1 500

11

Action integral (see Note 2)

10

2

3 × 10

2

10

3

5 × 10

600 3

3 × 10

4

10

5

5 × 10

ms 5

A 2 .s

NOTES: 1

The values shown in this table have been derived from a number of sources, and have been rounded in accordance with the accuracy with which these data are known. Values at the 1 percent and 99 percent levels are very uncertain, and are given only to indicate an order of magnitude.

2

The action integral, defined as ∫ i 2dt for the whole flash, is equivalent to the energy deposited in a oneohm resistor by the passage of the entire current for the duration of the flash.

The passage of lightning current through a conductor causes a force on the conductor given by the equation: F=B×l×i

. . . B3

where F = the force on the conductor, in newtons (N) B = the component of the magnetic flux density at right angles to the conductor, in teslas (T) l = the length of the conductor, in metres (m) i = the current through the conductor, in amperes (A) B4 POTENTIAL DIFFERENCES CAUSED BY LIGHTNING B4.1 General A lightning flash to a building or structure, or a flash to ground near a building or structure will cause a potential rise in the vicinity of the strike attachment point, and may also cause a potential rise of objects remote from the point of strike. For example, a lightning strike to a service conductor (electricity supply, telecommunications, or other metallic service) can cause current to be transmitted to the building, thus raising the potential of the building. A lightning flash to ground can also induce voltages and currents in remote conductors by electric and magnetic coupling (see also Section 5 and Appendix F). COPYRIGHT


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B4.2 Earth currents At the point where the lightning current enters the ground the current density is high. Hazardous earth potential gradients may be generated. Earthing electrodes should be distributed more or less symmetrically, preferably outside and around the circumference of a structure, rather than be grouped on one side. This will help to minimize earth potential gradients near the building, and tend to cause the lightning current to flow away from the building rather than underneath it. In addition, with earth connections properly distributed, the current from a lightning flash to ground near the building will be concentrated at the outer extremities. Thus current flow underneath the building, as well as earth potential gradients, will be minimized. B4.3 Side-flash If a lightning conductor system is placed on a building and there are unbonded metal objects of considerable size nearby, there will be a tendency for side-flashing to occur between the conductors of the LPS and the unbonded metal objects. To prevent damage from side-flash, interconnecting conductors should be provided at all places where sideflashes are likely to occur. This is referred to as equipotential bonding, although complete equalization of potential is never achieved. As the currents required to equalize potentials are considerably less than the full lightning current, conductors of relatively small cross-section are adequate for this purpose (see also Clause 4.16.2). B4.4 Potential (voltage) differences The impedance of the earth termination network to the rapidly changing lightning current influences the potential rise of the LPS. This in turn affects the risk both of side-flashing within the structure to be protected, and of dangerous potential gradients in the ground adjacent to the earth termination network. The potential gradient around the earth termination network, on the other hand, depends on the physical arrangement of the earthing electrodes and the soil resistivity. In Figure B1, a lightning flash is assumed to occur to the LPS of a building. For the purposes of the illustration, no equipotential bonding is shown although such bonding is required in accordance with this Standard. As the lightning current is discharged through the downconductor and the earthing electrode, the conductor system and the surrounding soil are raised, for the duration of the discharge, to a potential with respect to the general mass of the earth. The resulting potential differences as shown by ‘step’, ‘touch’ and ‘transferred’ potentials in Figure B1 may be lethal; hence the importance of keeping the impedance of the earth termination network low, and of preventing large local potential gradients by equipotential bonding, and by the manner in which the earthing electrodes are arranged.

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NOTES: 1

Person X is in contact with the ground at a and b: person Y is in contact with the ground at c and the conductor at d; person Z is in contact with the conductor at e and a metallic handrail f shown earthed at g.

2

Person X is subject to ‘step’ potential.

3

Person Y is subject to ‘touch’ potential.

4

Person Z is subject to ‘transferred’ potential.

5

The potential depends on the current magnitude and the impedance of the path of the lightning discharge.

6

Step potential increases with the size of the step a-b in the radial direction from the conductor and decreases with the increase in the distance between person X and the conductor.

7

The transferred potential increases with increase in the radial distance between the downconductor and the ground at g.

8

The diagram does not show equipotential bonding that may be necessary to protect persons from hazardous potential differences of the type described in this diagram (see Sections 4 and 5).

FIGURE B1 INSTANTANEOUS POTENTIAL DIFFERENCES DURING A LIGHTNING FLASH TO AN EARTHED CONDUCTOR

B5 PRINCIPLES OF LIGHTNING PROTECTION B5.1 Purpose of protection The purpose of lightning protection is to protect persons, buildings and their contents, or structures in general, from the effects of lightning, there being no evidence for believing that any form of protection can prevent lightning strikes. COPYRIGHT


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B5.2 Interception of lightning The function of an air terminal in an LPS is to divert to itself the lightning discharge that might otherwise strike a vulnerable part of the object to be protected. It is generally accepted that the range over which an air terminal can attract a lightning discharge is not constant, but increases with the severity of the discharge. The path of a lightning discharge near a structure is determined by the path of the successful streamer (see Paragraph B2.2) that will usually be initiated from a conducting part of the structure nearest to the downcoming leader. The initiation of streamers is also influenced by the local electric field. The upper outer edges and corners of buildings or structures, and especially protruding parts, are likely to have higher local electric fields than elsewhere, and are therefore likely places for the initiation of streamers. When the downcoming leader is within about 200 m of the building, the electric field at these protruding parts and corners will exceed the breakdown field strength of air, resulting in corona currents that cause these parts to be surrounded by ionized air. The resulting space charges influence the electric field in such a manner that the field is limited to the breakdown strength of air. However, these complicating factors do not alter the fact that the most probable strike attachment point on a building is the edge, corner, or other protruding part closest to the downcoming leader. This is the basic reason why the RSM gives a reliable guide to the most probable strike attachment points. Hence, if air terminals are placed at all locations where high electric fields and streamer initiation are likely, there will be a high probability that the discharge will terminate on some portion of the LPS . B5.3 Determination of lightning strike attachment points to buildings B5.3.1 The rolling sphere method The procedure for determining lightning strike attachment points is based on the RSM whereby a sphere of specified radius is imagined to be rolled across the ground towards the building, up the side, and over the top of the building, and down the other side to ground. This can be carried out in various orientations with respect to the building. Any point on the building touched by the sphere is a possible lightning strike attachment point. The physical basis for this method is as follows. As the lightning leader stroke approaches the ground, the electric field at various salient points, such as the upper corners of buildings, will help to launch electrical discharges, or streamers, that progress upwards toward the tip of the downcoming leader stroke. The position in space of the lightning discharge channel, and the location of the strike attachment point, is determined when the leader and one of the upward streamers join to complete the lightning discharge path. The upward streamer that determines the strike attachment point is generally that launched from the salient point or earthed conductor closest to the downcoming leader. The rolling sphere will tend to touch those salient points, and the method therefore provides a geometric means of identifying such points. B5.3.2 The striking distance The striking distance, ds, is the distance between the leader tip and the eventual strike attachment point at the moment when it has become inevitable that the gap, of dimension ds , will be bridged by the lightning discharge channel. The RSM is closely related to the electrogeometric method developed for predicting lightning attachment to electricity supply service lines, whereby the lightning leader is supposed to progress until it comes within the distance ds of an earthed object, when the final discharge path is determined to that object. There are theoretical and observational grounds for a relationship between d s and the i max. , where i max. is the peak return stroke current. The following relationship has been proposed: COPYRIGHT


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ds

=

10 × imax.0.65

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. . . B5.3.2

where ds

=

the striking distance, in metres (m)

i max.

=

the peak current of the return stroke, in kiloamperes (kA)

The advantage of the RSM is that it is relatively easy to apply, even to buildings of complicated shape. The limitation of the method is that no account is taken of the influence of electric fields in initiating return streamers, and the method therefore does not distinguish between likely and unlikely lightning strike attachment points. In particular, the enhancement of electric fields at the upper outer corners of a building makes these corners the most probable strike attachment points, whereas return streamers are unlikely to be initiated from a flat surface away from a corner or edge, even if on the roof and touched by the sphere. Therefore, in Clause 4.4.2, an increased radius of the rolling sphere was introduced for dealing with the protection of plane surfaces. Some qualitative indication of the probability of strike attachment to any particular point can be obtained if the sphere is supposed to be rolled over the building in such a manner that its centre moves at constant speed. Then the length of time that the sphere dwells on any point of the building gives a qualitative indication of the probability of that point being struck. Thus for a simple rectangular building with a flat roof, the dwell time would be large at the corners and edges, and small at any point on the flat part of the roof, correctly indicating a high probability of the corners or edges being struck, and a low probability that a point on the flat part of the roof will be struck. The RSM with its modification of an increased radius for plane surfaces is now applied with some account of electric field enhancement effects in mind, so that high priority is given to providing air terminals at the more probable attachment points. For a building of more or less rectangular shape with a flat roof, this means giving top priority to providing air terminals around the periphery of the roof. This could take the form, for example, of a metallic perimeter handrail. B5.4 Protection of the sides of tall buildings When the RSM is applied to a building of height greater than the selected sphere radius, then the sphere touches the vertical edges on the sides of the building at all points above a height equal to the sphere radius and a sphere of the corresponding increased sphere radius touches all flat surfaces on the sides about a height equal to the increased radius. This indicates the possibility of strikes to the sides of the building, and raises the question of the need for an air terminal network on the sides of the building. Practical experience indicates that strikes to vertical edges on the sides of tall buildings do occur but are uncommon in that strikes rarely occur to the flat side surfaces. There are theoretical reasons for believing that only flashes with low i max. and consequently low ds values are likely to be able to penetrate below the level of the roof of the building and strike the sides. The consequences of a strike to the sides of a building may result in damage of a minor nature. Unless there are specific reasons for side protection, as would be the case for a structure containing explosives, it is considered that the cost of side protection would not normally be justified. If it is decided that some protection for the sides of a building is justified, then conductors should be provided at the most probable lightning attachment points on the sides of the building. The most probable attachment points are at protruding corners and vertical edges of the sides of the building, including surfaces with changes greater than 20°. The conductors will generally serve both as air terminals and downconductors and will in general be connected to the roof air terminal network at their upper ends, and to the earthing termination network at their lower ends. The conductors may be made flush with the surface, and should be placed as near as practicable to the vertical edge to be protected. COPYRIGHT


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Where the building construction includes extensive metal objects on the vertical outer surfaces, such as large metallic window frames, then such objects can form part of the interception protection system. It is necessary to provide electrical connections between adjacent metal objects both in the horizontal and vertical directions, and to provide periodic connections between the surface metalwork and the reinforcing steel, or the downconductors if separate from the reinforcing. This provides multiple paths for the lightning current from any point on the surface metalwork to earth, and local potential differences will be reduced to an acceptable level. Return streamers are more likely to develop from a good conductor on the surface than from a poor conductor in a similar position. Thus if a wall consists mainly of a poorly conducting material, and there are isolated objects with earth connections made of a highly conductive material distributed over the surface, then the return streamers will tend to originate preferentially from the highly conducting objects with earth connections. A wall of poorly conducting material can therefore be substantially protected by earthed metallic studs placed on a grid pattern flush with the surface of the wall. Alternatively, protection can be provided by a system of metallic strips flush with the surface. Even if the lightning happens to strike a point on the non-conducting surface away from an earthed conducting point, it is likely that it will track across the non-conducting surface and terminate on the earthed conducting point. B5.5 Safe discharge to earth B5.5.1 General If the air terminal network is adequately connected to earth, the current will pass to ground without damage to the structure. Metallic parts of a building or structure may usefully be made part of the LPS, provided that the passage of lightning current will not cause harm (for bonding of metal in or on a structure, see Clause 4.16.2.2). B5.5.2 Use of reinforcing steel as a downconductor It is sometimes suggested that overlapped and tied reinforcing rods do not provide good electrical connections, and are therefore not suitable for carrying lightning currents. However, the situation differs greatly from that in which a conducting path for power currents is required. Even if there are thin films of iron oxides and cement between the bars, the voltage required to cause breakdown of these films would be less than 1000 V. Once breakdown has occurred, there would be localized arcing between the steel bars, with a voltage drop of a few tens of volts. The initial breakdown across the oxide and cement films would occur during the first few microseconds of the first stroke when there is a large inductive voltage drop from top to bottom of the building; this voltage would be very much larger than the voltage required to break down the oxide and cement films between bars. Thus there are good reasons for relying on the reinforcing bars to act as downconductors, even when no special precautions have been taken (such as welding the bars together) to ensure electrical continuity. The localized arcing referred to above would produce relatively small amounts of energy in relation to the thermal capacity of typical reinforcing bars, so heating effects should be negligible. Where the structural steel reinforcement of the building is to be utilized as the downconductor system, it is important that there be an effective electrical connection between the air terminal network and the steel reinforcement. Such connections should be made as close as practicable to the top of the building and preferably at a number of points around the building perimeter. Tall metal structures, such as chimneys, provide an adequate conducting path, but care must be taken to ensure that they are also suitably earthed. Special precautions are needed for the protection of structures containing explosives, highly-flammable materials and gases. The principles involved in such protection systems are given in Section 7. COPYRIGHT


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B5.6 Potential equalization As explained in Paragraph B4, lightning strikes may give rise to harmful potential differences within a building. Of particular concern is the occurrence of potential differences that may exist between local earth and incoming conductors such as metallic water services, telecommunication systems, and electricity supply systems. Reduction of these potential differences may be achieved by a system of coordinated bonding of all affected conductors contained in the building. This includes all incoming metallic services, protective earths associated with electricity supply and telecommunications systems, and the building lightning protection earth termination network (if provided). Potential equalization (understood to imply approximate potential equalization) may be effected by including in the bonding scheme earthed building metalwork such as reinforcement metals and metal framework, if any. In cases where the presence of dissimilar metals may create corrosion problems or for other reasons, the commoning path may be effected by using suitably rated SPDs. B6 ELEMENTS OF A PROTECTION SYSTEM The main parts of a typical LPS for a building or structure may be summarized as follows, noting that not all parts will be present in all systems: (a)

Air terminals are placed so as to achieve interception lightning protection, ensuring a high probability that lightning will attach to the air terminal network, and not to parts of the protected object that could be damaged by lightning current. Existing metalwork should be used as far as possible, supplemented by carefully positioned air terminals giving priority to high probability attachment points. These are the upper outer corners and edges of the building and any salient or protruding objects on the roof. The form of air terminal should be chosen for simplicity and low cost consistent with adequate mechanical strength, durability and aesthetic acceptability.

(b)

Downconductors are used to convey lightning current towards the earth. Existing building metalwork should be used as far as possible, especially steel frames and reinforcing steel in reinforced concrete columns, supplemented where necessary by external downconductors. If these downconductors are also to serve as part of the air terminal network for the sides of a tall building, they should preferably follow the outer vertical corners of the building. Where the number of downconductors required exceeds the number of vertical corners, the remaining downconductors should be placed uniformly between the ones at the corners.

(c)

Test links may be required between the downconductors and the earthing electrodes to facilitate the testing of the LPS.

(d)

The earth termination network consisting of one or more earthing electrodes, and any interconnecting conductors between earthing electrodes, serves the purpose of delivering the lightning current into the general mass of the earth. The footings of large reinforced concrete buildings will generally provide a better earth connection than can be provided by driven earthing electrodes around the periphery. Where the superficial layers of the earth have high resistivity, deep driven earthing electrodes may be needed to reach low-resistivity regions, and achieve an acceptable earth resistance.

(e)

Equipotential bonding is used to reduce or prevent hazardous potential differences between any pair of extended conducting objects in the building or structure. Equipotential bonding becomes particularly important in buildings or structures having a high earth resistance. In the extreme situation where an acceptable connection to earth cannot be achieved, it would be necessary to rely entirely on equipotential bonding to protect persons and equipment against hazardous potential differences caused by lightning. COPYRIGHT


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(f)

Equipotential bonding may, in some situations, be achieved by means of SPDs, where direct connection of the conducting parts results in an unwanted effect, for example, corrosion of metals, degradation or loss of communications.

(g)

Overvoltage protection is achieved by using various types of SPDs (e.g. spark gaps, gas-filled surge arrestors or MOVs) to prevent hazardous potential differences being applied to persons or equipment, while allowing correct operating potentials to exist (see Section 5 and Appendix F).

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APPENDIX C

NOTES ON EARTHING ELECTRODES AND MEASUREMENT OF EARTH IMPEDANCE (Informative) C1 GENERAL C1.1 Function of an earthing electrode The function of an earthing electrode is to provide an electrical connection to the general mass of earth. The characteristic primarily determining the effectiveness of an earthing electrode or group of interconnected earthing electrodes (earth termination network) is the impedance that it provides between the earth termination network and the general mass of earth. C1.2 Factors influencing earth impedance The impedance of the earth termination network to lightning currents varies with time and the magnitude of the current, and is dependent on— (a)

the resistance and surge impedance of the earthing electrode and the connecting conductors;

(b)

the contact resistance between the earthing electrode and the surrounding soil;

(c)

the resistivity of the soil surrounding the earthing electrode; and

(d)

the degree of soil ionization.

The resistance of the metallic conductors in the earth termination network can generally be neglected. In addition there are often fortuitous paths to earth, e.g. via bonded electricity reticulation low-voltage neutrals. These can mask the earthing electrode impedance by paralleling other routes of high surge impedance but low d.c. or low-frequency impedance to earth. It is essential to utilize measurement techniques, referred to later, to discriminate between these conditions. C1.3 Measures for reducing earth impedance Lightning current is considered to be a high frequency phenomenon with current rise times in the order of 10 kiloamperes per microsecond (10 kA/µs). In these circumstances, an earth termination network can best be regarded as a ‘leaky’ transmission line. Each conductor has resistance, inductance and capacitance to earth and leakage through non-insulated contact. An examination of earthing conductors using transmission line equations will show that the impedance of the earth termination network is lowered by the following: (a)

The use of flat strip rather than circular conductors. This increases surface area, reduces high-frequency resistance due to skin effect, increases both capacitive coupling and the earth contact area for a given cross-section of conductor.

(b)

The use of a centre point feed to create the effect of two parallel connected transmission lines is also effective. This concept can be further enhanced by using several radial conductors emanating from the injection point.

(c)

The use of short-length multiple conductors for example up to 30 m, is preferred over long buried systems.

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The use of ground enhancing compound around the conductor can reduce the impedance of the earth electrode (see Paragraph G2, Ref. 12). The reduction in impedance is achieved by greatly increasing the effective surface area and capacitive coupling of the conductor to the soil.

In areas of low to moderate soil resistivity, vertical earthing electrodes will, for an earthing electrode of given dimensions, usually be more effective in providing a low surge impedance. When trench (horizontal) earthing electrodes are installed, the initial surge impedance of two or more electrically paralleled wires or strips, radiating symmetrically from a central connection point, will be less than the equivalent length laid as one single unit. However, the multiped earthing electrode will be of higher d.c. or low-frequency resistance due to electric field interaction between the individual earthing electrode segments near the central connection point. The optimum surge performance for a single horizontal earthing electrode will usually be achieved when the downconductor attaches to its midpoint. The contact resistance between the earthing electrode and the soil can be up to about 10 percent of the total resistance of the earth termination network. This resistance may be reduced by ionization and arc-over in the soil in contact with the earthing electrode. The major part of the earth resistance of an earthing electrode arises from the resistance of the earth in the immediate vicinity of the earthing electrode. The value of this resistance depends upon the shape, size, and position of the earthing electrode and the resistivity, moisture content and degree of ionization of the soil in the vicinity of the earthing electrode. The ratio of resistance at peak impulse current to resistance at low current depends on the number and arrangement of the electrodes, the peak current and soil resistivity. Examples are given in Table C1. C2 RESISTIVITY OF SOIL C2.1 General Soil resistivity is another term for the specific resistance of soil. It is usually expressed in ohm metres (symbol Ω.m), i.e. the resistance in ohms between opposite faces of a cube of soil having sides 1 m long. The resistivity of the soil depends on its chemical and mechanical composition, moisture content and temperature. In view of this there is a very large variation in resistivity between different types of soils and with different moisture contents. This is illustrated in Tables C2, C3 and C4. NOTE: Earthing electrodes should not be located near brick kilns or other installations where the soil can be dried out by the operating temperatures involved.

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TABLE C1 EXAMPLES OF REDUCTION OF RESISTANCE OF EARTH CONNECTION UNDER IMPULSE CONDITIONS Soil resistance characteristic

No. of rods and arrangement Four rods at corners of square, 3.05 m sides

One isolated rod 10 2

10 3

Resistance at low currents, Ω

30

300

10.5

105

Resistance at current peak, Ω

11.3

54

6.8

37

Soil resistivity, Ω.m

Ratio of resistance at current peak/resistance at low current

0.38

0.18

10 2

0.65

NOTE: The table depends on the following earthing dimensions and conditions: Diameter of rods = 10 mm Depth in earth = 3.05 m Peak current injected = 80 kA Time to current crest = 4 µs

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TABLE C2 RESISTIVITY VALUES FOR VARIOUS MATERIALS Resistivity, Ω.m

Material Typical

Usual limits

Salt sea water

0.2

0.15 to

0.25

Estuarine water

0.5

0.2

to

5

Artesian water

4

2

to

12

Damp black inland soil (see Note 1)

8

5

to

100

Damp clay

10

2

to

12

Inland lake water, reservoirs

20

10

to

500

River banks, alluvium

25

10

to

100

Clay/sand mixture (see Note 2)

30

20

to

200

River water (upstream)

40

30

to

200

Concrete (see Note 3)

100

40

to

1000

Dry inland soil (see Note 1)

100

20

to

1000

Moraine gravel

2000

1000

to

10000

Coal

2000

1000

to

5000

Secondary rock

3000

1000

to

50000

Sand (see Note 2)

3000

1000

to

10000

20000

10000

to

50000

100000

10000

to

100000

Solid volcanic rock (see Note 4) Ice (see Note 5) NOTES: 1

‘Black soil’ is a non-specific term applicable to vast areas of Queensland and New South Wales. The soil is characterized by a high level of dissolved salts, and undergoes considerable contraction on drying out, thus causing a significant increase in volume resistivity when dry.

2

Resistivity values for a clay/sand mixture and for sand are based on measurements of a number of sites in Queensland. The resistivity of dry sand is intrinsically very high and it will serve to increase the resistivity of any material in which it may be interspersed.

3

Values of resistivity for concrete apply to the cast material and do not include the effect of any reinforcing bars. The values given will assist in determining the discharge resistance from steel reinforcement to the general body of earth.

4

Solid volcanic rock is often subject to fissures and faults the contents of which substantially reduce the resistivity, though not to a very satisfactory level for earthing electrode performance for lightning protection.

5

Ice is included for reference. It is not anticipated that ice will be of significance in design for the normal range of conditions.

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TABLE C3 VARIATION OF SOIL RESISTIVITY WITH MOISTURE CONTENT Moisture content (percent by weight)

Typical value of resistivity Ω.m Clay mixed with sand

0

Sand

10000000

—

1500

3000000

5

430

50000

10

185

2100

15

105

630

20

63

290

30

42

—

2.5

TABLE C4 VARIATION OF RESISTIVITY WITH TEMPERATURE IN A MIXTURE OF SAND AND CLAY WITH A MOISTURE CONTENT OF ABOUT 15 PERCENT BY WEIGHT Temperature

Typical value of resistivity

°C

Ω.m

20 10 0 0 −5 −15

(water) (ice)

72 99 138 300 790 3300

C2.2 Artificial reduction of soil resistivity Chemical additives can be used to reduce soil resistivity. These additives generally take the form of fully ionizable salts such as sulphates, chlorides or nitrates. Such chemical additives should not be used indiscriminately as— (a)

the benefit that they provide will lessen with time due to leaching through the soil; and

(b)

they may increase the rate of corrosion of the earthing electrode material.

Some of the chemical additives are also objectionable from an environmental viewpoint. A bland backfill material such as calcium or sodium bentonite clay, or montmorillonite with finely ground gypsum will reduce resistivity for a considerable period in high resistivity soils, maintain some moisture adjacent to the earth termination network, and provide a uniform and non-corrosive environment for the earthing electrodes. For further information see the recommendations in AS 2239 relating to the backfilling of galvanic anodes. C2.3 Determining soil resistivity by test It is fairly easy and useful to determine soil resistivity by test before commencing to install earthing electrodes. Testing procedures are given in Paragraph C10.1.

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C3 CALCULATION OF EARTH RESISTANCE OF AN EARTHING ELECTRODE If the soil resistivity is known (see Paragraph C2.3), the earth resistance R in ohms can be calculated as follows: (a)

Single vertical rod of length L and diameter d metres, top of rod level with surface: R

=

 ρ   8L  ln   −1   2πL   d  

. . . C3(1)

where R

=

resistance, in ohms

ρ

=

soil resistivity, in ohm metres

L

=

buried length of earthing electrode, in metres

d

=

diameter of earthing electrode, in metres

NOTE: Equation C3(1) is commonly referred to as the ‘modified Dwight formula’.

(b)

As above, but top of rod h metres below surface: R

(c)

 2h + L ρ   8L  ln   −1   2πL   d   4h + L

. . . C3(2)

Thin circular plate, diameter D metres, on surface: R

(d)

=

=

ρ 2D

. . . C3(3)

Thin circular plate, diameter D metres, buried h metres below surface: R

=

ρ  D   0.5 +  2D  4πh 

. . . C3(4)

For a vertical plate, h is measured from the centre of the plate. In the case of a square plate, the diameter can be replaced with 1.13 times the side of the square. (e)

Straight horizontal wire of length L and diameter d metres, on surface: R

=

 ρ   4L  ln   −1  πL   d  

. . . C3(5)

For a thin strip earthing electrode, the diameter can be replaced with a half-width of the strip. (f)

As for Item (e), but buried h metres below surface: R

(g)

=

  ρ   4L ln πL   (dh 12 )  

   − 1 

   

. . . C3(6)

Radial wires, number of wires n, on surface: R

=

 ρ   4L   − 1+ N (n)   ln  nπL   d  

. . . C3(7)

where m = n -1

N(n) =

∑

m =1

ln

1 + sin π m/n sin π m/n

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or for n

=

N(n) =

2

3

4

6

8

12

100

0.7

1.53

2.45

4.42

6.5

11.0

116

When the wires are buried at a depth of h metres then the diameter of the wire should be replaced by the equivalent diameter— d′ (h)

1 (dh ) 2

=

. . . C3(9)

Ring of wire, radius of ring r metres: R

16r ρ ln 2 4π r d

=

. . . C3(10)

or in terms of the circumference l R

ρ 8l ln 2πl πd

=

. . . C3(11)

When the wire is buried at a depth h, the diameter of the wire should be replaced by the equivalent diameter determined from Equation C3(9). NOTES: 1

The above equations assume that the longitudinal resistance of the earthing electrodes can be neglected. The resistance for an earth termination network, of such dimensions that the voltage drop along the earthing electrodes or buried conductors must be considered, may be obtained as follows: 1

1

R 2 R 2 R ′ = R  o  coth  o  R R

. . . C3(12)

= R when R o /R ≤ 0.4 1

= ( RRo ) 2 when R o /R ≥ 2 where R is the resistance calculated from the relevant equation and Ro is the longitudinal resistance of the total length of wire. 2

The above equations also assume that the earthing electrodes on the surface are buried to half their thickness or diameters; the lengths of rods and wires are much greater than their diameters; the thickness of the plates is much smaller than the plate diameters and the diameter smaller than the depth of burial; and that the angles between the radial wires are equal.

3

Because of interaction, the obtainable earth resistance of two or more radial earthing electrodes is higher than that for a single wire of the same length. The increase in resistance is approximately as follows: For two wires at right angles, energized at the joint, the earthing resistance is:

R+

3R 100

where R is the resistance of a single straight wire of the same total length and energized at one end. For a three-point star it is:

R+

6R 100

and for four, six and eight-point stars, all energized at the centre, the resistances are:

R+

12 R 100

,R+

42 R 100

and R +

65 R 100

respectively .

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The equations apply to direct current or power-frequency alternating current energization. For all practical purposes the resistance to a lightning surge of a typical structure LPS earth termination network discussed in this Standard can be considered as being somewhat lower than its direct current or power-frequency alternating current value.

C4 USE OF EARTHING ELECTRODES IN PARALLEL In situations where a desired earthing resistance cannot be achieved with one earthing electrode, a number of earthing electrodes may be used in parallel. The combined resistance of parallel earthing electrodes is a complex function of a number of factors, some of the more important being the number of earthing electrodes, their dimensions, the separation between the earthing electrodes, the soil resistivity and the configuration of the earthing electrodes. Where the desired resistance can be achieved with only a few additional earthing electrodes and if the separation between the earthing electrodes is greater than their lengths (see Note), then the resultant resistance may be calculated by using the ordinary equation for resistances in parallel. In other situations the combined resistance owing to the mutual interaction between the earthing electrodes, will be always higher than given by this equation. NOTE: For practical purposes, the separation between vertical earthing electrodes can be taken as twice the length of the earthing electrode.

For example, the combined resistance of two parallel earthing electrodes of diameter d separated by a distance s, which is small compared with the earthing electrode’s length L, is given by the following equation: =

 ρ   2L   ln  ′  −1  πL   a  

R

=

resistance, in ohms

ρ

=

soil resistivity, in ohm metres

L

=

buried length of earthing electrode, in metres

a′

=

equivalent radius of the earthing electrode at the surface, in metres, determined from the equation:

R

. . . C4(1)

where

1

a′

1

1

1

. . . C4(2)

[( dh) 2 ( ss ′ ) 2 ] 2 = [dhss ′ ] 4

=

where d

=

diameter of earthing electrodes, in metres

h

=

buried depth of earthing electrode, in metres

s

=

distance between two parallel earthing electrodes, in metres

s′

=

distance from one earthing electrode to the image of the other earthing electrode, in metres 1

1

NOTE: The term ( ss ′) 2 is the effective separation, and s′ = (4h 2 + s 2 ) 2 . An equation for radial conductors is given in Paragraph C3(g).

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C5 DRIVEN OR DRILLED EARTHING ELECTRODES C5.1 General The use of driven or drilled earthing electrodes combines economy of surface space with efficiency of performance, and accesses clays and other conductive layers at depth. In consequence, it is a preferred method of earthing electrode installation. C5.2 Safety The indiscriminate driving of earthing electrodes or drilling for their placement can lead to damage of other services and, in the case of electricity supply cables, to the creation of a significant hazard to the operator. As normally the drilled or driven rod will not be earthed to a low impedance system, it will usually remain live and dangerous if it contacts a live conductor. High pressure gas or hydrocarbon pipes also create a significant hazard, quite apart from service failure aspects. Consequently, appropriate searches for such services should be made before drilling or driving. C5.3 Installation The extent of the earthing electrode installation needed will depend on the variation in soil resistivity with depth, and the resistance to be achieved. Earthing electrodes may be driven directly into the ground or fitted into pre-drilled holes. In the latter case a bentonite/gypsum slurry or other drilling mud would normally be used as a permanent resistance and soil contact medium. C5.4 Materials for earthing electrodes Earthing electrodes should be made of metals not liable to be materially affected by corrosion. Clause 4.7.2 describes the considerations involved in selecting earthing electrode materials to minimize corrosion in service. C5.5 Earthing electrode diameter Although the resistance between the earth and the earthing electrode depends to a certain extent on the area of the earthing electrode in contact with the soil, a large earthing electrode of, say, 50 mm diameter does not decrease the resistance materially compared with earthing electrodes of 13 mm or 20 mm diameter, which need to be only slightly larger to achieve the same resistance. For a driven earthing electrode, provided the required current-carrying capacity is met, the minimum diameter will usually be determined by mechanical rather than electrical considerations. The usual practice is to select a diameter that will give enough strength to enable the earthing electrode to be driven into the soil of a particular location without bending or splitting. Large diameter earthing electrodes are more difficult to drive than small diameter earthing electrodes. For deep drilled earthing electrodes the size is selected in the light of available drill diameters, requirements for connections (if any) and economy. Strip earthing electrodes are commonly used. C5.6 Depth of installation The depth to which an earthing electrode is installed is usually the most important factor affecting its earth resistance, first because the area of soil contacted increases directly with the length of earthing electrode below the surface, and secondly because the soil resistivity usually decreases with depth.

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This is shown by the measurements plotted in Figure C1 for a number of sites. For curves 1 and 2 it was known, by tests, that the soil down to a depth of between 6 m and 9 m consisted of ballast, sand and gravel, below which was clay. The rapid reduction in resistance when the earthing electrode penetrated the latter is very marked. The mean resistivity up to a depth of 7 m in one case was 150 Ω.m, at 10 m the mean value for the whole depth was 20 Ω.m due to the low resistivity of the clay stratum. Similarly for curve 4, the transition from gravelly soil to clay at a depth of about 1.5 m is very effective. For curve 3, however, no such marked effect occurred although there is a gradual reduction in average resistivity with increase in depth, as can be seen by comparison with the dotted curves, which are calculated on the assumption of uniform resistivity.

FIGURE C1 CALCULATED AND MEASURED CURVES OF RESISTANCE OF 13 mm DIAMETER DRIVEN ROD EARTHING ELECTRODES

C5.7 Sleeving of exposed part of vertical earthing electrode Where side-flashing or step and touch potentials are a design problem on a driven earthing electrode, it is good practice to sleeve the upper part of the earthing electrode with a non-conducting pipe or heat shrink tubing of adequate weather resistance and electrical insulation properties. For example, this could take the form of 2 mm thickness of polyethylene or 4 mm thickness of PVC covering the upper 2 m to 3 m of the earthing electrode.

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C5.8 Comparison with other earthing electrode types The simplicity of driving an earthing electrode compared with making excavations for the burying of plates or strips is obvious. The problem of the adequate packing of the soil around the earthing electrode does not arise. The space occupied is small and, unlike plate earthing electrodes, connections may be above ground. Moreover, where the permanent moisture level or layer of low resistivity soil is available only at considerable depth below the surface, earthing electrodes may be driven to depths that would be far beyond that which would be practicable or economical for buried plate earthing electrodes. However, where soil resistivity increases with depth, there is no point in driving an earthing electrode any deeper as better results may be obtained by connecting a number of earthing electrodes in parallel or by using a buried strip earthing electrode. C6 BURIED STRIP EARTHING ELECTRODES Buried strip earthing electrodes provide a solution to the problem of obtaining a low resistance earth connection in locations where soil resistivity is high, particularly where there is a superficial layer of soil over a stratum of rock and it is impracticable to drive an earthing electrode. For a given cross-section, strip earthing electrodes have the advantage of a greater surface area in contact with the soil. The material for such earthing electrodes should be selected having regard to corrosion compatibility with the protected structure (see Clause 4.7.2). For example, for a galvanized steel tower, a 50 mm × 3 mm galvanized steel strip would be preferred. Where the earthing electrode is totally isolated from other metals, e.g. on an isolated stone or timber structure, any one of a variety of materials may be used. These include copper, galvanized iron, steel, stainless steel, Ni-resist. The last two materials offer some additional corrosion resistance in aerated soils, but with the disadvantage of higher electrical resistance and cost. Backfilling and compacting the trench will enhance the early resistance performance of the earthing electrode. The cross-section of the conductor has very little effect on the resistance of the earth connection so that the strip or cable size is not important provided it affords reasonable protection against mechanical damage and corrosion, and is of adequate current surge capacity. (see Table 4.6 for typical dimensions). The economics of depth of burial versus resistance performance do not warrant laying strip earthing electrodes below 500 mm, unless the risk of mechanical damage requires this additional protection. The optimum resistance for a given amount of earthing electrode material is achieved if the earthing electrode is buried in a straight single trench or in several trenches radiating from a point. If laid in parallel lines, the trenches should be widely separated (see Paragraph C1 and the Notes to Paragraph C3). C7 BURIED PLATE EARTHING ELECTRODES This form of earthing electrode is now mainly restricted to tower footings or the like where the civil works for the structure to be protected provide the facilities for the laying of the earthing electrode. The performance/cost relationship does not support other than such specific applications. C8 CONCRETE FOOTING EARTHING ELECTRODES This form of earthing electrode is one of the most effective both in cost and electrical performance of currently available earth termination networks. For a given site it provides a permanent, distributed, low resistance earthing electrode at very little cost above the structural civil work.

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The vital part of the exercise is the planning, design and supervision of the construction, as after concrete is poured it is impractical to address design deficiencies. If a sectional measurement of earth resistance is desired, this will only be feasible at the appropriate stage of construction. As this earthing electrode is likely to be bonded deliberately or fortuitously to the electricity supply service earthing system, and perhaps to other earthing systems, it is important to consider the possibility of corrosion arising from contact with dissimilar metals (see Clause 4.7.2). It may be necessary to address corrosion problems of rock anchors by cathodic protection (see AS 2832.2). The deliberate or fortuitous bonding of on-site fuel tanks, now relatively common adjacent to building foundations, should also be taken into account in the design of LPS earthing electrodes to ensure the earth discharge is from the earth termination network, i.e. the fuel tanks are electrically screened. C9 INSPECTION AND MAINTENANCE OF EARTHING ELECTRODES The scheduling of earthing electrode maintenance inspections is the prerogative of the system owner. However, the frequency of testing and the associated considerations are listed below as a guide to good engineering practice. Data on the layout, materials of construction data and electrical measurements pertaining to the original design should be prepared and preserved as a guide to later performance. Soil resistivity data are likewise useful for future comparison. The following practices are recommended: (a)

Inspections should be both physical and electrical.

(b)

The inspections should be carried out at intervals of not less than two years. Examples of the need for more frequent action would be if the earth termination network is in a marine environment, subject to a high rate of corrosion. Such installations may require inspection at intervals as frequent as once per year.

(c)

The physical inspections should address corrosion or mechanical damage to visible parts of the whole system, structural alterations that may have prejudiced the design or operation of the system, or changes in the usage of the structure, e.g. fuel storage added.

(d)

Electrical tests should cover the continuity of the downconductors, the integrity of bonding arrangements, and the resistance to earth of the earthing electrodes, preferably individually as well as collectively. Methods of testing resistance to earth are discussed in Paragraph C10. The continuity of downconductors should preferably be checked by a high-current testing system (approximately 10 A) in order to detect reduced current-carrying capacity resulting from fractures or other damage that may be obscured from view.

(e)

The enhancement or replacement of earthing electrodes to achieve a specified resistance may be necessary, and if this is done it should be recorded along with other test results. Restoration records of clamps, joints and fittings on downconductors or earthing electrode terminations should also be kept as a future maintenance guide.

(f)

Where uncertainty exists about the validity of inspection test results, comparison with original design figures and data, together with the historical test records, will often serve to indicate the extent of deterioration of the earthing electrodes. The change in soil resistivity with rainfall can at times be particularly misleading, and test results should be viewed with some suspicion if a significant reduction is observed in resistance figures.

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C10 MEASUREMENT OF SOIL RESISTIVITY, EARTHING ELECTRODE RESISTANCE AND EARTH TERMINATION NETWORK IMPEDANCE C10.1 Determination of soil resistivity by test C10.1.1 Four-pin method The Wenner or four-pin method of soil resistivity measurement is commonly used. It involves the use of four test pins (test electrodes) equally spaced in a straight line and driven to the same depth d, not exceeding 5 percent of their separation s and not more than 1 m in any case (see Figure C2).

LEGEND: s = test pin separation d = depth of test pin: this must be small in relation to s. s and not in any case greater than 1 m i.e. not greater than 20

NOTES: 1

The above configuration will give a reading for ρ, by calculation, that is equivalent to the resistivity at depth 0.75s.

2

If it is required to determine average resistivity to various depths at a given point ‘O’, the centre point O of the test configuration is kept fixed and the test pin separation s increased outward from that point.

3

As the effects of d.c. polarization on the test pins would give a superimposed error on V, of the same order as the small voltage being recorded, it is necessary to use an alternating electricity supply source, or if d.c., a cyclically-reversed source. The latter would also require a synchronous reversal of the indicating system.

FIGURE C2 FOUR-PIN METHOD OF SOIL RESISTIVITY MEASUREMENT

If a known current I is passed between the outer test pins, and the voltage drop V between the inner test pins is measured, the ratio V/I gives a resistance R. If the earth were perfectly homogeneous, i.e. of a constant resistivity ρ, then: ρ

. . . C10.1.1

=

2πsR

s

=

test pin separation, in metres

ρ

=

average soil resistivity to a depth of s metres, in ohm metres

where

The soil is rarely homogeneous and the value ρ calculated from Equation C10.1.1 (called the apparent resistivity) will be found to vary with the test pin separation s. It is from these variations that deductions can be made as to the variation in the nature of the underlying soil.

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Account should also be taken of seasonal variations of changing moisture content. By repeating the measurements separation, the average resistivity to various depths can indicate whether an advantage is to be gained by installing to reach strata of low resistivity.

the soil, primarily because of with different values of test pin be found and the results will deep-driven earthing electrodes

In practice an indicating ohmmeter or null-reading bridge is used to measure the resistance R from which soil resistivity ρ is calculated by using Equation C10.1.1. C10.1.2 Instrumentation for soil resistivity measurement Measurement of soil resistivity by the Wenner method (Paragraph C10.1.1) requires a fourpin earth resistance tester. There are two main types, the traditional null-reading analogue or the later microprocessor based digital. Originally the analogue types were powered by a hand wound generator but these have been largely superseded by battery-powered models. The majority of four-pin earth testers currently available are battery-powered digital instruments providing a square wave output. Test frequencies are usually around 128 Hz for the basic models. More capable models offer variable test frequencies to avoid possible errors from interference frequencies present on the earth system. These instruments can also provide automatic frequency selection which detects interference frequencies and automatically selects the optimum test frequency to avoid measurement errors. The output voltage of earth testers usually ranges from 20 to 50 V. Test current is governed by the resistance of the load and instruments are usually rated on their short circuit current. That is the current the instrument can generate with the terminals shorted. Typically the short circuit current ranges from 10 mA for basic models to 250 mA for more capable instruments. The connections for a four-pin measurement of soil resistivity is shown in Figure C3.

LEGEND: s = test pin separation d = depth of test pin: this must be small in relation to s. s i.e. not greater than and not in any case greater than 1 m 20 C 1 = Current terminal 1 on instrument (marked E on some instruments) P 1 = Potential terminal 1 on instrument (marked ES on some instruments) C 2 = Current terminal 2 on instrument (marked H on some instruments) P 2 = Potential terminal 2 on instrument (marked S on some instruments)

FIGURE C3 CONNECTIONS FOR EARTH RESISTIVITY TEST USING AN INDICATING OHM METER OR NULL BALANCE BRIDGE FIGURE

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C10.2 Earth resistance C10.2.1 General procedure Earth resistance is measured by applying a voltage to the earth electrode and measuring the current passing through the electrode to earth. Resistance is calculated by the earth tester using Ohm’s Law R=V/A. This is often referred to as the fall of potential method as shown in Figure C4. To create a path for the test current and to measure the voltage, a current is passed in a potential test stake, which is placed in the soil outside the potential gradient area of the electrode being measured. The conduction of current through the soil is non-linear. Consequently, earth resistance measurements will vary with different test voltages, currents and frequencies. Interference voltage and frequency can be picked up by the test stakes. An earth resistance tester ideally should provide variable test frequencies to avoid measurement errors from interference. Earth resistance can be measured by a three-pin or four-pin measurement. The difference between three-pin and four-pin testing is the number of connections to the electrode under test. A three-pin test makes one connection to the earth electrode while a four-pin test makes two. By making two connections to the electrode under test, the voltage is measured at the earth electrode and not at the instrument. This eliminates any error caused by resistance in the connecting lead and is generally regarded as the preferred method.

LEGEND: C 1 = Current terminal 1 on instrument (marked E on some instruments) P 1 = Potential terminal 1 on instrument (marked ES on some instruments) C 2 = Current terminal 2 on instrument (marked H on some instruments) P 2 = Potential terminal 2 on instrument (marked S on some instruments) E = Earth electrode under test P = Potential pin or stake C = Current pin or stake (sometimes referred to as auxiliary stake) R E = Resistance of earth electrode to ground R p = Resistance of potential pin to ground (sometimes referred to as R s ) R c = Resistance of current pin to ground (sometimes referred to as R h ) V = Test voltage applied by instrument I = Test current path G = a.c. generator in instrument

NOTE: For three-pin measurement terminals C1 and P1 are bridged at the instrument

FIGURE C4 PRINCIPLE OF FOUR-PIN EARTH RESISTANCE MEASUREMENT

The fall of potential follows the relationship: D=

Iρ 2πV

. . .C10.2.1

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where D

=

the distance from the test stake in metres

I

=

the test current in Amps

ρ

=

the soil resistivity in ohm-metres

V

=

the voltage at distance D from the test stake in Volts.

Consequently it is important that P be situated on the ‘flat’ part of the curve as in Figures C5 and C6. If C is not sufficiently distant from E, there will be no flat part of the curve. This can be established by moving P and retesting. If it varies, then C is too close to E. As a general rule, C should be separated from E by not less than 10 times the length of E, for homogenous ground, and P about half the distance from E to C.

FIGURE C5 FALL OF POTENTIAL AROUND AN EARTH ELECTRODE AND TEST STAKES

FIGURE C6 EFFECT OF THE FALL OF POTENTIAL ON THE MEASUREMENT OF EARTH RESISTANCE

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C10.2.2 Test lead considerations Because of the inhomogeneity or layering of the soil it is prudent practice to use as long a lead from E to C as practical. In fact, for extreme conditions, such as a mountain top, where the site may be on a volcanic core, it is not uncommon practice to use a 500 m or 1000 m test lead of physically substantial construction. Testing with short leads merely gives a test pin resistance to that small volume of ground encompassed by the electric field between E and C, in a roughly hemispherical volume. With very long leads a significant hazard arises. EPR from power fault currents or lightning pulse can give rise to dangerous voltages between different parts of the earth’s surface. The handling of leads of 500 m or greater should be accompanied by the careful use of insulating gloves suitable for working at voltages of up to 500 V a.c. Test instruments may require fitting with radiofrequency suppression devices to prevent the pickup of high-frequency radio communication signals damaging the electronic detection equipment, or producing erroneous readings. Another condition requiring very long leads is the situation of an earthing electrode of considerable dimensions, e.g. a 500 m strip earthing electrode, especially in high resistivity ground such as a mountain top or sandy plain. In this case, the concept of ‘resistance to the body of earth encompassed by the electric field’ above, requires a lead to C of the order of 500 m, at right angles to the run of the earthing electrode. The same safety considerations apply. C10.2.3 Instruments for earth resistance measurement C10.2.3.1 General The earth resistance of an earth electrode or group of electrodes can be measured by several methods using various types of earth testers. It cannot be assumed that each method or each type of instrument is suitable for every situation. A four-pin earth tester (see Figure C7) as used for soil resistivity can also be used for measuring earth resistance. Lower cost three-pin earth testers are also suitable for measuring earth resistance but cannot be used to measure soil resistivity. Users must be aware that measurements made with the three-pin method are susceptible to error from test lead resistance. Both three-pin and four-pin earth testers require the earth being measured to be disconnection during measurement. This poses a major safety hazard should there be a lightning strike or a fault current present during test. The use of a second downconductor and consequent earth electrode will ensure that the LPS is always earthed during system testing. (Refer to Clause 4.3.3(a)). To eliminate the hazard and inconvenience of disconnecting the earth to be measured, the selective method has been developed. Another alternative is the stakeless method. This has the advantages of not requiring the isolation of the earth electrode or the use of test stakes. This does however require the earth system to have a secondary reliable solid bond to earth, such as a bond to the electrical earth and/or metal water service. Users must be fully conversant with the earth circuitry, otherwise measurements by this method can be misleading. Each method has advantages and will produce reliable measurements if used in the appropriate situations. There is no one method which can be used for every situation. This has given rise to multi-method earth testers which combine all three methods into a single instrument.

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C10.2.3.2 The three and four-pin method

FIGURE C7 THE THREE-PIN AND FOUR-PIN METHOD OF MEASURING EARTH ELECTRODE RESISTANCE

C10.2.3.3 Selective method To address the hazard of isolating electrodes under test and the inconvenience of disconnecting and reconnecting, the selective method of measuring earth resistance was developed (see Figure C8). The principle of the selective method is the use of a clip-on current transformer (CT) to measure the test current flowing in the earth electrode under test. Resistance is computed from the actual current flowing to earth via the electrode. Current flowing to earth through the remainder of the earth network is not measured and has no effect on the result.

FIGURE C8 SELECTIVE EARTH RESISTANCE MEASUREMENT

Both the three-/four-pin and selective methods of measuring earth resistance rely on test stakes being placed, typically 50 to 100 m, or even further from the earth system being tested. This is generally of no consequence in rural areas but it is seldom possible in cities and towns where buildings are surrounded by pavement or the electrodes are in basements. C10.2.3.4 Stakeless or clamp-on method Stakeless testing provides benefits in developed areas as it is no longer necessary to find suitable soil to place test stakes or to need to run out long test leads and wind them up at the completion of tests. It provides a convenient method of measuring earth resistance in locations which were previously difficult or impossible to measure by other methods. No direct connection is made to the earthing system and maximim safety is achieved. COPYRIGHT


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Stakeless earth resistance testing requires a solidly earthed network in parallel with the electrode to be tested to provide a path of negligible resistance to earth for the test current (see Figures C9 and C10). For a valid reading, it is necessary that the resistance being measured (R x) is large compared to the parallel combination of the other interconnected paths to earth (R1 to R n ). More specifically, the actual value measured is given by— R measured = R x + (parallel combination of R 1 to R n) Two clip-on CTs are used for this method. The test voltage is applied to one CT which is used in reverse to induce a test current to flow in the earth network, the other CT is used as before to measure the component of test current flowing through the electrode under test. Earth resistance is computed in the same manner as for the selective method.

FIGURE C9 PRINCIPLE OF STAKELESS MEASUREMENT

Stakeless earth testing can be as an adaptation of the selective method by adding a second CT or as a specially designed clamp-on tester where the two coils are incorporated into a single jaw. Clamp-on earth testers can only function in this mode and cannot test by any other method.

FIGURE C10 MEASUREMENT OF EARTH RESISTANCE OF A LIGHTNING SYSTEM WHICH IS BONDED TO THE ELECTRICAL EARTH USING THE STAKELESS METHOD

C10.3 Isolation of surge impedance of an earth termination network from other fortuitous earth paths The measurement of electrode surge impedance in an LPS earth termination network requires that the electrical condition specific to a lightning pulse be addressed. Typically the rise time of a substantially unmodified lightning pulse is around 1 µs. This means that fortuitously-bonded path lengths of more than a few tens of metres will present a reactive component that prejudices the ability of such paths to divert a significant portion of the total earth path current. This in turn requires the measuring system to reject such paths and to measure the residual path—usually that to immediately adjacent earthing electrodes or earth features. COPYRIGHT


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This can be achieved fairly simply but at some cost by utilizing a relatively high frequency source of power for the three-pin test. Several excellent commercial instruments are available that operate in the 25 kHz to 50 kHz range. These are used in the same manner as other three-pin (and four-pin) test sets. The difference in readings between these and the low-frequency test sets will often be quite spectacular and will point out obvious reasons for observed catastrophic failure from lightning in systems thought to be adequately earthed. In particular, the electricity supply service neutral/earth connection bonded for 50 Hz equipotential protection to LPS earth termination network can give a grossly misleading sense of security if it is read with a 108 Hz test set. Typically, a one ohm reading can in reality be 100 Ω surge impedance, as measured by a high frequency test set. If a high frequency test set is unavailable, an alternative method with reasonable accuracy may be used, based on the fall of potential curve around the electrode (see Paragraph C10.2.1), similar to that used for testing substation earth mats. Alternating current is applied between the earthing electrode (or its downconductor) and an auxiliary test pin (see Figure C11). The portion of the current passing to earth via the earthing electrode under test is measured by a clamp ammeter placed between the current injection point and the earthing electrode entry to ground. The voltage to which this drives the earthing electrode is measured by a flying lead voltmeter to a test pin sited in the ‘flat’ portion of the fall of potential curve. The resistance of the earthing electrode is a simple R = V/I relationship, and is a good approximation to the surge impedance. A skilled operator may elect to measure approximate lightning surge impedance at a site on a long buried strip electrode by reduction of the length to the C2 electrode, using a conventional 3 terminal test set. This requires both experience with the set and its performance in various soil resistivities.

NOTE: No connections to bonding conductors should exist on the earth side of the current injection point.

FIGURE C11 MEASUREMENT OF EARTHING ELECTRODE SURGE IMPEDANCE

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APPENDIX D

THE CALCULATION OF LIGHTNING DISCHARGE VOLTAGES AND REQUISITE SEPARATION DISTANCES FOR ISOLATION OF A LIGHTNING PROTECTION SYSTEM (Informative) D1 GENERAL In the first one or two microseconds of a lightning discharge, transient voltages occur on the air terminal network and on the downconductors, which may be far greater than the discharge voltages that apply during the remainder of the discharge. This is because the discharge energy injected into the air terminal network at any instant is initially stored as thermal energy in the conductors of the protection system prior to discharging into the general mass of earth via the earth termination network. This transient voltage frequently determines the separation distance required for isolation of the LPS, if it is desired to isolate in accordance with Clause 4.16.2.3 as the preferred method of protection against side-flash (see Clause 4.16.2.1). The peak values of the transient voltages appearing at various points of the protection system differ according to location, and increase with distance from the earth termination measured along the route of the discharge through the protection system. At points very near to a concentrated earth termination, the transient voltages are suppressed by the discharge to earth and minimum values of discharge voltage and required clearance can be readily calculated (see Note 1). These lower limits apply only at the base of the structure and are given by the equations: Ve

=

150R

. . . D1(1)

De

=

0.3R

. . . D1(2)

where Ve

=

the discharge voltage at the base of the structure due to local EPR, in kilovolts

R

=

the combined earth termination resistance, in ohms

De

=

the required clearance in air at the base of the structure, in metres

The complete transient voltage waveshape at all points of the protection system can be calculated using travelling wave techniques and a computer, however substantial simplifications allowing helpful easily-calculated estimates can frequently be made (see Paragraph D2). An estimate of the transient voltage at any one point of an LPS can often be made using a conventional circuit theory approach (see Paragraph D3). This is possible because the transient voltages can often be neglected due to the high insulation strength of air to extremely brief voltage stresses. Transient voltages at points remote from the earth termination depend both on the lightning stroke current waveshape and the characteristics of the protection system. As indicated in Paragraph B2.2 and Table B1 of Appendix B, lightning flashes generally have a number of component strokes with differing waveshapes. The critical voltage may correspond to the highest peak current of a first stroke or to the steepest wavefront (di/dt) max. of a subsequent stroke.

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The design first and subsequent strokes used in this Standard are shown in Figure D1 (see Note 2). The more severe of these cases was adopted in arriving at the required clearances in Clause 4.16.2.3.

FIGURE D1 IDEALIZED LIGHTNING STROKE CURRENTS ADOPTED FOR DESIGN PURPOSES

The electrical breakdown strength of air depends on the polarity of the applied voltage and on the duration and shape of the voltage surge. In the studies conducted for this Standard, the required clearances were estimated using the breakdown strength of air, shown in Figure D2, which neglects some of these variables and should only be regarded as approximate.

FIGURE D2 TIME DEPENDENCE OF THE ELECTRICAL BREAKDOWN STRENGTH OF AIR (approximation for the purpose of this Standard only)

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In some cases where the transient voltage surges differed significantly from the waveshapes (chopped and triangular waves) upon which the graph is based, a further adjustment was made. NOTES: 1

The lower limit of design discharge voltage is based on the assumed peak lightning current of 150 kA. The corresponding required clearance is based on a minimum electrical breakdown strength of 500 kV/m in air.

2

The lightning flash is assumed to have a first stroke with I max. of 150 kA and (di/dt) max. of 32.6 kA/µs and a steepest subsequent stroke with I max. of 40 kA and (di/dt) max. of 200 kA/µs. A more severe case would occur not more than once in each hundred lightning strikes to the building, that is about once in a thousand years for a single 60 m high structure in a locality with a moderate level of lightning activity (one strike per square kilometre per year).

D2 TRANSIENT ANALYSIS

VOLTAGE

CALCULATIONS

BY

TRAVELLING

WAVE

D2.1 Simplified travelling wave characteristics An electric charge injected into one end of a conductor propagates along the conductor as a travelling wave with velocity v given by: v

1

=

. . . D2.1(1)

( L1C1 )

where v

=

velocity, in metres per second

L1

=

the inductance per unit length, in henries per metre

C1

=

the capacitance per unit length, in farads per metre

For a single bare conductor of radius r at a distance h above a perfect ground in free space, L 1 and C1 are given by: L1

=

C1

=

µo 2h 1n 2π r

. . . D2.1(2)

2πε o 2h 1n r

. . . D2.1(3)

where µ o and ε o are the permeability and permittivity of free space and the conductor is assumed to be non-magnetic, i.e. µo

=

4π × 10-7 H/m, and εo = 8.85 × 10-12 F/m

In air, µ and ε differ only slightly from µ o and ε o, and the velocity of the travelling wave becomes the velocity of light, c, in metres per second: v

=

1 ( µ o ε o)

= c = 3 × 10 8

. . . D2.1(4)

(In an insulated cable with continuously earthed sheath and dielectric or relative permittivity k (where k is typically 3 to 6), the velocity is reduced by the factor 1/ k and is typically 0.5 c.)

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For lossless surge propagation, the voltage e generated by a travelling wave with current i is given by: e

=

iZ

. . . D2.1(5)

where Z

 L1    is called the surge impedance of the conductor, and in free space  C1 

=

is given by: Z

=

60 1n

2h r

. . . D2.1(6)

where Z is in ohms. When a travelling voltage wave in a conductor arrives at an electrical discontinuity, such as an intersection of downconductors or the connection of a downconductor to an earth termination, part of the travelling wave is transmitted and part is reflected. If Z 1 is the surge impedance of the conductor on which the wave is travelling prior to reaching the discontinuity, and Z 2 is impedance seen at the termination or combined parallel surge impedance of conductors continuing beyond the junction or other discontinuity, then the reflected surge v′, i′ is related to the incoming surge by the equations: v′

=

bv

=

 Z − Z1   v  2  Z 2 + Z1 

. . . D2.1(7)

i′

=

−bi

=

 Z − Z 21   i  1  Z1 + Z 2 

. . . D2.1(8)

and the combined transmitted wave v″, i″ beyond the discontinuity or at the termination is given by: =

 2Z 2   = v(1 + b) v   Z 2 + Z1 

v″

=

av

i″

=

 2Z 1   = i(1−b) i   Z 2 + Z1 

. . . D2.1(9)

. . . D2.1(10)

where b

=

the reflection coefficient

a

=

the transmission coefficient

D2.2 Surge voltage calculation by lattice diagram The LPS is represented by a simplified model in the form of nodes and branches. The nodes are placed at junctions or impedance discontinuities. Each branch has a surge impedance, and a travel time determined from its length and the surge velocity. The earthing resistance at each earth termination is treated as a branch with surge impedance equal to the earthing resistance in ohms and of infinite travel time. Lattice diagram calculations are usually carried out by computer. Generally the program calculates the response of the system to a unit step function current and the response to any given input current wave is calculated as the superposition of the responses to the succession of such step functions of various magnitudes, polarities and input times whose sum closely approximates the desired input waveshape.

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For manual calculations only very simple models can be handled but triangular waveshapes can be readily used. It is therefore necessary to consider whether parallel downconductors can be represented by a single downconductor. Example D1 A building 35 m high is protected by four vertical air terminals placed at each corner of the flat roof and interconnected around the perimeter of the roof to four downconductors of surge impedance 480 Ω that run vertically to earth at each corner. Each earth termination has two to four driven earth stakes 3 m deep and 6 m apart to achieve a test resistance of 9 Ω to 10 Ω and a combined earthing resistance of 2.5 Ω. Calculate the first voltage peaks of the response to stroke currents i a(t) and i b(t) having waveshapes A and B of Figure D1, respectively. The system may be modelled as shown in Figure D3. The four air terminals and downconductors are represented by a single branch of surge impedance 120 Ω and length 40 m (say) terminated in a resistance (or reflectionless infinite branch) of impedance 2.5 Ω. Nodes 1 and 2 lie at the top and the base of the building respectively. For convenience of representation take i(t) as positive and adopt units of kiloamperes (kA), kilovolts (kV), and microseconds (µs). Then the surge velocity C may be taken as 300 m/µs and the branch (1, 2) travel time is T = 0.133 µs. The reflection coefficient b for surges arriving at node 2 is evaluated as −0.95 so that 5 percent of the incident surge current at node 2 leaks to earth with the remainder initially trapped on the protection system. The reflection coefficient for surges returning to node 1 from 2 is evaluated as unity (the surge impedance of the lightning discharge channel is neglected for this example). The lattice diagram is developed as indicated in Figure D3(b).

FIGURE D3 CALCULATION OF SURGE VOLTAGE BY LATTICE DIAGRAM

The surge voltage v(t) at node 1 depends simply on the incident current i(t) until the first reflection wave arrives at 0.266 µs(2T) and is given by— v(t)

=

Zi(t), for 0 ≤ t ≤ 2T

. . . D2. 2(1)

Because b is negative (−0.95) the first reflected voltage wave bv is negative and is doubled on arrival at the open-circuited node 1, i.e. from the instant 2T, the first reflected wave bv is again reflected as bv. The voltage at node 1 is varied by 2bv, and is given by— v(t)

=

Zi(t) + 2bZi(t − 2T), for 2T < t ≤ 4T

=

120[i(t) − 1.9 i(t − 2T)], for 2T < t ≤ 4T COPYRIGHT


. . . D2. 2(2)

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For the first stroke current ia(t), (where ia is in kiloamperes and t is in microseconds) the current waveshape in the period 0 < t ≤ 4T is a ramp of uniform slope: ia(t)

=

32.6t

. . . D2. 2(3)

The response is a triangular wave given by— v a(t) =

120 [32.6t − 61.9(t − 2T)]

. . . D2. 2(4)

for which the peak value occurs at t = 2T as shown in Figure D3(c). For the subsequent stroke current i b(t), the current waveshape reaches its crest value at t = 0.2 µs, which is prior to the arrival of the first reflection wave at t = T. The peak value occurs at t = 0.2 µs, and as the waveshape to this time is a ramp of uniform slope (i b(t) = 200t, t ≤ 0.2), the peak value is 4800 kV and the response takes the form shown in Figure D3(c). It should be noted that the transient oscillatory response is damped by the discharge to earth occurring at node 2. A travelling wave analysis for each stroke current was carried out by computer for a 10-metre structure with combined earthing resistance of 5 Ω, and for a case similar to the above. The downconductor surge impedance was arbitrarily reduced to 40 Ω to allow for corona and a lightning discharge channel surge impedance of 1500 Ω (a minimum value) was used, increasing the damping of the transient oscillations. The response up to 6 µs is shown in Figure D4. The travelling wave analysis permits calculation of the voltage response at any point on the LPS because of the distributed constant representation of the system. D3 SURGE VOLTAGE APPROXIMATIONS

CALCULATIONS

BY

LUMPED

CIRCUIT

The lumped circuit approximation precludes any assessment of the transient voltage oscillations associated with travelling waves generated on the protection system, however, elementary calculations generate the base lines about which any transient oscillations occur. Example D2 A building 35 m high is protected in a similar manner to that of the example in Paragraph D2.2. Each of four downconductors is assessed as having a length of 40 m and the inductance in microhenries per metre is given by: L1

2h = 1.5 r

=

0.2 1n

h

=

the average height above ground of the four downconductors, in metres

r

=

typical radius of the four downconductors, in metres

. . . D3(1)

where

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Curves A: Response to design first stroke waveshape A of Figure D1 Curves B: Response to design subsequent stroke waveshape B of Figure D1 FIGURE D4 VOLTAGES ON LPSs—ILLUSTRATIVE CASES CALCULATED BY SIMPLIFIED TRAVELLING WAVE ANALYSIS

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The total inductance of the protection system is given by: L

L1

=

H n

. . . D3(2)

where H

=

the average length of downconductors from the point struck to earth, in metres (this differs from the definition in Clause 4.15.2.3 because the voltage at intermediate points cannot be calculated for multiple downconductors using lumped circuit approximations)

n

=

the number of downconductors connected to a common air terminal network (spacing of downconductors is assumed large enough for mutual effects to be neglected)

The capacitance of the system is also neglected. The equivalent circuit for calculating the voltage at roof level (node 1) and ground level (node 2) is shown in Figure D5.

FIGURE D5 SIMPLIFIED LUMPED EQUIVALENT CIRCUIT

The voltage at node 1 with respect to remote earth is given by: v1 (t) =

i(t) R + L

d i (t ) dt

. . . D3(3)

The response to the idealized design stroke currents i a(t) and i b(t), calculated from this equation, is shown in Figure D6. It can be seen by comparison with Figure D4 that the simplified lumped circuit method is an extremely useful tool in estimating system responses to the various lightning stroke currents. In the case of first strokes, for which the transient oscillations have effectively been damped by the time of the current peak, the voltage waveform calculated by this method is also an adequate basis for estimating the required clearances for isolation.

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Curves A: Response to design first stroke waveshape A of Figure D1 Curves B: Response to design subsequent stroke waveshape B of Figure D1 FIGURE D6 VOLTAGES ON LPSs—ILLUSTRATIVE CASES CALCULATED BY SIMPLIFIED LUMPED CIRCUIT ANALYSIS

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APPENDIX E

EARTHING AND BONDING (Informative) E1 GENERAL This Appendix provides additional information to Clause 5.5.2 on acceptable methods of equipotential bonding. It is aimed at providing effective equipotential bonding between services and the electrical installation earth/local earth. E2 DEFINITIONS For the purposes of this Appendix, the following definitions apply, in addition to those of Clause 1.5. E2.1 Common bonding network (CBN) A common bonding network is formed by the interconnection of building steel and the reinforcing steel in concrete etc. A CBN is an effective method to provide earthing and bonding. E2.2 Common utilities enclosure A common utilities enclosure may contain, in separate compartments, the cabling and equipment associated with the provision of service for example, electricity, telecommunications, cable TV and water. E2.3 Earth potential rise (EPR) A rise in voltage of an earthing system and the surrounding soil with respect to a distant earth. NOTE: EPR is caused primarily when an earth fault on a HV power system produces a current flow through the earthing system of a HV site.

E2.4 EPR hazard zone The area around an earthing system bounded by a contour joining all points of EPR equal to the maximum acceptable voltage below which no special precautions need to be taken to protect telecommunication services, cabling providers and end-users. E2.5 Main earthing bar (MEB) Installed in the main switchboard (MSB), it provides a termination point for the main earthing conductor, equipotential bonding conductors and protective earthing conductors. E2.6 Main earthing conductor The conductor connecting from the MEB to the electricity supply service earthing electrode. E2.7 Main switchboard (MSB) Associated with the electricity supply service to the building. Contains the main earthing bar (MEB), main neutral earth link and has a connection to the electricity supply service earthing electrode. Where the multiple earthed neutral (MEN) system of earthing is employed, a link exists between the MEB and the main neutral link (MEN link). E2.8 Main distribution frame (MDF) A distributor that provides, or is intended to provide, an electrical termination point for a carrier’s lead-in cabling. COPYRIGHT


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E3 HAZARDOUS CONDITIONS ASSOCIATED WITH HV POWER EPR E3.1 General Placement of equipment, telecommunications plant and a connection to the structure earthing system shall not take place where a hazardous EPR exists. If a building is only supplied by 230 V a.c. single phase power or 400 V a.c. three phase power , there will be no need to consider EPR unless the proposed installation is within the EPR hazard zone of a HV site, as determined by AS/NZS 3835, Parts 1 and 2. E3.2 HV sites of particular concern A hazardous EPR may occur in the following sites: (a)

In or near a power generating station or power substation.

(b)

Near a HV transformer or SWER transformer.

(c)

In or near electrical traction systems.

(d)

In or near any HV site located in an area of high soil resistivity (e.g. rocky or dry, sandy terrain).

For requirements for safety in these situations refer to AS/NZS 3835, Parts 1 and 2. E4 METHODS OF EQUIPOTENTIAL BONDING E4.1 General As stated in Clause 5.6.2, the objective of equipotential bonding is to reduce the potential difference between various parts of the structure and the main earth bar (MEB). To coordinate with the equipment insulation as specified in IEC 60950.1, the requirement is to limit the potential difference to less than or equal to 1.5 kV under direct strike conditions. It is particularly necessary for telecommunication line SPD bonding to ensure coordination with the requirements of IEC 60950.1. To achieve less than or equal to 1.5 kV, based on the assumed ∆U = 1 kV per m length for direct lightning strikes, the total length of bonding conductor between the part being bonded and the MEB should be equal to or less than 1.5 m. This length is easy to achieve if all services enter at the same point in a small structure. In Australia, AS 4262.1 allows bonding conductors up to 10 m length, for safety of persons using a telephone during a lightning storm, to coordinate with the handset breakdown voltage of 7 kV required by AS/NZS 60950.1. Therefore in a poorly designed new small structure, or in larger structures, bond wires up to 10 m may be used. However every attempt should be made to achieve as short a bond wire as possible to ensure maximum protection of people and equipment. In those exceptional cases where the total bonding conductor length would to be greater than 10 m, an engineered solution should be applied. E4.2 Using a bonding bar To aid in the bonding of the various utility services it is preferable to use a bonding bar. This bonding bar can be the MEB in the MSB but, for easy access, preferably a separate bar provided in a common utilities enclosure or mounted as close as possible to the MSB. Figure E1 shows the bonding bar and the connection of the various bonding conductors. The bonding conductor requirements are— (a)

the bonding bar should be bonded to the MEB in the MSB with a short bonding conductor, preferably less than 0.5 m long, via a disconnect link;

(b)

the MEB should be bonded directly to the electrical installation earth electrode with as short a conductor as possible and in accordance with AS/NZS 3000; and

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all other bonding conductors should be kept short, preferably less than 1 m long, and be bonded to the bonding bar. NOTE: To achieve a high level of protection for telecommunications services, the total bonding conductor length, between the telecommunication SPD and the MEB, should be no more than 1.5 m. Where due to practical reasons, this 1.5 m requirement cannot be met, a total bonding conductor length of no more than 10 m will provide a reasonable level of protection for people using a telephone. In those exceptional cases where the total bonding conductor length would be greater than 10 m, a risk assessment according to AS 4262.1 should be performed. Where the risk according to AS 4262.1 is considered particularly high, an engineered solution should be applied which will give protection equivalent to that using short bond wires. Some examples of engineering solutions which could be applied in these particularly high risk situations are given in Paragraphs E4.3 and E4.4.

The LPS, antennas and other earthed objects likely to be struck by lightning should be bonded so as to comply with the requirements of Clause 4.16.2.2. This LPS earth termination network should be directly bonded to the electrical installation earth electrode electrode. All bonding conductors should be labelled at the point of connection to the bonding bar, the main earthing conductor or the electricity supply service earthing electrode. Bonded joints should be accessible for maintenance.

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FIGURE E1 BONDING OF SERVICES NOTES: 1

Refer to Paragraph E4.2 for bonding conductor lengths and other requirements.

2

Refer to AS/NZS 3000 for connection of the MEB to the electricity supply service earthing electrode.

The preferred method for new small buildings, e.g. domestic premises, is to co-locate the entry of services next to the MSB or to use a combined utilities enclosure, see Figures E2 and E3. For larger buildings an MDF will normally be installed and it may not be practicable to achieve a short bonding conductor to the main earthing bar. Ideally, the MDF should be installed as close as possible to the MSB. However, when the MDF is installed on a different follor to the MSB, a bond to the nearest switchboard earthing system shall be installed.

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NOTE: An LPS earthing system would be bonded to the electricity supply service earthing electrode.

FIGURE E2 CO-LOCATION OF SERVICES NEXT TO A SWITCHBOARD

See AS/ACIF S009 for further methods of connecting the telecommunications SPD to the MEB.

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FIGURE E3 COMBINED UTILITIES ENCLOSURE

E4.3 Use of a common bonding network A building with a properly bonded reinforced concrete floor effectively provides a common bonding network (CBN). In this case, bonding can be achieved by bonding the various services and SPDs directly to the CBN. This connection must be made by connecting directly to the reinforcing mesh by a suitable method. The mesh must be electrically continuous between the points of attachment. See Figure E4 which is an example showing a building with a reinforced concrete floor. In a new building the reinforcing sheets should be tied together with wire in accordance with Clause 4.5.2.4. In an existing building an attempt should be made to measure the resistance of the reinforcing steel from one side of the slab to the other. If continuity of the reinforcing steel is in doubt, e.g. the measurement was made with the soil wet, a ring earth should be installed and bonded to the slab at every earthing electrode.

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FIGURE E4 COMMON BONDING NETWORK (CBN)

E4.4 Use of a ring earth A ring earth system may need to be installed for one or more reasons as follows: (a)

To interconnect an LPS electrode system when installed.

(b)

To provide effective bonding of services to the MEB/local earth when this cannot be achieved by one of the preferred methods shown in Figures E2 and E3.

The same ring earth system can be used for multiple purposes. When used as an LPS the resistance to ground should be 10 Ω or less. When used only to bond incoming services, there is no maximum resistance specified. A ring earth, when required, should be provided by installing a bare conductor below ground, see Figure E5. This conductor should encircle the building(s) and additional earthing electrodes should be installed at each bonding point to the ring earth. Where an LPS is installed, a ring earth should be used to interconnect all downconductors unless the LPS is an integral part of the building. When a ring earth is installed as part of an LPS system, equipotential bonding of services can be achieved by any one of the methods shown in Figures E2 to E5 providing the conditions of the chosen method are achieved and it complies with the requirements of AS/NZS 3000.

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NOTE: Where an LPS is installed the LPS downconductors would be bonded to this ring earth system. In this case the 10 Ω resistance requirement applies.

FIGURE E5 RING EARTH

The method in Figure E5 is not as effective as the methods in Figures E2 to E4 but is an attempt to minimize the potential difference between the telecommunications service SPD, the MEB and ground. Other valid engineered options can be used providing they reduce the potential difference to an acceptable level.

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APPENDIX F

WAVESHAPES FOR ASSESSING THE SUSCEPTIBILITY OF EQUIPMENT TO TRANSIENT OVERVOLTAGES DUE TO LIGHTNING (Informative) Due to the random nature of lightning disturbances and the variable characteristics of the transmission media (such as electricity supply and telecommunications service lines) these transients exhibit wide waveshape variations. However, field and laboratory measurements, confirmed by theoretical calculations, have led to the selection of a small number of waveshapes that are representative of the majority of transients encountered in practice. The value of these standard waveshapes lies in the uniform specification of transient protection equipment. By using the same waveshapes and conditions to test the equipment, manufacturers can quote results that may be directly compared between brands, enabling the user to select an appropriate device. The most common waveshapes used to represent transients on electricity supply service lines are the 1.2/50 µs voltage waveform and the 8/20 µs current waveform. These two waveshapes are shown in Figure F1, and Table F1 indicates their recommended applications and magnitudes. Guidance on the use of these waveshapes in the testing of equipment is given in AS 1931.1. It is important to note that these waveshapes represent line input conditions expected under practical conditions. Purchasers of protection equipment should ensure that equipment side output voltages are reduced to be within the input tolerance envelope of the specified equipment. Tests have shown that the tolerable input voltage variations to electronic equipment can be both time and magnitude dependent. Figure F2 shows two magnitude/time curves derived for computing equipment. Long period variations can generally be corrected by line conditioners while fast transients due to lightning need special devices. These usually comprise non-linear devices to clamp overvoltages and subsequent filtering stages to modify the residual waveshape. The purpose of such devices is to bring the residual voltages from a lightning surge to within the safe operating zone. The test voltage of Figure F1(a) can represent input levels to protection devices. The residual voltage level is that seen at the output, or equipment side of the protection device, when the impulse is applied at the crest of the a.c. voltage. This voltage level will be a function of both the input pulse characteristics and the technical performance of the protection device under test.

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FIGURE F1 STANDARD UNIDIRECTIONAL WAVESHAPES

FIGURE F2 TYPICAL VOLTAGE/TIME TOLERANCE OF COMPUTING EQUIPMENT * See Paragraph G2, Ref 13.

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TABLE F1 RECOMMENDED APPLICATION FOR WAVESHAPES OF FIGURE F1 Location (see Figure F3) Category A

B

C1

Waveshape

Medium exposure peak amplitude

Figure F1(a)

6 kV

high impedance

Figure F1(b)

500 A

low impedance

Figure F1(a)

6 kV

high impedance

Figure F1(b)

3 kA

low impedance

Figure F1(a)

6 kV*

high impedance

Figure F1(b)

10 kA

low impedance

Description Long final subcircuits and electricity supply outlets Major submains, short final subcircuits and load centres Service entrance, other than below

Type of load

C2

Service entrance, building fed by long overhead service lines, or is a large industrial or commercial premises

Figure F1(b)

20 kA

low impedance

C3

Service entrance, building in a high lightning area, or fitted with a LPS

Figure F1(b)

50 kA

low impedance

* The 6 kV amplitude shown here is a minimum value. Higher amplitudes may be used depending on generator construction. NOTES: 1

These test waveshape amplitudes were considered when formulating the recommended SPD surge ratings of Table 5.1. In general, an SPD selected in accordance with Table 5.1 will handle a considerable number of the corresponding test impulse amplitudes listed above.

2

Categories C2 and C3 do not show a voltage waveshape, as this large value current impulse is intended to provide a measure of the SPD robustness, rather than allow a precise measurement of the let-through voltage. See discussion of U p in Clause 5.6.3.5.

3

For testing the performance of SPDs on long, twisted pair telecommunications lines, a standard 10/700 µs voltage surge waveform is often used. The standard 10/700 µs voltage surge generator is defined in ITUT-Recommendation K.44 (Figure A.3-1).

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185

FIGURE F3 LOCATION CATEGORIES FOR APPLICATION OF THE WAVESHAPES IN TABLE F1

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APPENDIX G

REFERENCED DOCUMENTS (Informative) G1 REFERENCED STANDARDS AND REGULATORY DOCUMENTS AS 1074

Steel tubes and tubulars for ordinary service

1214

Hot-dip galvanized coatings on threaded fasteners (ISO metric coarse thread series)

1222 1222.1

Steel conductors and stays—Bare overhead Part 1: Galvanized (SC/GZ)

1397

Steel sheet and strip—Hot-dipped zinc-coated or aluminium/zinc-coated

1432

Copper tubes for plumbing, gas fitting and drainage applications

1531

Conductors—Bare overhead—Aluminium and aluminium alloy

1565

Copper and copper alloys—Ingots and castings

1566

Copper and copper alloys—Rolled flat products

1746

Conductors—Bare overhead—Hard-drawn copper

1874

Aluminium and aluminium alloys—Ingots and castings

1931 1931.1

High voltage testing techniques Part 1: General definitions and test requirements

2187 2187.2

Explosives—Storage, transport and use Part 2: Use of explosives

2239

Galvanic (sacrificial) anodes for cathodic protection

2738

Copper and copper alloys—Compositions and designations of refinery products, wrought products, ingots and castings

2832 2832.1 2832.2

Cathodic protection of metals Part 1: Pipes and cables Part 2: Compact buried structures

4070

Recommended practices for protection of low-voltage electrical installations and equipment in MEN systems from transient overvoltages

4262 4262.1 4262.2

Telecommunication overvoltages Part 1: Protection of persons Part 2: Protection of equipment

AS/NZS 1020

The control of undesirable static electricity

1567

Copper and copper alloys—Wrought rods, bars and sections

1866

Aluminium and aluminium alloys—Extruded rod, bar, solid and hollow shapes

2053 2430

Conduits and fittings for electrical installations (all Parts) Classification of hazardous areas (all Parts)

3000

Electrical installations (known as the Australian/New Zealand Wiring Rules)

3004

Electrical installations—Marinas and pleasure craft at low-voltage COPYRIGHT


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AS/NZS 1768:2007

AS/NZS 3100

Approval and test specification—General requirements for electrical equipment

3191

Electric flexible cords

3835

Earth potential rise—Protection of telecommunications network users, personnel and plant

3835.1 3835.2

Part 1: Code of practice Part 2: Application guide

4853

Electrical hazards on metallic pipelines

5000 5000.1

Electric cables—Polymeric insulated Part 1: For working voltages up to and including 0.6/1 (1.2) kV

60950 60950.1

Information technology equipment—Safety Part 1: General requirements

NZS 3501

Specification for copper tubes for water, gas, and sanitation

4403 IEC 60950 60950.1 61643 61643-1 61643-12

Code of practice for the storage, handling, and use of explosives (Explosives Code) Information technology equipment—Safety Part 1: General requirements Low-voltage surge protective devices Part 1: Surge protective devices connected to low-voltage power distribution systems—Requirements and tests Part 12: Surge protective devices connected to low-voltage power distribution systems—Selection and application principles

62305 62305.1 62305.2 62305.3 62305.4

Protection against lightning Part 1: General principles Part 2: Risk management Part 3: Physical damage to structures and life hazard Part 4: Electrical and electronic systems within structures

ISO 10134

Small craft—Electrical devices—Lightning-protection systems

BS 1473

Specification for wrought aluminium and aluminium alloys for general engineering purposes—Rivet, bolt and screw stock

6651

Code of practice for protection of structures against lightning

ANSI/NFPA 780

Standard for the installation of lightning protection systems

ASTM A240M AUSTEL AS/ACIF S009 ITU-T K.44 K.66

Standard specification for chromium and chromium-nickel stainless steel plate, sheet and strip for pressure vessels and for general applications Installation requirements for customer cabling (Wiring Rules) Resistibility tests for telecommunication equipment overvoltages and overcurrents—Basic recommendation Protection of customer premises from overvoltages COPYRIGHT


exposed

to

AS/NZS 1768:2007

UL 1449

188

Standard for Transient Voltage Surge Suppressors

G2 REFERENCED TECHNICAL PAPERS AND PUBLICATIONS 1

HOLLE, R., LOPEZ, R.E. and ZIMMERMANS, D. Updated recommendations for lightning safety–1998. Bulletin of the American Meteorological Society. 1999, vol 80, 1999, pp 2035-2041; see also HOLLE. R et al. Distances and times between flashes in a storm. Proc. Int. Conf. on Lightning and Static Electricty (ICOLSE). Blackpool Set 2003. pp. 8.

2

ANDREWS, C.J., COOPER, M.A., DARVENIZA, M. and MACKERRAS, D. (Eds), Lightning injury: Electrical, medical and legal aspects. Baton Rouge, Florida: CRC Press, 1992. pp. 32.

3

HARTANO, Z.A. and ROBIAH, I. The collection surface concept as a reliable method for predicting lightning strike location. Proc. 25th ICLP, Rhodes, Greece, 2000. pp. 328-333.

4

HARTANO, Z.A., ROBIAH, I. and DARVENIZA, M. A database of lightning damage caused by bypasses of air terminals on buildings in Kuala Lumpur. Proc. 6th SIDPA, Santos, Brazil, 2001. pp. 211-216.

5

D’ALESSANDRO, F. Improved placement of protective lightning rods on structures. International Conference on Grounding and Earthing and 1st International Conference on Lightning Physics and Effects. Brazil. 2004.

6

DARVENIZA, M. A modification to the ‘rolling sphere’ method for positioning air terminals for lightning protection of buildings. Proc. 25th ICLP, Rhodes, Greece, 2000. pp. 904-908.

7

THOMSON, E.M. A critical assessment of the U.S. code for lightning protection of boats. IEEE Transactions on Electromagnetic Compatibility. Vol 33, No. 2. 1991, pp 132-138.

8

RAKOV, V.A. and UMAN, M.A. Lightning: Physics and Effects. Cambridge University Press. Cambridge USA. 2003 (Chapter 4.2).

9

KULESHOV, Y. and JAYARATNE, E.R. Estimates of lightning ground flash density in Australia and its relationship to thunder-days. Aust. Met. Mag. 53, 2004. pp 189196.

10

NASA website, http://thunder.msfc.nasa.gov/data/OTDsummaries/

11

KULESHOV, Y., MACKERRAS, D. and DARVENIZA, M. Spatial distribution and frequency of lightning activity and lightning flash density maps for Australia. Journal of Geophysical Research. Vol 111, D19105, doi:10.1029/2005JD006982. 2006.

12

D’ALESSANDRO, F., JUDSON, W. AND HAVELKA, M. Long-term study of a ground enhancing material. International Conference on Grounding and Earthing and 1st International Conference on Lightning Physics and Effects. Brazil. 2004.

13

KEY, T.S. Diagnosing power quality-related computer problems. IEEE Transactions on Industry Applications. Vol 1A-15, No. 4, July/August 1979, pp. 381-384.

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