GP 12-25 14 November 2003.pdf

Document No.

GP 12-25

Applicability

Group

Date

14 November 2003

Guidance on Practice for Earthing/Grounding

GP 12-25

BP GROUP

ENGINEERING TECHNICAL PRACTICES

14 November 2003

GP 12-25 Guidance on Practice for Earthing/Grounding

Foreword This is the first issue of Engineering Technical Practice (ETP) GP 12-25. This Guidance on Practice (GP) is based on the following heritage documents from merged BP companies:

Amoco ACES EL-GR-00-G EL-GR-00-E EL-GR-00-C

Electrical—Grounding—Guide Electrical—Grounding—Engineering Specification Electrical—Grounding—Installation Specification

ARCO APCES ES 401-90

Electrical Grounding

BP GOMDW EL-SP-4056

Neutral Grounding Resistor

BP Chemical-US CP 17-10-1 CP 17-15-1

Neutral Grounding Resistor Equipment Grounding Details

BP RPSE RP 12-16

Electrical Systems and Installations Earthing and Bonding

Copyright  2003, BP Group. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient’s organization. None of the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of Manager, Standards, BP Group, unless the terms of such agreement or contract expressly allow.

Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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Table of Contents Page Foreword ........................................................................................................................................ 2 1.

Scope .................................................................................................................................... 5

2.

Normative references............................................................................................................. 5

3.

Power system earthing/grounding philosophy ........................................................................ 5 3.1. General....................................................................................................................... 5 3.2. Un-earthed/grounded (isolated) neutral....................................................................... 7 3.3. Solidly earthed/grounded neutrals (for systems below 1000 V) ................................... 7 3.4. Impedance earthed/grounded neutrals (for systems rated 1000 V and above)............ 7 3.5. Earthing/grounding resistors ....................................................................................... 8 3.6. Generators.................................................................................................................. 8 3.7. Substations ................................................................................................................. 9 3.8. High-voltage motors.................................................................................................. 11 3.9. Low-voltage equipment ............................................................................................. 11 3.10. Offshore installations ................................................................................................ 11

4.

Earthing/Grounding methodology......................................................................................... 12 4.1. General..................................................................................................................... 12 4.2. Common earthing/grounding system......................................................................... 13 4.3. Steel structures (onshore)......................................................................................... 14 4.4. Vessels ..................................................................................................................... 14 4.5. Storage tanks............................................................................................................ 14 4.6. Floating roof tanks without rolling ladders ................................................................. 15 4.7. Metallic stacks and towers (not applicable to flare stacks) ........................................ 16 4.8. Non-metallic structures ............................................................................................. 16 4.9. Metallic guy ropes ..................................................................................................... 16 4.10. Pipelines and valves ................................................................................................. 16 4.11. Machine sets with non-electric drive.......................................................................... 17 4.12. Machine sets with electric drive................................................................................. 17 4.13. Road tanker loading bays ......................................................................................... 17 4.14. Rail car loading bays................................................................................................. 18 4.15. Sea tanker loading jetties.......................................................................................... 18 4.16. Portable container filling............................................................................................ 18

5.

Earthing system design........................................................................................................ 19 5.1. Soil resistivity ............................................................................................................ 19 5.2. Earth electrodes........................................................................................................ 19

6.

Earthing/grounding when cathodic protection is applied....................................................... 20

FIGURE 1A................................................................................................................................... 22 Typical Methods of Earthing Electrical Equipment Onshore .......................................................... 22 FIGURE 1B................................................................................................................................... 23 Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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Earthing Principles Onshore ......................................................................................................... 23 FIGURE 2A (1 of 2)....................................................................................................................... 24 Typical Methods of Earthing Electrical Equipment Offshore .......................................................... 24 FIGURE 2B (1 of 2)....................................................................................................................... 26 Earthing Principles Offshore ......................................................................................................... 26 FIGURE 3 ..................................................................................................................................... 28 Bonding Principles Cable Glands.................................................................................................. 28 FIGURE 4 ..................................................................................................................................... 29 Static and Lightning Earthing Systems (Onshore) General Principles ........................................... 29 FIGURE 5 (1 of 2) ......................................................................................................................... 30 Typical Earth Rod and Earth Bar Details....................................................................................... 30 FIGURE 6 ..................................................................................................................................... 32 Typical Connections for Double Roof Tank with 4" Outlet ............................................................. 32 FIGURE 7 ..................................................................................................................................... 33 Typical Connections for Double Roof Tank with 6" Outlet ............................................................. 33 FIGURE 8 ..................................................................................................................................... 34 Typical Roof Connection for Double Roof Tank ............................................................................ 34 FIGURE 9 ..................................................................................................................................... 35 Typical Connections for Single Roof Tank..................................................................................... 35 FIGURE 10 ................................................................................................................................... 36 Typical Roof Connection for Single Roof Tank.............................................................................. 36 Bibliography .................................................................................................................................. 38


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1.


Scope This GP document provides guidance for design, application, and installation of earthing/grounding.

2.

Normative references The following normative documents contain requirements that, through reference in this text, constitute requirements of this technical practice. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this technical practice are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies.

American Petroleum Institute (API) RP 2003

Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents

British Codes and Standards BS 7430 BS EN 1474

Code of Practice – Earthing Installation and equipment for liquefied natural gas – Design and testing of loading/unloading arms

Institute of Electrical and Electronic Engineers (IEEE) IEEE Std. 80 IEEE Std. 141

Guide for Safety in AC Substation Grounding Recommended Practice for Electric Power Distribution of Industrial Plants Recommended Practice for Grounding of Industrial and Commercial Power Systems

IEEE Std. 142

International Electrotechnical Committee (IEC) IEC 60364-5-54 IEC 61892

Electrical installations of buildings - Part 5-54: Selection and erection of electrical equipment - Earthing arrangements, protective conductors and protective bonding conductors Mobile and fixed offshore units - Electrical installations - All parts

National Fire Protection Association (NFPA) NFPA 70

3. 3.1.

National Electrical Code (NEC)

Power system earthing/grounding philosophy General a.

Earthing/grounding and/or bonding for electric power distribution systems should be designed and installed in accordance with relevant parts of IEC 60364-5-54, IEC 61892, API RP2003, NFPA 70, IEEE Standards 80, 141, and 142 or BS 7430. Additional earthing/grounding details are given in PIP ELIGD000 and Appendix A of this GP. Typical earthing and bonding installations for both onshore and offshore are shown in Fig 1A, 1B, 2A, and 2B. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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b.


Earthing/grounding of each part of a power system operating at a specific voltage should be considered individually. If such systems are required to be earthed/grounded solidly, or via an impedance, the neutrals should be connected to a common plant earthing/grounding system. This system should have a resistance to earth that ensures operation of protective devices in the various circuits and should not be in excess of 4-ohms. If connected to a Public Utility supply HV system earth, the combined earth resistance of the Public Utility earth and this earthing/grounding system should be less than 1-ohm. The selection of 4 Ohms as power system resistance to earth is relatively arbitrary and therefore this value may vary depending upon the basic design and safety requirements). However, the possibility of earthing/grounding system resistance variation due to varying soil conditions through time needs to be considered also. The requirement for 1-ohm resistance of a combined Public Utility HV system earth and the interconnected earth for solidly earthed neutrals arises from the need to minimise neutral potentials under Public Utility HV earth fault conditions. A 1-ohm earthing/grounding resistance is not considered mandatory if HV and LV distribution systems are both owned and operated by BP and are earth bonded together with negligible probability of HV faults giving rise to LV system neutral potential rise with respect to 'true' earth. (If true earth represents the potential of the earth's core. This may be transferred to structures if these are not bonded to the site earthing/grounding system).

c.

The neutral connections for earthing/grounding equipment should be provided at generators, transformers or both, which have their neutral brought out, and which are the source of power to the distribution system. If such power sources are delta connected, and do not have neutrals brought out, neutral earthing/grounding may be carried out at other star connected power transformers on the system or by the provision of earthing/grounding transformers.

d.

Power systems operating at a common voltage, which may be normally or abnormally run un-paralleled, should have a neutral earthing/grounding connection facility provided for each system. The neutral earthing/grounding system should be designed to ensure that there is no possibility of inadvertently operating a system with an isolated neutral. Under some circumstances, an alarm for the condition of power system in operation without neutral earthing/grounding should be satisfactory. Typical of such circumstances is the condition whereby a single earthed neutral is desirable (perhaps in order to limit circulating currents between power generators) and there would be a complex and difficult automatic system for earthing/grounding switching in the event of power system disconnection.

e.

The potential between immediate metalwork and a ' true earth'(which may be transferred to the immediate locale typically by telephone services) should be calculated for all representative single phase to earth conditions. Calculations should be presented as support to earthing/grounding system design. The earthing/grounding arrangements of a site are a fundamental feature of safety and it is therefore necessary that the operator is assured that no dangerous potentials can be obtained. It is important that touch potentials be less than 430 V under any single phase-earth fault if there is overcurrent protection arranged to deenergise the live conductor. The 430 V level relates to telephone system limits of exposure. Fundamental safety to personnel should be considered separate to this requirement. IEEE 80 is a useful document for this. If communications and/or protection cables run outside the earthing/grounding system to another remote site, the need for voltage isolation measures at all termination sites should be considered. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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It is common for Control Systems to demand 'clean' or 'reference' earth points which are associated with earthing/grounding electrodes not connected to the general site earthing/grounding system. This practice can give rise to dangerous situations under HV system faults and should be avoided. 3.2.

Un-earthed/grounded (isolated) neutral a.

An un-earthed/grounded or isolated neutral system should be used only subject to specific design approval. Such systems may be used if the highest integrity against faults is required. (e.g. Unearthed systems are often used for oil well drilling power supplies and for d.c. shutdown system power supplies). If unearthed/grounded systems are installed it should be recognised that the power system conductors (phase conductors) could constitute a danger of electrical shock or fire as a result of contact with them and earth. The danger is a result of capacitance coupling of the power system conductors and earth.

b. 3.3.

Unearthed/ungrounded power systems should be provided with an earth fault detection system and operator action to clear any earth faults that occur as a matter of priority.

Solidly earthed/grounded neutrals (for systems below 1000 V) Low voltage systems should have their neutrals solidly connected to the plant earthing/grounding system. Impedance earthing/grounding of systems with a voltage below 1000 V should only be used subject to design approval. Impedance earthed LV systems may be considered if the system will be 3 wire without neutral connections and there is benefit in limiting damage during earth fault conditions. (E.g. If a special voltage 690 V may be used in preference to say 3.3 kV and in effect replaces the HV system in its application). If single-phase supplies are taken from LV systems, the neutral should be solidly earthed at the supply point only in accordance with IEC 60364-5-54. If PME (Protective Multiple Earth) power supplies are taken from a Public Utility it is considered that there should be no need for isolation transformers to achieve the TN-S arrangement within the premises, however, the design should conform to national legislation and this may require that under certain circumstances, isolation transformers are required.

3.4.

Impedance earthed/grounded neutrals (for systems rated 1000 V and above) a.

Neutral earthing/grounding equipment to limit earth fault currents should be provided in the neutral connection to the plant earth system on all high-voltage power systems. Such provision limits the overall transient system disturbance caused by earth faults and also limits the amount of damage caused by this most common type of fault.

b.

c.

If the power system at the specific voltage contains no direct connected generators under any mode of operation, the provision of the earthing/grounding resistor and its earth/ground connection should consider the following possibilities: 1.

At the source star connected transformers.

2.

At other star connected power transformers.

3.

At earthing/grounding transformers.

Neutral earthing/grounding equipment should comprise a resistor with a minimum 10-second fault rating selected to reduce the fault current to the full load rating of the Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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power source transformer. The use of reactors to earth/ground neutral systems should only be used subject to design approval Systems employing reactor earths are uncommon but have been used in some countries or under special circumstances. One special circumstance is if a 'Petersen Coil' is used to avoid a circuit trip due to a transient earth fault e.g. lightning induced flashover in overhead lines. Such special circumstances are unlikely in industrial installations. If reactance earthing/grounding is considered, its use should be tested against the cost/benefit compared with resistance earthing/grounding. Special attention should be taken of the possible tuning effects between the earthing/grounding reactor and the power system. d.

If direct connected generators may be operated in parallel with source transformers, the neutral earthing/grounding arrangements should provide for either system operating independently. Neutral earthing/grounding equipment should be identically rated for all power sources. If resistors are provided, these should comprise 10-second fault rated, solid resistors. Resistors should reduce the fault current to a convenient level one half to full load rating of the lowest rated power source, provided this fault current is sufficient to operate the distribution system earthing/grounding protection and provide suitable discrimination. If normal ratings of the source transformer and parallel running generators are significantly different, the resistor rating selection should be dictated by the requirement to ensure that the most insensitive earth/ground fault protection on any incoming or outgoing circuit operates positively with the smallest possible source of earth/ground fault current connected to the system. If there is a 4-wire supply system, solid earthing/grounding will be provided in order to keep neutral voltages as close to earth as possible. In this case the neutral earthing/grounding arrangement should be simplified by arranging for the neutral to be earthed at the switchboard.

3.5.

Earthing/grounding resistors Earthing/grounding resistors should be provided with insulation suitable for the phase-to-phase voltage of the systems to which they are connected. They should be designed to carry their rated fault current for the times specified in the preceding clauses, without any destructive effect to their component parts. It is expected that resistors will be located in an outdoor environment where transformer earthing/grounding is involved but could be indoors for generators. There is no mandatory requirement for locating these devices indoors other than normal economic factors.

3.6.

Generators a.

Neutrals of generators directly connected to the distribution switchgear in 3-wire systems (i.e. HV systems or special LV systems) may be solidly or resistance earthed/grounded. However, solid earthing/grounding should be limited to generators if the earth/ground fault capacity of the generator is approximately equal to the current level required to operate the distribution system protection. If solid earthing/grounding of the neutral is applied to a system with a (relatively) low capacity generator in parallel with a larger power source, which has resistance earthing/grounding, the generator should have a neutral circuit breaker, which should be automatically controlled. However, where the influence of the fault on other equipment is not unduly compromised by too high an earth/ground fault current, the generator neutral could be allowed to remain solidly connected. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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b.

Resistor earthing/grounding should be used for generators directly connected to distribution switchgear. Resistor should be of the 10-second (minimum) fault rated, solid type, and its resistance should be selected to reduce the fault current to a level between the half and full load current rating of the machine, provided this is sufficient to operate the distribution system protection system selectively.

c.

If generators directly connected to the distribution switchgear are operated in parallel and earthed/grounded either solidly or through resistors, the provision of neutral switchgear for each generator should be considered to avoid third harmonic current flows and their heating effect. If such neutral switchgear is provided, it should be connected and operated in a manner to ensure that only one generator neutral switch is always closed at a time. Neutral switchgear may comprise contactors on resistor-earthed/grounded schemes instead of circuit breakers. The provision of neutral switchgear schemes may be avoided if identical pitch generators are paralleled, or if the provision of neutral earthing/earthing resistors sufficiently reduces the prospective third harmonic currents to acceptable levels. A 1% or less de-rating as a result of third harmonic currents should be considered acceptable. A possibility would be for the neutral of each generator to be permanently earthed/grounded via a resistor where the resistance thus installed would limit the circulating currents to acceptable levels. However, this type of earthing would mean that earth/ground fault current levels on the power system would vary depending upon how many power generators were operating. Such an installation would be acceptable if it showed economic advantages.

d.

If generators are connected to the distribution switchgear system via unit transformers, the generator neutral should be connected to earth/ground via the primary winding of a singlephase distribution transformer. Secondary winding of this transformer should be shunted by a resistor with a resistance value, which is calculated to be approximately the same as the zero sequence capacitance for the generator winding system.

e.

Manufacturers standard arrangements for high impedance earthing/grounding systems for these types of generators will be acceptable. It is expected that the arrangements will be aimed at raising 5 to 10 A under earth/ground fault conditions. Selection of a resistance equal to the system-to-earth/ground capacitance under earth/ground fault conditions will ensure that the system capacitance will discharge energy reasonably and over voltages will be eliminated even for arcing earth/ground faults.

f.

Rated primary voltage of generator earthing/grounding transformers should be taken as the generator phase voltage although voltages of at least 1.5 times generator line to neutral voltages will be acceptable to obtain standard transformers. Transformer ratings should be the product of the primary current and rated primary voltage using a 30-second (6 times overload factor) duty cycle. Conservative sizing of the transformer is preferred as its reactance has little effect on the earthing system. The secondary resistor should have a continuous duty as its size and cost will not normally be significant.

3.7.

Substations a.

Substation safety criteria are different in different countries, with two main approaches. American and European methods assess “step” and “touch” voltages, while the UK, focus is given to evaluating the Rise of Earth Potential. UK method is based on the fact that if the rise of earth potential is within specific safety limits, then the maximum ‘step’ and ‘touch’ voltages are also sufficiently low not to yield any hazard. UK maximum permissible substation potential rise is 430 V. On systems protected by high-speed protection (typically transmission lines), this limit may be increased Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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to 650 V, provided that protection intervention is guaranteed to be of 200 ms or faster. These values, although derived from experience and common practice, are universally adopted by the electricity supply industry in the UK, and are included in EA TS 41-24 and EA S.34. In the UK, sheath to earth voltages at sealing ends and joint positions should be limited to 50 V. Doing so, the system safety is enhanced by complying with the provisions of a functional extra low voltage system. Step and touch potential requirements for American practices are covered in IEEE Std 80. b.

For onshore sites, earth electrodes should be installed in ground below each main substation. Although their main functions are related to electrical safety, the earth electrodes are also used in this case to create a reference point for the supply system. They are not generally required to carry full fault currents, as all power circuits entering and emanating from the substation will have earth continuity provided (i.e. by sheaths/armours and/or separate earth conductors). This screening effect reduces the ground return current and therefore the rise of substation potential, which shall however be assessed and within the safety limits for all soil conditions likely to occur clause.

c.

At least one more than the required minimum number of electrodes shall be installed. By this means, each electrode can be disconnected one at a time for testing without affecting the integrity of the power earthing/grounding system.

d.

A bar of high conductivity hard drawn (hchd) copper should be used as main earthing/grounding terminal, to which earth/ground bars of switchboards and metallic enclosures of low-voltage ancillary equipment (e.g. battery charger, lighting distribution board, etc.) should be connected.

e.

Main switchboards should be connected to substation earthing/grounding system at two separate points.

f.

Main substation earth/ground bar should be connected to one or more earthing/grounding busbars to form a complete ring. Alternatively, stranded copper cable may be used to connect each item to the earthing/grounding busbars.

g.

Earth/ground bars and earthing/bonding conductors should be “sized” respective of minimum sizes prescribed in standards.

h.

Star points of delta-star and similarly connected transformers and all alternator windings should be connected to earthing/grounding system (either directly, via a current limiting impedance and/or earthing/grounding switchgear). Either hard-drawn high-conductivity copper bar or stranded copper cable with green/yellow PVC covering should be used.

i.

Transformer tanks should be connected directly to an earthing/grounding bus-bar.

j.

The neutral of low-voltage three phase four wire systems shall be connected to earth/ground via the relevant main switchboard earth/ground bar. A bolted link should be provided between the neutral bar and the earth/ground bar.

k.

Armouring and metallic sheath of multi-core cables should be bonded to switchboard earth/ground bar, via the termination or gland and gland plate.

l.

Armours and/or metallic sheaths on single-core cables should be bonded together and earthed/grounded at each termination unless cable de-rating is unacceptable. A metallic gland plate provides an adequate bonding connection between sheaths/armours of three phases, but care must be taken to reduce eddy current effects. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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In single-core cable installations, the terminating gland plate surrounds each of the three cable cores. The load current in each of the three phases generates a magnetic field which extends into the volume external to the cable core and therefore into the gland plate. If a ferrous material is used alternating magnetic field induces hysteresis and eddy current effects, which heat the gland plate, with possible insulation stress. Insulation stress effects are reduced by introducing an air gap between adjacent cables (or other means of producing high magnetic reluctance). This reduces the magnetic field intensity and the magnitude of eddy currents. Alternatively, a non-ferrous gland plate could be employed, however, corrosion due to dissimilar metals and the method of sheath earth bonding/grounding need to be carefully considered. Problems associated with eddy currents and hysteresis in single core cables also affect ferrous armour. Therefore single core cables should always have a non-ferrous armour applied. m.

3.8.

3.9.

3.10.

If single point (or cross) bonding is required, unearthed/ungrounded terminations must be insulated and shrouded. Earthed/grounded end should be at the hazardous area end (when applicable). Maximum permissible sheath voltages to earth/ground are specified in referenced industry standards for both sealing ends and joint positions.

High-voltage motors a.

Enclosures of high-voltage motors should be connected directly to local static and lightning earthing/grounding system, or to local earth/grounding electrodes. A common earth/ground electrode system may be used for several motors in same area. When highvoltage motors are in permanent electrical contact with steelwork forming part of common earthing/grounding system, no additional copper bonding connections are required.

b.

Metallic enclosures of local control stations and other associated electrical devices local to the motor, should be bonded to motor enclosure or to earthing/grounding system to which motor is connected. This may be achieved through the mounting bolts and earthed/grounded steelwork.

Low-voltage equipment a.

Enclosures in permanent direct metallic contact, e.g. via pump bedplates, vessels, piping, structures, etc., with earthed/grounded plant steelwork require no further connection to earthing/grounding system.

b.

Enclosures not in direct metallic contact with earthed/grounded plant steelwork or pipework should be bonded to static and lightning earthing/grounding systems or to the adjacent earthed/grounded steelwork by means of a copper conductor.

Offshore installations a.

Earthing/grounding should comply with local, national, and international industry standards. e.g. the IEE Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations, Section 2.

b.

Enclosures of each main generator, transformer and switchboard should be solidly bonded to steel structure.

c.

Location of earthing/grounding connections should be visible, easily accessible and protected against corrosion.

d.

Additional bonding connections should be made at welded bosses local to equipment to avoid long cable runs. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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4. 4.1.


e.

Low-voltage electrical equipment in contact with earthed/grounded steelwork and protected in accordance with IEE Recommendations for the electrical and electronic equipment of mobile and fixed offshore installations or equivalent standard do not require additional bonding connections.

f.

Parts of the plant not in metallic contact with an earthed/grounded structural steelwork should be bonded to platform steelwork. A separate earthing/grounding system for lightning and static protection is not required.

g.

A reel of single core flexible cable, of suitable size, should be installed in an area classified as non-hazardous by helideck, for bonding helicopters to platform during refuelling. Reel should incorporate a slip ring to pick up connection to earth/ground.

Earthing/Grounding methodology General a.

b.

Earthing/grounding conductors may consist of one or a parallel combination of: 1.

Dedicated earthing/grounding conductor

2.

Cable armouring or metallic sheath

3.

Rigid screwed conduit

4.

Earth core within a multi-core cable

5.

Structural steelwork.

Sole reliance on cable armouring and/or metallic sheathing should only be made if it is adequately fault rated as a protective conductor. This is a fundamental requirement, which should be met in case of faults within electrical equipment. In the case of faults within cable, it may not be possible to have armour which is fully fault-rated, particularly if armour is steel braid type. However, the advantages of steel braid armour over steel wire armour on cables used offshore are considered to outweigh this possible disadvantage.

c.

Structural steelwork may be considered as a protective conductor provided it is part of the common static and lightning earthing/grounding system and there is a permanent metallic path for fault current via such steelwork and other earthing/grounding conductors back to the power supply. Use of structural steelwork as part of earthing/grounding system and as a path for fault current is acceptable practice, particularly offshore where welded steel structure of platform is regarded as 'earth'. The term only applies to permanent structures such as an offshore platform or an onshore process plant. Connection points should be made available for periodic testing of electrical integrity and current carrying capacity of steelwork alone. When a complex structure is used as (one of) the protective conductors, its impedance cannot be readily calculated. However, it can be assumed that a very large structure such as an offshore platform has negligible impedance compared with that of cable armouring. For the purpose of calculations, the impedance of the return path through such a structure can be assumed to be zero. A consistency in choice of protective conductors for a particular plant should be maintained when extensions to the plant are designed.

d.

Cross sectional area of earthing/grounding conductors and bonding connections between exposed and extraneous conductive parts should be capable of carrying earth fault current, or an appropriate fraction of total fault current, for the duration of the fault without damaging conductor or associated insulation. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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e.


The cross sectional area of protective conductors should comply with the minimum requirements determined by methods given in clause 543.1of IEC 60364-5-54. If 'protective conductor' consists of a large steel structure such as an offshore platform, or an extensive mesh of interconnected copper earthing/grounding conductors, its earth impedance may be ignored for fault current calculation. However, for short cable circuits, earth impedance may be of comparable magnitude to that of the cable sheath and should be considered. Also, if evaluating its earth potential rise under fault conditions, the voltage rise across such impedances should be accounted for. Minimum sizes without mechanical protection are prescribed. Use of multi-stranded protective conductors should be considered (e.g. 2.5mm2, 7/0.67). In practice, it may be desirable for any one project, to limit the number of sizes of protective/bonding conductors to make purchasing more economical and/or to simplify design and installation.

f.

Buried earthing/grounding conductors should normally be bare copper cable. However, when there is a likelihood of corrosion (for example particularly acidic soil), an overall protective covering, e.g. green/yellow PVC, should be provided. Note: When conductor is used as (part of) an earth/ground electrode, its earthing/grounding performance will be reduced by the protective covering (no transverse current leakage).

4.2.

g.

Joints in protective conductors should be avoided.

h.

Design of high voltage cable glands or terminations should incorporate a lug for bonding the cable armour to earth, or to equipment enclosure.

i.

If a low-voltage cable enters a metallic enclosure, a bonding connection between the gland and enclosure is not required, provided there is no electrical discontinuity of the enclosure.

j.

If non-metallic enclosures are used, means shall be provided to preserve the electrical continuity of the armouring and/or metallic sheaths of cables.

k.

Enclosures, of any type, should be maintained at ground potential or be protected by a system of double insulation.

Common earthing/grounding system a.

Protection against static electricity and lightning is by a common earthing/grounding system to which all structures and items of process equipment are connected either directly or indirectly. Conductors designed according to fault current requirements are normally adequate for carrying short time surges caused by lightning. BS 5958 treats the control of undesirable static electricity. Guidance is given in BS 6651 on how the requirement for lightning protection for various structures can be assessed. For American applications, lightning protection requirements for buildings, miscellaneous structures, stacks and structures containing flammable liquids and gasses are treated in NFPA 70 and API RP 2003.

b.

Common earthing/grounding systems should comprise either of, or a combination of following elements: 1.

Buried copper strip or single core copper conductor [e.g. 70 mm2 (No. 2/0 AWG)].

2.

A steel structure to which, all equipment to be earthed/grounded is in direct contact or is bonded. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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4.3.

4.4.

4.5.


c.

Onshore earthing/grounding systems should comprise independent connections to earth/ground electrodes (minimum of 2).

d.

Static and lightning earthing/grounding systems should be connected to power earthing/grounding system at two points for facilities located in proximity to process area substations.

Steel structures (onshore) a.

An earth lug or boss should be welded to main columns at regular intervals and at specific heights above ground level.

b.

If a steel structure forms common earthing/grounding system, or part thereof, each earth lug or boss should be directly connected to an adjacent earth electrode.

c.

If steel structures do not form part of the common earthing/grounding system and/or direct earthing/grounding for lightning, each lug or boss should be connected to common earthing/grounding system.

Vessels a.

When vessels are mounted directly on, and in metallic contact with, an earthed steel structure, no further bonding is necessary. However, additional earthing/grounding may be required for lightning protection or to reduce overall earth/ground impedance.

b.

When vessel mounting is insulated from steelwork by materials of poor conductivity such as wood, concrete, rubber etc., two earthing/grounding connections should be taken from vessel to common earthing/grounding system. If vessel is remote from plant and connection to the common earthing/grounding system is impractical, two connections should be taken from vessel to separate earth/ground electrodes and the resistance to earth of each electrode should be less than 10 ohms.

c.

Looping earthing/grounding conductors between vessels is permitted provided a connection is taken from each end of ' looped'system to general earthing/grounding system or earth electrodes.

d.

Direct earthing/grounding for lightning should be applied when necessary.

e.

Earthing/grounding connections should be copper conductor and sized as specified in section 4.1.

f.

If a vessel has insulation and an outer metal cladding or wire reinforcement, the metal cladding or reinforcement should be electrically continuous and bonded to the vessel.

g.

Armouring of armoured cables, which enter the vessel should be bonded to shell at point of entry.

Storage tanks a.

Tanks up to 30 m diameter should be provided with two, and tanks over 30 m diameter should be provided with three, equally spaced earthing/grounding bosses these bosses should be positioned near base of the tank.

b.

Earthing/grounding lugs or bosses on tanks should be connected to separate earth/ground electrodes. Earth electrodes should be located close to tank base.

c.

Earth electrodes common to a group of tanks should be installed so that each tank has, as a minimum of two paths to earth/ground. This ensures that during testing of one electrode, the tank will remain earthed/grounded by a system with an earth resistance of appropriate value.

d.

Tank internals, e.g. mixers, gauge floats and sling arms, should be bonded to tank shell at one or more locations depending on size of internal object. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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Bonding can preferably be achieved by direct bolting. Care should be taken to avoid the formation of re-entrant loops. e.

On floating roof tanks, multiple shunt connections comprising stainless steel strips (e.g. 50x0.6x400 mm long), should be provided between the floating roof and the tubbing shoe at adequate intervals around the roof periphery or one per pantograph if these are fitted.

f.

If high winds prevail shunt strips may be replaced with cables bolted in position. Shunts should be fitted above the sealing arrangement.

g.

Spacing of shunt connections should avoid risk of discharge from roof to tank wall directly across the gap rather than via a shunt, due to formation of re-entrant loops. The risk is increased when path length of the loop exceeds eight times the width of the open side. In this case the maximum loop length is half the peripheral distance of the closed path between adjacent shunts. Although hanger linkages on the pantograph offer an earthing/grounding path from the floating roof to the shell, they can be a source of arcs during a lightning strike if placed too close together (accepted minimum separation on an empirical basis is 1m). The arcs occur from sharp points or joints, but may be avoided installing short insulated jumpers around each pinned hanger joint and covering sharp points of hangers with insulating material.

h.

When a rolling ladder is installed, a flexible copper-bonding conductor [e.g. 35 mm2 (No. 2 AWG)] should be applied across the ladder hinges, between ladder and tank top, and between ladder and floating roof. (This is in addition to shunt connections.) The earthing/grounding provisions assume that the ladder is articulated in the centre in a manner, which allows continuity of earthing/grounding. If there may be appreciable movement of the ladder at either the roof or rim, which makes earth/ground bonding non-feasible, another approach may be necessary. No specific advice is available for this eventuality, but past instances have led to arrangements, which stowed ladders away from the roof when not in use.

i.

When a rolling ladder is not installed, a flexible earth/ground cable [e.g. 70 mm2 (No. 2/0 AWG)] should be installed along roof drain of tank. Previously, fitting of flexible earthing/grounding cable between the floating roof and tank shell was practised as a supplement to the pantograph earthing/grounding system. However, experience showed that these cables frequently became entangled and broke with the movement of the roof. Also, the tangling of the cable produced re-entrant loops. Thus, this practice is no longer recommended. As an alternative to the provisions of this section, lightning protection may be achieved by using an air termination network. A suitable system is described in BS6651.

4.6.

Floating roof tanks without rolling ladders a.

Earthing/grounding of floating roofs should be provided by flexible earth cables [e.g. 70 mm2 (No. 2/0 AWG)] neoprene covered) laid along the roof drain. One earth cable should be installed for each roof drain.

b.

Roof connections for double and single roof tanks should be to a similar material specification to tank itself.

c.

Roof connections should be sited close to roof drains with suitable facilities to securely fix earth/ground cable to roof. Connections should be protected against corrosion.


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4.7.

4.8.

4.9.


d.

A detensioner should be installed if the earth/ground cable exits the roof connection. The earth/ground cable should be fixed to the roof drain using suitable clips (e.g. stainless steel cable ties) with due allowance for movement of all swivels.

e.

When earthing/grounding cables have been installed, tests should be undertaken to ensure a low resistance (less than 0.1 ohm) between roof and wall.

Metallic stacks and towers (not applicable to flare stacks) a.

No air terminals or down conductors are required when metallic stacks and/or towers are of welded, bolted or riveted construction.

b.

Two earth/ground lugs or bosses located near the bottom and on opposite sides of the equipment, should be provided and independently connected either to general earthing/grounding system or to two earth electrodes located near base of equipment. The method used will depend on the need for protection against direct lightning strokes.

c.

Armouring of cables entering metallic stacks should be bonded to stack at point of entry.

Non-metallic structures a.

In classified areas, steelwork such as stairways, cable racks, handrails, etc., mounted on or attached to non-metallic structures should be bonded to general earthing/grounding system directly.

b.

When steelwork is not bonded as an extraneous conductive part an alternative to bonding should be carried out to ensure: 1.

That steelwork is sufficiently isolated from other potential lightning discharge carrying conductors to ensure no side flashing will occur.

2.

No static build up above minimum ignition energy of hydrocarbons present is possible.

c.

Non-metallic structures less than 9 m (30 ft.) in height do not require lightning protection or earthing/grounding. When greater than, 18 m (60 ft.) in height, non-metallic structures should be provided with lightning protection.

d.

Lightning protection for non-metallic structures between 9 m (30 ft.) and 18 m (60 ft.) in height will be determined taking into account heights of adjacent structures, nearness of flammable materials, consequences of damage, etc. Standards of the country of installation should be referred to for guidance.

Metallic guy ropes Metallic guy ropes used for supporting metallic or non-metallic stacks or other structures should be bonded at their upper ends to stack or structure if metallic, or to lightning protective system if non-metallic stacks or structures. Lower end of guy ropes should be directly earthed/grounded. If a guy rope is comprised of two ropes in parallel, they should be bonded together at the upper and lower ends and then treated as one rope.

4.10.

Pipelines and valves a.

It is not necessary to bond across flanges of pipe joints, nor earth/ground pipelines separately, since adequate earthing/grounding is provided via vessels and other equipment to which pipes are connected. However, long pipelines crossing open ground should be earthed/grounded at or near plant boundary. Spindles of ball valves should be bonded to their pipeline if the ball valve is controlling a two-phase mixture, the valve is not fitted with a special earthing/grounding washer, and/or the valve is immediately downstream of fine filtration facilities.


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Charge generation is increased in a two-phase mixture due to the increased surface area of the individual phases, and is increased downstream of fine filters due to extensive charge separation. b.

An exception to valve earthing/grounding above should be made if liquefied petroleum gas is handled, such as tanker loading bays. Particular attention should be paid to earthing/grounding across flexible connections. LPG is contained in closed systems and there is no danger as gas concentration will be well above upper flammable limit. However, release and/or rapid expansion of LPG results in the formation of a mist capable of generating static electricity. Therefore, danger exists where LPG is handled and there is risk of static build up when metallic parts are not adequately bonded.

4.11.

4.12.

Machine sets with non-electric drive a.

When driving and driven machines are in direct metallic contact with an earthed/grounded steel structure, no additional earthing/grounding is required.

b.

When driving and driven machines are bolted to a common metallic bedplate on a concrete or other poorly conducting foundation, one connection should be taken from bedplate to earthing/grounding system.

c.

When driving and driven machines are on separate bedplates mounted on separate concrete plinths or other poorly conducting material, bedplates should be bonded together and one connection (minimum) taken to general earthing/grounding system.

d.

Earth connections should be copper conductor of appropriate size [e.g. 35 mm2 (No. 2 AWG)].

Machine sets with electric drive No specific earthing/grounding connections are required when driven machines are mounted on a metallic bedplate providing for dissipation of static.

4.13.

Road tanker loading bays a.

Each loading gantry should have a connection to earth/ground at each end. Connections may be direct to independent earth/ground electrodes or to general earthing/grounding system of adjacent plant. Individual resistance to earth/ground should be less than 10 ohms.

b.

Product pipelines should be bonded to loading gantries either by pipe clamps or pipe flange bolts.

c.

Loading hoses should be electrically continuous from product pipeline to loading nozzle or flange. A bond should be installed across each swivel joint in metallic loading arms.

d.

Loading bays should be provided with a number of flexible copper connections bolted to an earth/ground boss in the loading gantry at one end, and have a robust earthing/grounding clamp for bonding tanker to earth during loading and discharge.

e.

For LPG loading bays, vehicle earthing/grounding arrangements should ensure that earth/ground connections are completed in areas classified as non-hazardous via an earthing/grounding switch in a suitably certified enclosure.

f.

Earthing devices and interlock arrangements should be provided so tanker loading or discharge is possible only when effective earthing/grounding has been achieved.

g.

Connections to earth should be stranded copper conductor of suitable section [e.g. 35 mm2 (No. 2 AWG)]. Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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4.14.

4.15.


Rail car loading bays a.

Requirements of section 4.11 of this Guidance on Practice apply, except that a flexible cable connection is not required since the contact between the tanker wheels and the track provides sufficient earthing.

b.

Rails should be bonded together and connected to loading gantry or earthing/grounding system.

c.

Insulated joints should be inserted in rails to isolate loading gantry from rail system to guard against stray currents.

d.

On electrified rail systems live contact rails or overhead conductors should terminate outside loading compound.

Sea tanker loading jetties a.

Earthing/grounding and bonding systems on tanker loading jetties should comply with International Safety Guide for Oil Tankers and Terminals and BS EN 1474 or equivalent.

b.

Loading arms on jetties should have an insulated flange inserted in the ' outboard'end to prevent flows of stray current and hence potential sparking when connection is made. That section of loading arm, which is ' downstream'of the flange should be connected to the ship and the ' upstream'section connected to the jetty earthing/grounding system.

c.

For jetties using hose loading gantries, the jib handling rig hook should be insulated.

d.

Breasting dolphins, fenders and quays having metallic parts connected to jetty earthing/grounding system should be protected from direct contact with ships hulls (e.g. by wooden linings).

e.

Mooring dolphins should be insulated from jetty earthing/grounding system or located in a safe area but insulated from the shore earthing/grounding system.

f.

Insulating flanges should be inserted in each pipeline at shore end. Jetty structure and earthing/grounding system should also be isolated from shore earthing/grounding system.

g.

Steel jetties, which have impressed current cathodic protection cables entering jetty area from shore should have their metallic sheaths and/or armours bonded to jetty earthing/grounding system but isolated from shore earthing/grounding system. An alternative insulation arrangement should be used when this connection presents excessive diversion of impressed current, resulting in protection potential not being maintained. It is no longer considered safe practice to bond the tanker to the jetty earthing/grounding system, because of potential difference, which may exist due to cathodic protection of the ship's hull and/or the jetty structure. When such a bond is broken, an incendive spark can be produced (see International Safety Guide for Oil Tankers and Terminals). The resistance across insulated flanges should be at least10x103 ohm when new, but in service a resistance as low as 103 ohm is acceptable. Periodic measurements of the insulation resistance should be carried out with a multi-meter, which should be of a type of protection suitable for the classification of the area.

4.16.

Portable container filling a.

In addition to earthing/grounding items already mentioned in this section, the following should be electrically continuous and bonded to the common earthing/grounding system or to an earthing/grounding system installed specifically for the filling installation: 1.

Weighing machine platforms and bases Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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5. 5.1.


2.

Conveyor tracks

3.

Other ancillary equipment.

b.

Filling hoses should be electrically continuous. A separate flexible earthing/grounding lead with a robust clip or clamp should be provided for connecting to drum during filling. This arrangement is not necessary for filling of pressure containers.

c.

An earthing/grounding unit, interlocked with product delivery pumps should be installed.

Earthing system design Soil resistivity Resistance of an earth/ground electrode of given dimensions and geometry is dependent on soil resistivity, which varies according to the type of soil, moisture content, degree of compaction and chemical composition. Resistivity values for various types of soil is given in table 1 of BS 7430.

5.2.

a.

Resistivity measurements should be made at proposed electrode locations. More than one measurement traverse should be considered to account for local resistivity variations in soil adjacent the site. These measurements and subsequent electrode design should be carried out at an early stage of project so that electrode locations are compatible with plot layouts, foundations, etc.

b.

A site which is not naturally well drained should be chosen for electrodes. However ground need not be waterlogged. Locations where ground is kept moist by water flowing over it should be avoided. Dry, sandy or rocky ground should also be avoided.

c.

Effect of possible seasonal increases in electrode resistance due to drying out or freezing of ground shall be taken into account. Whenever possible, earth electrode should be installed deep enough to reach water table or permanent moisture level, deeper than frost is likely to penetrate and to reach stable ground conditions.

d.

Bentonite or similar material may be used to improve contact efficiency in difficult ground conditions.

Earth electrodes a.

Earth/ground electrodes consist of a number of rod sections coupled together and driven vertically into the ground/earth. A perimeter electrode may also be installed depending on the extent of the plant. A number of rods may need to be connected in parallel to obtain required electrode earth impedance. Distances between rods should be greater than their depth.

b.

When earth/ground rods are installed at an angle (15-20 degrees) due to underground obstructions, they should equal vertical rod length. In soils of uniform resistivity, it is more economical to install a number of rods connected in parallel than to attempt to obtain the required resistance from a single deeply driven rod. However if the rods are driven too closely together, their effectiveness is reduced due to proximity effects. In practice, a separation of the order of 3 m should be considered.

c.

Earthing materials should comply with requirements of national standards or codes of practice employed in the country of installation. Materials for grounding electrodes (and conductors) should be selected to meet requirements of soil resistivity and corrosion resistance based on both soil analysis and facility site experience. See table 2 of BS 6651 for recommended materials for the manufacture of lightning protection components Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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6.


d.

Due to galvanic coupling between copper and steel, an extensive copper grounding system grid may accelerate corrosion of steel piping and other buried structures that are connected to the system. Under this condition, galvanized steel ground rods and insulated or coated copper conductors may be used. However, earth electrodes should not be allowed to corrode or rust and reduce their effectiveness, and use of insulated or coated conductors should not compromise overall system safety by increasing its overall earth impedance.

e.

Earth/ground rods may be of the following materials providing that they are entirely suitable for the application and ground conditions: 1.

Solid hard drawn copper

2.

Phosphor bronze

3.

Copper clad steel

4.

Stainless steel

5.

Galvanised steel

6.

Cast iron pipe.

f.

Earth rods are generally available in standard lengths. Copper is commonly used for grounding system grids because of resistance to corrosion as well as high conductivity. Copper clad steel rods are available in specific lengths. An appropriate minimum diameter should be considered.

g.

Copper clad steel rods should be of molecularly bonded type. Minimum thickness of copper recommended in relevant standards should be respected and coatings should be maintained over entire length of rod, including threaded portion. Couplings should be made of silicone aluminium bronze and should be of sufficient length to enclose completely threads with rods in end-to-end contact.

h.

Each earth/ground rod should be protected against corrosion and terminated in an inspection pit.

Earthing/grounding when cathodic protection is applied a.

Bonding between a cathodically protected pipeline and/or storage tank and any earthing/grounding system can reduce efficiency of an impressed current cathodic protection system by diverting flow of impressed current. Cathodically protected sections of pipeline should be isolated from unprotected sections and from any earthing/grounding systems. Presence of electrically operated valves may make complete isolation impractical.

b.

Isolation can be achieved when electrically operated valves or other devices are installed by inserting insulating bushings between cable gland and device. Device should be bonded to pipeline. It should be established that a return earth/ground path via the pipeline is sufficient to ensure correct operation of protective devices on supply to equipment.

c.

Sometimes it is necessary to earth/ground buried pipelines which are impressed current cathodically protected, e.g. to alleviate the effects of local overhead power lines. In such cases pipeline should be earthed/grounded by polarisation cells or alternatively by use of earthing/ground rod materials of a suitable galvanic potential.

d.

When plant is cathodically protected, either by sacrificial anodes or by an impressed current system, the design of earthing/grounding systems should be agreed with suppliers and designers of cathodic protection system.


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Annex A

(Informative)

Earthing/grounding details Note: Some details forming part of this Annex contain references to heritage BP RP 12-16. It is intended that these details will be revised in a future update of this GP 12-25, but until then, if required, it will be possible to check the references with the heritage document.


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NOTES: 1. Numbers shown thus 2.2.1 indicate the relevant clause of RP 12-16. 2. For earthing details see figure 5.

FIGURE 1A Typical Methods of Earthing Electrical Equipment Onshore Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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NOTES: 1. Apparatus which is bonded to an earthed steel structure by means of its holding down bolts or by welding requires no additional bonding connection. 2. Apparatus which is otherwise isolated from the earthing system should be bonded as shown. The cross section of the earth bond should be in accordance with sub-section 2.1.5 or 2.1.6 of RP 12-16. 3. If the plant/apparatus is remote from the sub-station the earthing systems need not be interconnected. In this case earth fault current will return via the cable armour or earth core. Account should be taken of parallel earth paths via the armour of other cables.

FIGURE 1B Earthing Principles Onshore


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FIGURE 2A (1 of 2) Typical Methods of Earthing Electrical Equipment Offshore Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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NOTES 1. Fixed off shore installations are grounded or earthed into the seabed by steel pipes, well conductors etc. Mobile units are earthed by the conductivity of the seawater 2. All topsides metalwork is either bolted or welded onto the structural support steelwork to extend the earth continuity to the extremities of the installation. 3. All tanks, vessels, pipe, duct , and traywork, handling and access ways are bolted or welded to the structural steelwork to ensure that any static potential build up will be avoided by continuous drain to ‘earth’ 4. All live conductors are protected against touching by personnel, corrosion and possible mechanical damage 5. All metallic enclosures, gland plates and cables glands have earth continuity by physical connection. 6. All insulated enclosures have earth continuity facility for internal metal work, via cable armouring system. 7. Neutrals of low voltage systems are earthed at the switchboard 8. Exposed metallic parts of electrical equipment are in electrical contact with the structure by holding down bolts, bedplates etc, but additional bonding connections are required where indicated 9. Numbers shown thus :- 2.2.1 indicate the relevant clause of RP 12-16. 10. This drawing is based on fig 2.1 of the IEE recommendations for the electrical and electronic equipment of mobile and fixed offshore installations.

FIGURE 2A (2 of 2) Typical Methods of Earthing Electrical Equipment Offshore


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FIGURE 2B (1 of 2) Earthing Principles Offshore


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NOTES

1. The steel structure of the platform jacket and modules forms the earthing system and the principle protective conductor

2. Exposed metallic parts of electrical (and non-electrical ) apparatus are normally effectively

bonded to the steel structure by holding down bolts, bedplates welds etc But additional bonding connections are required where indicated 3. All tanks, vessels . pipes, ducts, traywork, handrails, etc. are bolted or welded to the steel structure ensuring electrical continuity throughout installation and preventing any build up of static. 4. Arrows indicate route of current for earth fault at point ‘F’ in addition fault current will return via pipe work and the armouring of other cables.

FIGURE 2B (2 of 2) Earthing Principles Offshore


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NOTES

1. Where cables enter a metallic enclosure it is not necessary to install additional bonding connections

between glands or between glands and the equipment, provided that the entry is tapped or a ‘ star ‘ washer is fitted under the backnut to ensure good electricity 2. Where cables enter a non-metallic enclosure means should be provided for bonding the cable armour to each other and to earth. ( normally achieved with an earth continuity plate located inside the enclosure ).

FIGURE 3 Bonding Principles Cable Glands


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Notes 1. Numbers shown thus 3.4.1 refer to relevant clause of RP 12-16 2. For earthing details refer figure 5 3. This diagram is intended to illustrate a typical static and lightning earthing scheme. In practice the design of the earthing system will depend on site conditions, soil resistivity, plant layout, need for lightning protection, etc

FIGURE 4 Static and Lightning Earthing Systems (Onshore) General Principles


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FIGURE 5 (1 of 2) Typical Earth Rod and Earth Bar Details


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NOTES

1. Minimum size and type or earth conductor for general use shall be 25 x 3 copper tape or 70mm2 bare

copper cable. Minimum size and type of flexible earth conductor should be 35mm2 276/0 single core 600/1000 volt grade. Whilst copper conductors are preferred other equivalent materials may be used subject to approval. For mechanical protection or where corrosive conditions may exist an overall covering, e.g. pvc, shall be provided 2. If rods cannot be driven to a suitable depth their installation must be preceded by drilling. Care must be taken to ensure the satisfactory consolidation of the backfill soil. Resistivity can be reduced by the use of a suitable soil conditioning agent. Salt agents should be avoided as they encourage corrosion. 3. The use of cad weld jointing technique is permitted for earth conductor joints, and is the preferred method for underground joints. 4. Earthing bosses welded to pressurised vessels shall be of the same material quality as the vessel

FIGURE 5 (2 of 2) Typical Earth Rod and Earth Bar Details


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FIGURE 6 Typical Connections for Double Roof Tank with 4" Outlet Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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FIGURE 7 Typical Connections for Double Roof Tank with 6" Outlet Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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FIGURE 8 Typical Roof Connection for Double Roof Tank Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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FIGURE 9 Typical Connections for Single Roof Tank


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FIGURE 10 Typical Roof Connection for Single Roof Tank Downloaded Date: 6/17/2008 10:11:56 PM The latest update of this document is located in the BP ETP and Projects Library

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FIGURE 11 Typical Cable Detensioner


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Bibliography [1]

BS 5958

Code of Practice for the control of undesirable static electricity

[2]

BS 6651

Code of Practice for protection of structures against lightning

[3]

BS 7671

Requirements for electrical installations. IEE Wiring Regulations. Sixteenth edition

[4]

EA TS 41-24 Guidelines for the design, installation, testing and maintenance of main earthing systems in substations

[5]

EA S.34

[6]

IEE Recommendations for the electrical and electronic equipment of mobile and fixed offshore installations

[7]

NFPA 780 Standard for the Installation of Lightning Protection Systems

[8]

PIP ELIGD000

A guide for assessing the rise of earth potential at substation sites

Grounding Installation Details


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GP 12-25 14 November 2003.pdf

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