AODC035 - Safe use of electricity underwater.pdf

IMCA

Guidance Note No: AODC 035

Code of Practice for

The Safe Use of Electricity Under Water

September 1985 This Code was reviewed in 1993 and again in 1996 and no amendments were considered necessary. Its content therefore remains as relevant now as when it was first published

The information contained herein is given for guidance only and endeavours to reflect best industry practice. For the avoidance of doubt no legal liability shall attach to any guidance and/or recommendation and/or statement herein contained.

The International Marine Contractors Association Incorporating AODC and DPVOA

Carlyle House, 235 Vauxhall Bridge Road, London, SW1V 1EJ, UK Tel: +44 (0) 20 7931 8171 Fax: +44 (0) 20 7931 8935 E-mail: [email protected] Website: http://www.imca-int.com

Recognition This Code: ♦

has been "Accepted by the United Kingdom Department of Energy for Use in Underwater Operations".

♦

has been "Accepted by the United Kingdom Department of Transport for Use in Underwater Operations".

♦

is regarded "As a Recommended Standard for the Safe Use of Electricity Underwater" by the Norwegian Petroleum Directorate.

♦

is "Acceptable to the Canada Oil and Gas Lands Administration for Use in Underwater Operations".

♦

has been "Recognised by the Industrial Inspectorate of the Department of Labour in Ireland for Use in Underwater Operations".

Every reasonable effort has been made to ensure that the guidance given in this document is based on the best knowledge available at the time of finalising the text. However, no responsibility of any kind for any injury, delay, loss, damage or cost whatsoever, however caused, resulting from the use of the document can be accepted by AODC or any of the Companies. Organisations or individuals involved with its preparation or publication. Any person or organisation using the information within the document should satisfy itself that the guidance is directly applicable to their specific situation and should seek specialised electrical advice if any doubt exists.

Code of Practice for the Safe Use of Electricity Under Water Contents Foreword Scope Introduction Discussion

3 5 6 7

GLOSSARY

11

SECTION 1 - APPLICATIONS OF ELECTRICITY UNDER WATER

14

1.0 1.1 1.2

14 17

1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 1.12 1.13 1.14 1.15

Introduction Deck Compression Chamber (DCC) and Hyperbaric Evacuation System Transfer Chamber, Submersible Compression Chamber (SCC) and Diver Lock-Out Submersible Diver Heating (Electrical) Hand Held Equipment Habitat Wet Welding and Cutting Sea Bed Equipment Impressed Current Systems Remotely Operated Vehicle (ROV) Manned Submersible Umbilicals High Power Equipment Surface Electrical Distribution Fresh Water Explosives

18 20 21 22 24 25 26 27 28 29 30 31 32 33

Summary Table of Active Protection Practices

34

SECTION 2 - INSTALLATION AND USE OF EQUIPMENT

38

2.0

38 38 38

Introduction 2.0.1 Protection Against Electric Shock 2.0.2 Passive Protection

2.0.3 2.0.4 2.0.5 2.0.6

2.0.2.1 Insulation 2.0.2.2 Fixed Barrier 2.0.2.3 Protective Clothing 2.0.2.4 Shielding 2.0.2.5 Suitability of Earthing Active Protection 2.0.3.1 Residual Current Devices 2.0.3.2 Line Insulation Devices Toxicity of Materials Installation Practice Batteries

AODC 035 – September 1985

38 39 39 39 40 40 40 42 42 42 43

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2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15

Deck Compression Chamber (DCC) And Hyperbaric Evacuation System Transfer Chamber, Submersible Compression Chamber (SCC) and Diver Lock-Out Submersible Diver Heating (Electrical) Hand Held Equipment Habitat Wet Welding And Cutting Sea Bed Equipment Impressed Current Systems Remotely Operated Vehicle (ROV) Manned Submersible Umbilicals High Power Equipment Surface Electrical Distribution Fresh Water Explosives

46 48 49 50 52 54 57 59 61 63 63 63 63 64 64

SECTION 3 - SYSTEM DESIGN 3.0 3.1 3.2

3.3

Introduction New Techniques Reduction In Toxicity Hazard 3.2.1 Selection Of Insulating Material 3.2.2 Terminal Blocks And Circuit Boards 3.2.3 Quantity Limitation 3.2.4 System Design Protection Against Explosion And Fire Risk

65 65 65 65 66 66 66 66

APPENDICES Appendix A:

Bibliography

69

Section 1: Some Relevant Codes Of Practice, Recommendations, Standards

69

Section 2: Some Technical Reference Papers

71

Appendix B:

Useful Formulae

73

Appendix C:

Detailed Graph Of Allowable A.C. Current In The Body

75

Appendix D:

Earth Fault Current Restriction

77

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FOREWORD In September 1982 the UK Department of Energy published a 'Code of Practice for the Safe Use of Electricity Under Water'. This was intended as general guidance on the safe use of electricity under water and not as a mandatory or contractual document. The Department of Energy funded a number of research projects which formed part of the background to the 1982 code. Other projects were still in hand at the time of publication, and various assumptions and safety factors were built into that code. Subsequent experimental results and work carried out elsewhere, showed that some of these were unnecessarily conservative, and early in 1983 the Department of Energy invited industry to take over responsibility for development of the code, taking into account new material and experience of working with the 1982 document. AODC accepted this invitation. This new Code is based on the earlier document but its format has been altered to make it more readily usable by designers, manufacturers, users and others. It incorporates the latest information available from known research projects and the experience of a wide cross section of interested organisations. The preparation of this code was overseen by a steering group comprising representatives from a wide cross section of interested organisations. These were: T. A. Hollobone C. W. Logan L. E. Virr

Chairman Association of Offshore Diving Contractors Admiralty Research Establishment/ Experimental Diving Unit R. M. Mavin Department of Energy, Diving Inspectorate T.D. White Department of Energy, Electrical Inspectorate M. Robertson Department of Transport (Marine Directorate) L. A. Mollinder/ J.F. Wilson Det Norske Veritas (representing the certifying authorities) F. D. S. McCrudden Institution of Electrical Engineers R. J. Moulton MaTSU/UKAEA W. B. Norton NIFO B. A. Mathisen Norwegian Petroleum Directorate R. W. Barrett Underwater Engineering Group D. L. Judd UKOOA, Engineering and Development Committee J. E. Hendrick UKOOA, Diving Advisory Committee

The first draft of this code was prepared by a working party made up of representatives from AODC member companies assisted by a number of steering group members. The working party included (at various times): I. J. Murray C. W. Logan M. D. Collar D Beedie, A W Bissett T. E. Shore J. Malone P. Downs J. Low A. D. Lamont, K. J. Gill, A. J. Cameron

Thalassa (Chairman) Technical Secretary, AODC Comex Houlder Diving Oceaneering International OSEL Sub Sea Offshore Thalassa UDI Group Wharton Williams Taylor

This working party continued to provide input into various drafts as the code was developed, based on their practical experience. AODC 035 – September 1985

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Meetings of the steering group were held between November 1983 and October 1984, and several drafts of the new code were considered. Copies of the final draft were sent to over fifty organisations for detailed review and comment in July 1984 and comments received were considered by the steering group. Set out below is a list of those organisation (in addition to steering group members) who submitted comments. BEAMA (Federation of British Electrotechnical and Allied Manufacturers Associations) British Rig Owners Association Clinical Research Centre Department of Transport ERA Technology Limited General Council of British Shipping Health and Safety Executive Institution of Electrical Engineers Institute of Marine Engineers Lloyd's Register of Shipping Nautical Institute NUTEC Royal Institution of Naval Architects Shell UK Exploration and Production UDI Group Limited UK Offshore Operators' Association Limited University of Manchester, Department of Occupational Health

The new code is based on the 1982 code, the most recent research work in the field, and a wealth of experience provided by the Working Party, the Steering Group, and those who submitted comments on the final draft. Sincere thanks are extended to all of these, but in particular to Professor W. R. Lee of the University of Manchester and Dr. H L. Green of the Clinical Research Centre for their advice on basic electrical safety and physiological criteria: Ian Murray for chairing the Working Party; David Judd of UKOOA; Bob Moulton of MaTSU; Frank McCrudden of the Institution of Electrical Engineers and Crawford Logan, Technical Secretary of AODC, for their support throughout the project and in particular in assessing detailed technical comments on the final draft and ensuring that the code incorporates in a meaningful way all those that were agreed by the steering group. Finally, thanks are due to the Department of Energy and MaTSU for arranging for the production of the final drawings and circuit diagrams. The new code should provide sound guidance to all concerned with the use of electricity under water whether designer, manufacturer or user. It will be reviewed as a result of experience gained in its use and future research.

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SCOPE This Code deals with the various hazards which may arise from the use of electricity under water. The most obvious of these is electric shock. In addition, degradation of electrical insulating material by heat can result in the emission of toxic or explosive products, and hot surfaces or electric arcs from faulty equipment or switching devices can ignite some gas mixtures. All these hazards are included in the Code. All other risks associated with the use of electric power under water (mechanical risks, nonelectric burns, ionising radiation, and generation of sound, ultra-sound and shock waves) are excluded. The Code covers all types of electrical equipment used by the diver or submersible pilot or employed for his benefit and under his control or that of his support team. In addition, it covers some electrical equipment unrelated to the underwater operation but capable of creating a risk to the diver or submersible pilot. The Code considers the risks arising from the various environments encountered. It makes recommendations for the selection, installation and maintenance of electrical apparatus used to enable an adequate level of safety to be achieved. The Code does not address electrical safety above water. This subject is adequately covered in other documents (see sections 1.13 and 2.13 for references) however personnel involved in maintenance or modification of surface equipment used in connection with electricity under water should remember that the measures outlined in this Code are designed to protect man under water and may not on their own provide adequate protection for surface crew. The question of surface back-up supplies and levels of redundancy is also not covered. The limits set on insulation material are based on information relating to the decomposition of materials and the physiological effect of the decomposition products at normal atmospheric pressures. The effect of higher pressure on these processes is not yet fully known and when further information becomes available the Code may be amended. The recommendations apply to underwater operations in sea water, but the differences which exist in fresh water are also addressed to a limited extent.


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INTRODUCTION This Code of Practice is in three sections. Section 1 gives an overview of the use of electricity in an underwater environment. Section 2 is aimed at the installer and user with particular reference to present day standards and practices. It also covers the selection and installation of equipment, the provision of protective systems and the correct selection of materials. Section 3 gives guidance to the system designer and when read in conjunction with Sections 1 and 2 provides the designer with an overall understanding of the environmental constraints and details of current practice. The Code describes practices aimed at minimising the risk associated with the use of electricity under water whilst at the same time allowing maximum flexibility in the supply and use of electric power. The Code addresses the environments met in the various applications of electricity under water. The values used in the Code are based mainly upon the values contained in document IEC 479 : 1974 (International Electrotechnical Commission) which are used internationally in many industries. Pertinent information from work carried out by ERA Technology Limited and the Clinical Research Centre under contract for the U.K. Department of Energy (D.En) specific to diving has also been incorporated. In addition a wide range of published literature and technical papers have been consulted. The response of living tissue to electric shock, on land, has been well researched. This data has been applied to the special conditions of divers working under water.

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DISCUSSION Electrical Safety - Derivation Of Values Research over many years has been aimed at determining the levels of safety for electricity in contact with humans. A number of different levels of electric shock have been established causing respectively, Sensation (or Perception), Pain, Involuntary Contraction of Muscles and Ventricular Fibrillation (when the heart pumping muscles become out of synchronisation). Electric shock can be fatal and the different levels often overlap. In some circumstances Involuntary Contraction alone can cause death. Individuals exhibit different reactions to electricity, and an electric shock which is only just perceptible to one person can cause pain to another. For this reason researchers have used as large a number of subjects as possible, in order to arrive at average values and safety levels have been set to allow for the most susceptible. The factors causing these levels of shock are variable, for example, the duration of the shock is critical. The most widely accepted international standard on the effect of electricity on the human body is report number 479 of the International Electrotechnical commission published in 1974 (IEC 479). This document gives values for effective resistance of an individual for various contact paths and also values of current which can flow safely through the body in different circumstances. These values are intended for use on land and, when they have been applied under water, arbitrary safety factors have been applied to the IEC values to allow for possible unknown effects such as pressure. IEC 479 was based on experimental work by many researchers, including Professor Lee in the U.K., Professor Dalziel in the U.S.A., and Professor Biegelmeier in Austria. Doctor Green of the Clinical Research Centre in London, has shown that many of the values in IEC 479 can confidently be applied to the underwater situation directly without the need to add safety factors (Appendix A Sect. 2 ref. 18). Additional Research has demonstrated certain areas in which the values in IEC 479 can be extended although some of the values in the document may need to be reviewed (Appendix C). This Code takes full account of all the experimental work and its effect on the values of current given in IEC 479, particularly the fact that the hyperbaric condition does not affect the fibrillation level, and uses IEC 479 as the basis of the figures given. Ohm's Law can be used to calculate the 'safe' voltage in any situation if the current which is acceptable to flow through the body can be established and the current route resistance is known. This can be applied to the underwater situation using the following. Current Route Resistance (This is the resistance offered by the diver's body). In the past, because of uncertainty of the facts relating to underwater use, a value of 500 ohms has been used for a typical, worst case, limb to limb contact irrespective of voltage. More recent research has shown that while this value is correct for high voltages it is conservative for lower voltages, where resistances of over 1000 ohms AODC 035 – September 1985

Page 9

have been demonstrated. Although work remains to be done on exact values in the underwater situation, the value of current route resistance for low voltages can safely be raised in this Code to 750 ohms for voltages up to 50 volts. The value of 500 ohms is retained for voltages over 50 volts. Both these values are conservative and are based on the latest evidence. The only exception to these values is where there is a possibility of a front to back of the chest contact path, when a resistance of 100 ohms has been used because of the possible larger area of skin in electrical contact than in the limb to limb case. Similarly, although resistance in a "dry" environment, such as a Habitat or a Deck Compression Chamber, should be much higher, it was felt that realistic situations would involve significant levels of humidity and thus the "wet" values were also used. Safe Body Current It is important to differentiate between fault current and allowable current passing through the body. Based on the maximum fault current the resulting voltage gradient in the surrounding water can be calculated using the conductivity of the sea water, which can in turn be used to estimate the current likely in the divers body, assuming the worst case. IEC 479 gives the relationship between shock duration and allowable current passing through the body, in the form of two graphs, one for 50/60 Hz a.c. current and the other for d.c. current. These curves start at a short duration shock of 10 ms and become constant for shocks lasting more than 10 seconds. (Note: Higher frequencies of a.c. current are safer on long shock durations but are not in common use. No figures are therefore given for higher frequencies, although data are available (Appendix A Section 2 Ref. 16 & 20). The curves were drawn as a composite of three separate effects. If the shock is short, the only limiting value necessary is the level of current likely to cause heart fibrillation. As the shock duration increases, levels of current, well below that causing fibrillation, will induce involuntary muscle contraction and in particular severe breathing difficulties and the curves used these lower values. When the shock duration exceeds 10 seconds, an even lower value of current will produce the phenomenon in which a person cannot let go of an electrical conductor due to localised involuntary muscle contraction in the forearm. At this current level most people suffer no other short term ill effects, as it is less than half that necessary to cause fibrillation or breathing difficulties. Improved understanding of the "safe" current flowing through the body has allowed these curves to be modified as follows: D.C. Current The d.c. curve (Figure 1) is unaltered for shock durations of 200 ms up to the straight line section over 10 seconds at a value of 40 m.A., although the section from 200 to 500 ms is dotted to indicate reservations in fully endorsing it. The curve stops at 200 ms as in shorter shocks than this there is little significant difference between a.c. and d.c. The single point value of 570 m.A. at 20 ms is included, however, as it is a widely used reference value which is considered safe and is only slightly higher than the equivalent a.c. value. Page 10


A.C. Current The a.c. curve (Figure 1) has not been altered for durations greater than 20 ms. Experimental work subsequent to the publication of the curve, particularly by Dr. H. Green of the Clinical Research Centre in London, has however shown that at durations less than 20 ms the IEC curve comes close to levels which can cause fibrillation, and thus a horizontal line has been drawn at the value of 500 m.A. Dr. Green's work has also demonstrated 'safe' values for extremely short shock durations and these are shown on the graph as 'provisional extension'. This modified graph, including some relevant experimental findings, is shown as Figure 13 in the Appendices. These values have been proved to be applicable to the underwater situation without any further modification (See Appendix C for references). Preparation of this Code has involved a number of assumptions, such as 20 ms being a typical tripping time for actively protected circuits. It is possible to carry out the specific calculations for any given situation using Fig 1 to establish the acceptable current in the body and then applying the appropriate resistance value to give the maximum safe voltage.


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Page 13

GLOSSARY Active Protection The provision of protection against electric shock by a system which detects an actual or potential shock condition, and responds by actuating a protective device. Constant-current Source A source of electric power which supplies a current that is independent of the load within a specified working range. Constant-voltage Source A source of electric power which supplies a voltage that is independent of the load within a specified working range.

Deck Compression Chamber (DCC) A pressure vessel consisting of one or more compartments, not suitable for immersion in the water, in which divers slowly return from the pressure of their dive to surface pressure, or in which they live under pressure during saturation diving operations. Differential Transformer A current transformer which delivers an output current proportional to the vector sum of the input current in two or more conductors. Diver Lock-out Submersible A diver lock-out submersible is a submersible craft capable of deploying divers from a separate lock-out compartment. Explosive Mixture A mixture of flammable gas or vapour and an atmosphere containing oxygen in proportions which, after ignition, will lead to rapid combustion spreading throughout the unconsumed mixture. Flammable Material A substance which can react continuously with oxygen which may therefore sustain fire when such a reaction is initiated by a suitable spark, static discharge, hot surface or adiabatic compression. Fully-Protective Diving Suit A diving suit which is fully insulating or fully conductive, or may incorporate both insulating and conductive layers, thus providing protection from shock arising from voltage gradients in the water. Habitat This refers to a structure which is located around or over the area to be worked on and inside which there is a substantially dry but humid gaseous atmosphere at ambient sea water pressure.

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Hyperbaric Evacuation System A system by which divers can be evacuated under pressure in an emergency. It is commonly based upon a pressure vessel which may or may not be housed in a seagoing hull. Involuntary Contraction In the context of this Code it refers to the phenomenon whereby an electric shock applied to the hand or forearm causes the muscles of the forearm to contract involuntarily such that the victim cannot let go of the object giving the electric shock. Isolating Transformer A transformer, the input and output windings of which are electrically separated to limit hazards due to accidental simultaneous contact with earth and live parts or metal parts which may become live in the event of an insulation fault. Isolation In electrical terms this refers to total disconnection from the supply. Isolator : Optically Coupled A coupling unit for communications, data or control signals, which incorporates an optical link to provide electrical isolation between the input and output. Isolator : Transformer Coupled A coupling unit, for communications, data or control signals, which incorporates a transformer to provide electrical isolation between the input and output. Line-Insulation Monitor (LIM) An active device which continuously monitors the integrity of the insulation between live conductors and the earth return circuit, and provides a read-out of the insulation level which can be used to trigger either an alarm or a tripping device. If the LIM is contained in one unit with a circuit breaker then it is known as a LINE INSULATION CIRCUIT BREAKER (LICB). Ohms' Law This states that if a voltage of magnitude V is applied across a resistor R then the current I through the resistor is related to V by the equation V = IR. Partial Pressure The partial pressure of a gas in a mixture of gases is that pressure which the gas would exert if it occupied the same volume alone at the same temperature. Passive Protection The provision of protection against electric shock by a system which inherently reduces the possibility of the occurrence of any shock condition. Residual Current Device (RCD) (Formerly known as an Earth Leakage Circuit Breaker). An active protection device which detects earth-leakage current as a difference between the supply and load currents, and responds by tripping a circuit breaker which interrupts the supply. AODC 035 – September 1985

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Current-operated RCD

An RCD connected directly, or through a differential transformer, to the supply circuit and responding to leakage current.

Voltage-operating RCD on RCD connected to the screen of a screened and earthed system and responding to voltage on the screen. Type 1 RCD

An RCD which uses a differential transformer to detect the out-of-balance current in the conductors connecting the supply network to the load.

Type 2 RCD

An RCD which detects leakage current to earth, being directly connected between supply and earth.

Current-Sensitive RCD

A Type 1 or Type 2 RCD which responds to current.

Voltage-Sensitive RCD

A Type 1 or Type 2 RCD which responds to voltage

NB: The term RCD, as used in this document, covers all fault operated circuit breakers. Safe Distance The distance beyond which the voltage gradient in the water presents no hazard to the diver. Submersible Compression chamber (SCC) - also known as a Diving Bell A manned pressure vessel used under water to support divers. Transfer chamber A Transfer Chamber is a chamber connected to the Deck Compression Chamber by means of which divers under pressure can transfer from a Submersible Compression Chamber or Diver Lock-out Submersible to the Deck Compression Chamber or vice versa whilst maintaining pressure. Trip Device This refers to any device (RCD, LIM with trip facility etc.) which can interrupt the electrical supply on detection of a fault. In this Code an overall system operating time of 20 ms has been assumed in deriving the values given in the various tables. Voltage Clipper (or Limiter) A component which prevents the voltage in the circuit from rising above a predetermined level by drawing off the excess current at voltages reaching that level.

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SECTION 1 - APPLICATION OF ELECTRICITY UNDER WATER 1.0

INTRODUCTION

The application of electricity during a diving operation has been sub-divided as shown in Fig. 2. Each of these sub-divisions reflects the relevant environment and the specific protection requirements. The sub-divisions are : 1.

Deck Compression Chamber (DCC) and Hyperbaric Evacuation System

2.

Transfer Chamber, Submersible Compression Chamber (SCC), and Diver Lockout Submersible.

3.

Diver Heating (Electrical).

4.

Hand Held Equipment.

5.

Habitat.

6.

Wet Welding and Cutting.

7.

Sea Bed Equipment.

8.

Impressed Current Systems.

9.

Remotely Operated Vehicle (ROV).

10.

Manned Submersibles.

11.

Umbilicals.

12.

High Power Equipment.

13.

Surface Electrical Distribution.

14.

Fresh Water.

15.

Explosives.

For each application, specific acceptable values are given. Their derivation is explained in the DISCUSSION. The values given in this section are based on 20ms maximum shock duration. Total resistance values are also chosen as explained in the DISCUSSION. Reference to the DISCUSSION will allow specific situations to be assessed, for example, the use of active protection devices with reaction times other than 20ms. In this Section three paragraphs are given for each sub-division. Paragraph a) gives a brief definition to allow full identification, paragraph b) identifies the environment which the electrical equipment may encounter and paragraph c) lists acceptable active protection practices. This last section is in tabular form with four headings. RECOMMENDATION This summarises the manner in which electricity can be used safely. If none of these methods is possible then the electrical equipment should not be used or else alternative arrangements should be made to ensure safety. Where a trip device is specified, it is based on an overall operating time of 2Oms.


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SAFE BODY CURRENT This is the maximum current which can be allowed to flow through the diver's body safely, and has been derived from IEC 479: 1974. It is NOT the current flowing in the electrical equipment. CURRENT ROUTE RESISTANCE This is the resistance offered by the diver's body. The values are based on experimental data and are for limb to limb contact, except in the case of diver heating where front to back of the chest was chosen. VOLTAGE This is the voltage derived from the maximum current allowable through the diver's body and the route resistance. It is expressed as a maximum value which should never be exceeded and also as a commonly used nominal value. The voltages stated are the values to which the diver may be subjected without serious physical harm. Note: 1. Where d.c. is referred to, it is assumed that the ripple content is not more than 5%, otherwise it should be treated as a.c. (Appendix A, Section 2 Ref 19). 2. Where a.c. is referred to, the voltage is RMS. 3. All pressures given in this code are gauge pressures. This means that O Bar is the pressure at sea level, otherwise known as atmospheric pressure.

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1.1 Deck Compression Chamber : Hyperbaric Evacuation System a. A Deck Compression Chamber is a pressure vessel consisting of one or more compartments, (not suitable for immersion in the water) in which divers slowly return from the pressure of their dive to surface pressure, or in which they live at pressure during Saturation Diving Operations. A Hyperbaric Evacuation System is a system by which divers can be evacuated under pressure in an emergency. It is commonly based upon a pressure vessel which may or may not be housed in a seagoing hull. b. Internal pressure can vary between O and 50 bar but is commonly in the range 0 - 25 and can change at various rates. The atmosphere is normally compressed air in the range O to 5 bar or a mixture of oxygen and helium up to 25% 02 or 0. 5 bar ppO2 whichever is the lower. Higher oxygen concentrations may be encountered in special circumstances. Occupants are likely to be lightly clad with no electrical protection from their clothing. The internal temperature range is normally 25°C to 35°C in a saturation chamber and 5°C to 25°C in an air chamber. Humidity in such chambers is normally in the range 50 - 75%, but can exceptionally rise to 100%. c. The acceptable practices are: Recommendation

Safe Body Current Current X Route Resistance mA OHMS

a.c. with Trip Device (1) d.c. with Trip Device a.c. without Trip Device d.c. without Trip Device

500 570 10 40

500 500 750 750

= Voltage Maximum V

Nominal V

250 285 7.5 30

220 250 6 24

Note 1. Significantly higher frequencies of a.c. current provide higher safety levels. The advantage of using higher supply frequencies rapidly decreases for shorter duration shocks and no change in the safe level is recommended in the region where most trip devices operate. For long duration shocks, a limit is set by the internal heat generated (Appendix A, Section 2 Ref. 16 & 19).


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1.2

Transfer Chamber, Submersible Compression Chamber (SCC) and Diver Lock-Out Submersible a. A Transfer Chamber is a chamber connected to the Deck Compression chamber (DCC) by means of which divers under pressure can transfer from a Submersible Compression Chamber (SCC) or Lock-Out Submersible to the DCC or vice versa whilst maintaining pressure. It usually contains washing and toilet facilities during saturation diving. A Submersible Compression Chamber (SCC), or a Diving Bell, is a manned pressure vessel used under water to support divers. A Diver Lock-Out Submersible (DLO) is a submersible craft capable of deploying divers from a separate lock-out compartment. b. Transfer Chambers, Submersible Compression Chambers and Diver Lock-Out Submersibles are very similar in their electrical requirements and are therefore considered together. Submersible Compression Chambers and Diver Lock-Out Submersibles may be subjected to rough handling during launch and recovery and will also be immersed for a long time in sea water. Internal pressures will be in the range of 0 to 50 bar but normally 0 to 25 bar. This pressure could be subject to rapid change. Externally the Transfer Chamber will be subject only to atmospheric pressure but the Submersible Compression Chamber and Diver Lock-Out Submersible will be subject to sea water pressure according to the depth, which will be in the range of 0 to 50 bar, but normally 0 to 25 bar. The internal atmosphere is either compressed air at a pressure of 0 - 5 bar or an oxygen and helium mix at a pressure of 0 to 50 bar. The oxygen concentration will normally be 25 % by volume or 0.5 bar ppO2, whichever is lower. Divers in Submersible Compression Chambers and Diver Lock-Out Submersibles will normally wear divers' rubber suits, but in Transfer Chambers they could be lightly clad with no electrical protection from their clothing. The temperature inside all vessels will lie in the range of 25°C to 35°C, but could drop to 10°C in exceptional circumstances. External temperatures will vary from 0°C to 30°C but will normally be between 5°C and 15°C. There will always be a high humidity internally and equipment mounted inside Transfer Chambers, Submersible Compression Chambers and Diver Lock-Out Submersibles will be subject to exposure to sea water splash, and possible total immersion in the case of Submersible Compression Chambers and Diver Lock-Out Submersibles.

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c. The acceptable practices are: Recommendation


= Voltage Maximum V

a.c. with Trip Device d.c. with Trip Device a.c. without Trip Device d.c. without Trip Device

500 570 10 40

500 500 750 750

250 285 7.5 30

Nominal V

220 250 6 24

N.B. If Trip Devices are used then they should be able to be reset by the Diving Supervisor after necessary safety checks. They should have an override facility which may only subsequently be operated by the Diving Supervisor if he considers the danger to the diver as a result of loss of power to be greater than the possible electrical hazard.


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1.3

Diver Heating (Electrical) a. This refers to electrically heated undersuits where the current flows over a large area close to the diver's body. b. Electrical Diver Heating is not in common use within commercial diving, although it has been used in the past and may be used again in the future. An electrically heated diver's suit will be exposed to the same environment as the diver, a pressure range of 0 to 50 bar although normally 0 to 25 bar. This pressure is not likely to change rapidly. The ambient environment when energised will normally be seawater but for brief periods during exit from or entry to a diving bell, the environment could be compressed air at a pressure of 0 to 5 bar or an oxygen and helium mixture at a pressure from 0 to 50 bar. In the case of surface supported diving this brief period could be at 0 bar in air while the diver enters or leaves the surface of the sea. A diver's clothing would always include a rubber suit but normally worn on top of the electrically heated garment. The temperature of the water surrounding the diver will normally be in the range of 5° to 15°C. c. The acceptable practices are: Recommendation


= Voltage Maximum V

a.c. with Trip Device (3) d.c. with Trip Device

200 228

100 (1) 100

20 22.8

Nominal V

18 18

N.B. The heating element in a suit or gas heater should be completely enclosed in an earthed conducting screen. d.c. without Trip Device (2)

70

100

7

6

Notes 1. These values have been divided by 2.5 to allow for the concentration of current in the region of the heart, in this case in the ratio of 2.5 to 1. (Appendix A Section 2 Ref. 19). 2. This is based on the level of shock leading to involuntary contraction of breathing muscles and not to the level causing forearm contraction which is the basis for the figures in other cases without trip devices. 3. Significantly higher frequencies of a.c. current provide higher safety levels. The advantage of using higher supply frequencies rapidly decreases for shorter duration shocks and no change in the safe level is recommended in the region where most trip devices operate. For long duration shocks, a limit is set by the internal heat generated. (Appendix A Section 2 Ref. 16 & 19). Page 24


1.4

Hand Held Equipment a. This refers to equipment held by the diver during routine operations, such as cameras, small hand tools and NDT equipment. b. Hand held equipment for subsea use must be extremely robust to withstand normal rough handling. It will be exposed to a pressure equivalent to the depth at which it is to be used. This will be in the range 0 to 50 bar but normally 0 to 25 bar. This pressure can be subject to very rapid change. Its surroundings will include total immersion in sea water but it may also be taken into the gaseous environment of a diving bell or habitat, when it would be in an atmosphere of compressed air at a pressure of up to 5 bar or an oxygen helium mix at a pressure of up to 50 bar. The diver's clothing while the unit is energised will normally be a rubber suit. The temperature range to which the equipment will be exposed is 0°C to 30°C. c. The acceptable practices are: Recommendation

Safe Body Current Current X Route Resistance MA OHMS

= Voltage Maximum V



500 570 10 40

500 500 750 750

250 285 7.5 30

Nominal V

220 250 6 24

Page 25

1.5

Habitat a. This refers to a structure which is located around or over the area to be worked on and inside which there is a substantially dry but. humid gaseous atmosphere at ambient sea water pressure. The commonest use of such items is to provide facilities for hyperbaric welding. b. Welding habitats are by their very nature subject to wide extremes of environment. During operations, equipment within a habitat will be subject to internal pressure varying between 0 to 50 bar but normally in the range of 0 to 25 bar. This pressure could be subject to rapid change. The atmosphere is either compressed air at a pressure of 0 to 5 bar or an oxygen and helium mix at a pressure of 0 to 50 bar. The oxygen concentration will normally be the lower of 25% by volume or 0.5 bar ppO2 Occupants' clothing will be divers' rubber suits, conventional welding clothing or even fire resistant boiler suits. The internal temperature will vary from 5°C up to 40°C and exceptionally, particularly in very small habitats, could rise to 60°C. During the deployment phase, the habitat may be flooded and any electrical equipment must be capable of withstanding total immersion in salt water at ambient pressure. During operations the humidity level will vary from 70100%.

Page 26


c. The acceptable practices are: (Note For welding practices in habitats see Section 1.6) Recommendation


= Voltage Maximum V


250 285 7.5 30

Nominal V

500 570 10 40

500 500 750 750

220 250 6 24

A supply fed from an a.c. isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker.

-

-

In this case, a single fault does not present a hazard and thus no maximum voltage need be stipulated provided the protective devices are able to prevent the occurrence of a second fault constituting a hazard.

A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A, and trip device.

-

-

No voltage limit is stated as the diver is protected by the fault current limit and the associated trip device.

Note If Trip Devices are used then they should be able to be reset by the Diving Supervisor after necessary safety checks. They should have an override facility which may only subsequently be operated by the Diving Supervisor if he considers the danger to the diver as a result of loss of power to be greater than the possible electrical hazard.


Page 27

1.6

Wet Welding And Cutting a. This covers any welding, burning or cutting operation where both the diver and the workpiece are totally immersed in water. b. The environment is considered to be seawater. The pressure range will be 0 to 50 bar but normally 0 to 25 bar. The temperature could be in the range 0°C to 30°C but normally 5°C to 15°C. The diver will normally be wearing a rubber suit and heavy rubber gloves, over a pair of light rubber gloves. The gloves in particular will give a measure of electrical protection where the voltage gradient is highest. c. The acceptable practices are: Recommendation


= Voltage Maximum V

d.c. without Trip Device

40

750

30

Nominal V

24

It is recognised that 30V dc is not a high enough voltage to be practical in many circumstances. In such cases it is recognised that it would be difficult to provide active protection to ensure that the diver is safe from electric shock at all times. Safety is achieved by rigid adherence to good operational procedures. A description of the necessary operational practices is given in Section 2.6.

Page 28


1.7

Sea Bed Equipment a. This covers large items of equipment such as pumps or power packs used to provide power on the sea bed for a variety of tools. b. The environment is considered to be seawater. The pressure range will be 0 to 50 bar but normally 0 to 25 bar. The temperature range will be 0°C to 30°C but normally 5°C to 15°C. The diver will normally be wearing a rubber suit. c. The acceptable practices are: Recommendation


= Voltage Maximum V

Nominal V


-

-

In this case a single fault does not present a hazard and thus no maximum voltage need be stipulated provided the protective devices are able to prevent the occurrence of a second fault constituting a hazard.

A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A and trip device.

-

-


Note In all cases, such equipment should have a suitable seawater earth as outlined in section 2.0.2.5.


Page 29

1.8

Impressed Current Systems a. These are systems installed to protect vessels or structures from corrosion by means of electrically supplied anodes in the sea which protect the parent structure. The protective effect takes some hours or days to establish and they are therefore rarely switched off unless special circumstances require. b. Impressed current anodes are considered as surrounded by seawater at a pressure dependent on the depth but normally 0 to 30 bar. The water temperature is likely to be in the range 0°C to 30°C but normally 5°C to 15°C. Divers who may approach impressed current anodes will be swimming in sea water and normally wearing a rubber suit. c. The acceptable practices are: Recommendation


= Voltage Maximum V

d.c. without Trip Device Physical barrier at suitable distance to prevent diver entering area of potential hazard.

Page 30

40

750

-

-

30

Nominal V

24

No maximum voltage is stated as the safe voltage will depend on the distance of the physical barrier. Appendix B, Section 2 gives more detail.


1.9

Remotely Operated Vehicle (ROV) a. These vehicles are controlled from the surface by an operator who has visual control by means of a camera mounted on the ROV. The vehicles are linked to the surface by an umbilical. b. As ROVs are launched and recovered through the air/sea interface electrical components must be robust to withstand the inevitable rough handling. Once totally immersed, the pressure on the ROV is that of the surrounding water, ranging from 0 to 100 bar but normally 0 to 30 bar. This pressure will change rapidly during raising and lowering. The water temperature varies from 0°C to 30°C. Any diver who may come in contact with an ROV can normally be assumed to be wearing a rubber suit. c. The acceptable practices are: Recommendation


= Voltage Maximum V

Nominal V


-

-

In this case a single fault does not present a hazard and thus no maximum voltage need be stipulated provided the protective devices are able to prevent the occurrence of a second fault constituting a hazard.

A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A and trip device.

-

-


Note Trip Devices should be able to be reset by the Diving Supervisor after necessary safety checks. They should have an override facility which may only subsequently be operated by the Diving Supervisor if he considers the danger to the diver as a result of loss of power to be greater than the possible electrical hazard.


Page 31

1 .10 Manned Submersible a. A manned free swimming submersible craft is designed to maintain some or all of its occupants at or near atmospheric pressure and carries all of its electrical power onboard. A one man tethered submersible is a small submersible craft tethered to the surface and designed to maintain the occupant at or near atmospheric pressure. Electrical power is normally fed from the surface through the umbilical. b. The environment within a one man tethered submersible and within the pilot's compartment of a free swimming submersible is normally air at atmospheric pressure, although in extreme circumstances the oxygen concentration may rise above 25% by volume. Care should be taken with electrical equipment which may cause a fire in these circumstances. The internal pressure will remain at 0 bar although exceptionally it may rise by a few millibar. External pressure due to depth of sea water could be up to 100 bar. Occupants will normally be lightly clad with no electrical protection from their clothing. Internal temperature will normally be in the range of 10°C to 25°C although this could drop to 2°C or rise to 35°C for short periods. External temperature will be that of the sea water that is 0°C to 30°C. There will always be high humidity levels. c. The acceptable practices both for the internal occupant and for any external electrical equipment which may be in contact with a diver are: Recommendation


= Voltage Maximum V


Page 32

500 570 10 40

500 500 750 750

250 285 7.5 30

Nominal V

220 250 6 24


1.11

Umbilicals

a. These are any cable or bundle of cables connected between the underwater work site and the surface or between two underwater work sites (but excluding equipment described in section 1.12). b. An umbilical is normally connected at the surface to an electrical source and at its lower end to the electrical load. The environment to which the umbilical is exposed varies from the dry atmosphere, through the surface of the sea where pressure is still atmospheric (but with splashing or immersion) to its extreme in sea water at pressures up to 50 bar. The umbilical is likely to receive rough handling and to be exposed to water temperatures from 0°C to 30°C, and, for its exposed length between the surface of the sea and its topside connection, at temperatures well below 0°C or up to 50°C. It will be subjected to regular flexural stresses and may also be exposed to tensile stress. Any diver exposed to an electrical hazard from the umbilical can normally be assumed to be wearing a rubber suit. c. The acceptable practices are: Recommendation


= Voltage Maximum V


500 500 750 750


-

-



-

-



250 285 7.5 30

Nominal V

500 570 10 40

220 250 6 24

Page 33

1.12

High Power Equipment

a. Such equipment is not normally deployed by, or under the control of, the Diving Contractor but may need to be worked on by divers, a manned submersible craft or an ROV. It includes subsea wellhead control devices and power cables. b. Such equipment will be considered to be in contact with, or at the surface interface to be immersed in, sea water at a pressure equivalent to its depth and normally in the range 0 to 50 bar. The water temperature is likely to be in the range 0°C to 30°C. Divers who may be at risk from high power equipment can normally be assumed to be wearing a rubber suit. c. It should be recognized that power cables exist which cannot readily be disconnected. Detailed investigation and assessment of possible hazards should be carried out before diving operations commence and the necessary safety protections provided. The acceptable practices are: Recommendation


= Voltage Maximum V

Page 34

Nominal V

Physical barrier at suitable distance to prevent diver entering area of potential hazard.

-

-

No maximum voltage is stated as the safe voltage will depend on the distance of the physical barrier. Appendix B, Section 2 gives more detail.


-

-



-

-



1.13

Surface Electrical Distribution

a. This includes all equipment on the surface (support vessel or similar) which is under the control of the Diving Contractor and necessary for the underwater operation. b. The environment will vary from the dry, warm and air conditioned accommodation of a fixed platform to the exposed after deck of a ship where wind, rain, vibration and sea-spray may be encountered. The area in which the equipment is to be installed should be studied to determine the likely environment. c. It is not within the scope of this Code to specify protection practices for Surface Electrical Distribution as it is concerned only with the use of electricity under water. There are adequate guidelines available through national and international standards. In particular the "Institution of Electrical Engineers Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations", First Edition 1983; and "Regulations for Electrical and Electronic Equipment of Ships", Fifth Edition with recommended practice for their implementation published by the Institution of Electrical Engineers with amendments 1 to 4 give sound advice. All electrical supplies for use under water should be electrically isolated from the distribution system of the ship or installation by, for example, the use of isolating transformers or a separate generation source. Personnel involved in maintenance or modification of equipment used in connection with electricity under water should remember that the measures outlined in this Code are designed to protect man under water and may not, on their own, provide adequate protection for surface crew. The question of surface back-up supplies and levels of redundancy is also not covered.


Page 35

1.14

Fresh Water

a. This covers any operation carried out in water which is not seawater. b. Fresh water is normally much shallower than seawater and the pressure to which equipment is likely to be subjected is 0 to 5 bar although in exceptional circumstances this could be as high as 25 bar. The temperature of fresh water will be 0°C to 30°C. c. This Code does not specify practices for use in fresh water due to the considerable difference in its conductivity, compared to salt water, and the greater hazard in fresh water. It is recommended that for any application in fresh water a calculation be carried out from first principles for each specific application and if any doubt exists, expert advice should be obtained. Active protection values of voltage will remain the same as for seawater but other practices may be different. In particular the protective effect provided by seawater providing earthing will be reduced and any recommendation in this Code relying on this will not apply in fresh water. Provided the conductivity of the fresh water is known then this Code provides enough basic data to assess any particular situation.

Page 36


1.15

Explosives

a. This refers to any explosive charge or device which is initiated by electrical means. b. The environment to which any electrical device used to initiate an explosion will be subjected is considered to be sea water at a pressure of 0 to 50 bar, but normally 0 to 25 bar. The water temperature will vary from 0°C to 30°C. c. In normal explosive operations, the electrical device will only be energised when the explosive is to be detonated, and divers or submersible craft should not be in the vicinity at this time. Therefore it is not necessary to specify protective practices against electrical hazard. However it must be remembered that, unless specially designed detonators are used, the energy from radio frequency transmissions may initiate an electric detonator.


Page 37

SECTION 1 – APPLICATION OF ELECTRICITY UNDER WATER

Summary Table Of Acceptable Practices (Numbers in Brackets refer to explanatory notes at the end of this table) Application (1)

Recommendation (2) (3)

Safe Body Current Current X Route Resistance mA (4) OHMS (5)

= Voltage (6)

Maximum Nominal 1.0 Deck Compression Chamber a.c. with Trip Device (8) 500 500 250 220 (DCC) and Hyperbaric d.c. with Trip Device 570 500 285 250 Evacuation System a.c. without Trip Device 10 750 7.5 6 d.c. without Trip Device 40 750 30 24 2.0 Transfer Chamber a.c. with Trip Device (7) 500 500 250 220 Submersible Compression d.c. with Trip Device (7) 570 500 285 250 Chamber (SCC) and Diver a.c. without Trip Device 10 750 7.5 6 Lockout Submersible d.c. without Trip Device 40 750 30 24 3.0 Diver Heating (Electrical) a.c. with Trip Device (8) 200 100(9) 20 18 d.c. with Trip Device 228 100 22.8 18 N.B. The heating element in a suit or gas heater should be completely enclosed in an earthed conducting screen. d.c. without Trip Device (10) 70 100 7 6 4.0 Hand Held Equipment a.c. with Trip Device 500 500 250 220 d.c. with Trip Device 570 500 285 250 a.c. without Trip Device 10 750 7.5 6 d.c. without Trip Device 40 750 30 24 5.0 Habitat a.c. with Trip Device (7) 500 500 250 220 d.c. with Trip Device (7) 570 500 285 250 a.c. without Trip Device 10 750 7.5 6 d.c. without Trip Device 40 750 30 24 A supply fed from an a.c. (11) isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker (7) A supply fed from an a.c. (12) isolating transformer with the secondary earthed through an impedance to limit fault current to 1A, and trip device (7) 6.0 Wet Welding And Cutting d.c. without Trip Device 40 750 30 24 N.B. As this d.c. voltage will not be high enough in most cases it is clear that the process cannot be made safe electrically. Refer to sections 1.6 and 2.6 for detailed advice.

Page 38


Application (1)

7.0 Sea Bed Equipment (13)

8.0 Impressed Current System

9.0 Remotely Operated Vehicle

10.0

Manned Submersible



A supply fed from an a.c. isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A and trip device. d.c. without Trip Device Physical barrier at suitable distance to prevent diver entering area of potential hazard. A supply fed from an a.c. isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker (7). A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A and trip device. (7) a.c. with Trip Device d.c. with Trip Device a.c. without Trip Device d.c. without Trip Device


= Voltage (6)

-

-

Maximum (11)

-

-

(12)

40 -

750 -

(14)

-

-

(11)

-

-

(12)

500 570 10 40

500 500 750 750

Nominal

30

24

250 285 7.5 30

220 250 6 24

Page 39

Application (1)

11.0

12.0

13.0 14.0 15.0

Umbilicals


Surface Electrical Distribution Fresh Water Explosives

Page 40


a.c. with Trip Device d.c. with Trip Device a.c. without Trip Device d.c. without Trip Device A supply fed from an a.c. isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A, and trip device Physical barrier at suitable distance to prevent diver entering area of potential hazard. A supply fed from an a.c. isolating transformer with nonearthed secondary. Using a line insulation monitor with circuit breaker A supply fed from an a.c. isolating transformer with the secondary earthed through an impedance to limit fault current to 1A, and trip device Refer to text


= Voltage (6) Maximum 250 285 7.5 30 (11)

500 570 10 40 -

500 500 750 750 -

-

-

(12)

-

-

(14)

-

-

(11)

-

-

(12)

Refer to text Refer to text


Nominal 220 250 6 24

Notes 1. APPLICATION This is the item of equipment in which the electricity is to be used. 2. RECOMMENDATION This summarises the manner in which electricity can be used safely. If none these methods is possible then the electrical equipment should not be used or else alternative arrangements should be made to ensure safety. Where a trip device is specified it is based on an overall operating time of 20 ms. 3. Where d.c. is referred to it is assumed that the ripple content is not more than 5% otherwise it should be treated as a.c. (Appendix A. Section 2 Ref 19). Where a.c. is referred to the voltage is RMS. 4. SAFE BODY CURRENT This is the maximum current which can be allowed to flow through the diver's body safely which has been derived from IEC 479: 1974. It is NOT the current flowing in the electrical equipment. 5. CURRENT ROUTE RESISTANCE This is the resistance offered by the diver's body. The values are based on experimental data and are for limb to limb contact except in the case of diver heating where front to back of the chest was chosen. 6. VOLTAGE This is the voltage derived from the maximum current allowable through the diver's body and the route resistance. It is expressed as a maximum value which should never be exceeded and also as a commonly used nominal value. The voltages stated are maximum values to which the diver may be subjected without serious physical harm. 7. Trip Devices should be able to be reset by the Diving Supervisor after necessary safety checks. They should have an override facility which may only subsequently be operated by the Diving Supervisor if he considers the danger to the diver as a result of the loss of power to be greater than the possible electrical hazard. 8. Significantly higher frequencies of a.c. current provide ids higher safety levels. The advantage of using higher supply frequencies rapidly decreases for shorter duration shocks and no change in the safe level is recommended in the region where most trip devices operate. For long duration shocks a limit is set by the internal heat generated (Appendix A Section 2 Ref 16 & 20). 9. These values have been divided by 2.5 to allow for the concentration of current in the region of the heart, in this case in the ratio of 2.5 to I. 10. This is based on the level of shock leading to involuntary contraction of breathing muscles and not to the level causing forearm contraction which is the basis for the figures in other cases without trip devices. 11. In this case a single fault does not present a hazard and thus no maximum voltage need be stipulated provided the protective devices are able to prevent the occurrence of a second fault constituting a hazard. 12. No voltage limit is stated as the diver is protected by the fault current limit and the associated trip device. 13. In all cases such equipment should have a suitable sea water earth as outlined in section 2.0.2.5. 14. No maximum voltage is stated as the safe voltage will depend on the distance of the physical barrier. Appendix B Section 2 gives details.


Page 41

SECTION 2 - INSTALLATION AND USE OF EQUIPMENT 2.0

INTRODUCTION

This section has been laid out under the same headings as Section 1 to identify the relevant environmental and specific protection requirements. For each application, where practical, a simple block diagram is given showing typical present day practice, using the techniques identified in section 1, in order to achieve a 'safe' system (where necessary the main electrical considerations are explained in a subsequent paragraph). It must be stressed that the block diagram shows a typical circuit only and alternative circuits would be acceptable provided the level of protection is at least that required by this Code and any relevant legislation. 2.0.1

Protection Against Electric Shock Available methods of protection against electric shock fall into two groups, passive and active. Passive methods (insulation, screening and earthing) constitute a first line of defence against shock, and one or more of them should always be used. When a passive method fails, due for example to water ingress or deterioration of earthing connections, the system may be in an undetected dangerous condition and constitute a shock risk. Consequently, a passive method alone may be inadequate where a high level of protection is needed and the use of active protection in addition, such as a residual current device, should be considered.

2.0.2

Passive Protection There are five ways of providing passive protection.

2.0.2.1 Insulation The primary means of providing passive protection against shock is by insulation of the power system and the appliance it serves. Insulation is less effective under water than on dry land, because a defect at any point might allow current to flow in the water and part of this current may be intercepted by the diver or submersible craft. The effectiveness of insulation as a means of protection may be improved by using two layers of insulation with a conducting screen in between. Two defects are then needed to constitute a risk. The first defect can be detected by continuously monitoring the insulation level between the conducting screen and the load, and between the screen and the outer casing. Entry of water could cause simultaneous failure of both sections of insulation; consequently sealing arrangements (such as 'O'-rings and pressure balance terminations) should be incorporated if double insulation is to be more effective than single insulation.

Page 42


Insulation should be further improved by supplying the load via an isolating transformer, the whole of the electrical system (including the transformer secondary winding and all the appliances) thus being insulated from earth. It is then possible to make contact with any point on the secondary circuit without receiving a shock. This prevents, for example, contact with a crane wire from the surface, the leg of a steel platform or a ship's hull from forming an earth return path and thereby creating a hazard. If a first defect is not rectified, contact with the circuit or the occurrence of a second defect may then cause current to flow through the diver. However, this can be avoided by incorporating an active protection device for detecting the first defect. The whole of the circuit (transformer, secondary winding, connected cable and the load) should have a high insulation resistance to earth. It is also necessary to restrict the capacitance of the circuit to earth. In practice, the capacitance usually imposes a limit on the maximum length of cable connecting the transformer secondary winding to the load - (See Appendix B.1). 2.0.2.2 Fixed Barrier When electrical equipment requires direct contact with sea water to function correctly (e.g. an impressed current anode) a fixed barrier can be installed to keep the diver a specific safe distance away from it. This barrier should be non-metallic and non-conducting if possible. In addition to such equipment, high-power fixed installations (e.g. cables and motors) can feed large currents into the water if a fault occurs. Again fixed barriers can be used to keep the diver at a safe distance - (See Appendix B.2). The safe distance can be reduced by incorporating an impedance in the star point to earth line of the supply to limit the fault current. Care should be taken to ensure that all protective devices will function at the low level. A fault current limit of 1 A is recommended. 2.0.2.3 Protective Clothing Any practice which limits the flow of current through the diver is beneficial. Rubber gloves should be worn, particularly during welding, and should have a cuff to give the wrist area some degree of protection. The degree of protection offered by existing diving suits varies and they should not be relied upon for protection. 2.0.2.4 Shielding The electrical equipment may be enclosed within a conducting shield to prevent current from flowing into the water. Where a shield is fitted it should be suitably connected to earth, to prevent a dangerous voltage by an internal fault. Protective screens should be constructed from high-conductivity material and have low-resistance joints, otherwise a fault current flowing in the screen can AODC 035 – September 1985

Page 43

produce a dangerous voltage gradient over its external surface. However, this deficiency can be greatly reduced by the use of a double screen. The conducting screen (the external screen in double-screened systems) should also be in contact with the water to restrict the voltage difference between the screen and the surrounding water. 2.0.2.5 Suitability of Earthing On any unit which operates at a voltage which is higher than the unprotected safe limit (30V d.c. maximum or 7.5V a.c. maximum) then the conductive structure or frame should be connected to earth to dissipate any fault current. The connection should have a low impedance to minimise any rise of voltage on the conductive structure or frame, and sufficient mechanical strength to prevent accidental breakage when the equipment is operated within the stated limits. The connection should be through purpose-designed conductors in the power cables. The earth return path can be augmented by an area of bare metal (even corroded steel) in contact with the water; such an arrangement can be many times more effective than normal earth leads.

2.0.3

Active Protection

In addition to passive protection, and wherever practicable, the system should provide active protection against shock resulting from direct contact with the live circuit of the equipment and indirect contact via the structure or the sea. Active protection may be provided by a residual current device (RCD) or a line insulation monitor (LIM) coupled to a circuit breaker device. 2.0.3.1 Residual Current Devices (RCDs) Commonly used RCDs have a typical operating time of 15 to 25 ms. The trip currents of RCDs should be selected to be as low as possible consistent with freedom from accidental tripping which is inconvenient and can be dangerous. A trip current of 30 mA at 20 ms has been found to be suitable.

Page 44



Page 45

Two alternative types may be used; one uses a differential transformer to detect out-ofbalance current and the other is connected between an isolated supply and earth. For d.c. systems, Differential Transformer RCDs are not applicable, and isolated supply RCDs should be designed for use in d.c. circuits. Because of practical limitations in their design, differential transformers may have an upper limit on the rated value of the line current. Isolated supply RCDs are not subject to any limits of load current. 2.0.3.2 Line Insulation Devices A line-insulation monitor (LIM) may be used to monitor the insulation level of an umbilical cable as it enters or leaves the water and any developing electrical leakage. A read-out of insulation level should be provided with warnings of low levels if appropriate. In order to use a LIM as part of an active protection device it should be connected to a circuit breaker to give suitable overall system operating characteristics.

2.0.4

Toxicity Of Materials In the event of a fire or even serious overheating, many commonly used electrical materials give off noxious and toxic fumes. As a diver or submersible pilot is confined within his environment and unable to escape from any fumes, it is important always to use low-toxicity cables and other materials. Full guidance on selection of such materials is given in section 3.2.

2.0.5

Installation Practice To maintain the integrity of all forms of protection, portable and fixed electrical equipment should be regularly inspected by competent staff. Only such staff should carry out installation or rewiring, and any temporary electrical equipment used should be to the standard described in this Code. Contractors responsible for underwater electrical equipment should authorise staff in writing as competent for specific functions. Competent staff should be familiar with proper installation procedures and be aware of the hazards and problems particular to underwater work. Frequent inspections should be made for signs of mechanical damage on cables and for any general deterioration of equipment. The following measures should be adopted for the installation, modification and repair of electrical equipment for use under water. 1. All conductors should be adequately protected by suitable fuses and/or circuit-breakers. 2. Conductors should have an adequate cross section, based, not only on full load current and voltage drop along its length, but also on sustained

Page 46


overload current if applicable, and the fault current which can flow for the time taken for any protective devices to operate. 3. A minimum number of joints should be used in a circuit to reduce the possible number of poor connections. 4. Terminal connections of proven integrity should be used, and conductor 'tails' should be supported to avoid fatigue failures. 5. Wiring should be spaced to avoid cross-circuit breakdown or tracking. 6. Conductors should be routed clear of areas where they may be liable to mechanical damage, or else provided with some form of mechanical protection. Fixed cables should be positioned so that they do not form a convenient hand grip. 7. Terminal chambers should be sealed against entry of moisture. 8. Common power return circuits should not be used. Each circuit should function independently. Similarly each protected circuit should be separate so that a fault on one cannot interact with a second circuit. 9. Identification sleeving is potentially toxic when heated and should be fitted at a distance from the termination. 10. A good tracking index material should be used and adequate tracking distance should be incorporated in the design of plugs, sockets, connections and printed circuit boards to make allowance for any salt which may be present on the surface. Penetrators, where continued tracking erosion can cause failure of the pressure seal, are particularly vulnerable. 11. Solder should not be used on stranded or flexible cables, unless the wiring is fully supported to avoid any stress or fatigue at the joints resulting from vibration. 12. Records should be maintained to ensure that the design safety standards are met. 13. Records should be kept of maintenance carried out and any modifications made to equipment. 14. Records should be kept of routine testing of active protection devices. 2.0.6

Batteries In the past, batteries have usually been considered to be electrically 'safe'. As in many cases they are not used as a primary power source, but rather as a reserve or back-up, they are frequently omitted from electrical safety assessments. In practice batteries can present very real hazards and considerable care should be taken when using them. Primary cells (Non-Rechargeable Batteries) have a limited life and when discharged are notorious for producing corrosion products. Short-circuiting of primary cells can be potentially hazardous and adequate short-circuit protection should be provided.


Page 47

Secondary cells (Rechargeable Batteries) are normally of higher power than primary cells, so the same basic recommendations apply. In addition, however, there is an explosive hazard from hydrogen gases produced during recharging and discharge. Secondary cells should normally be recharged out on the surface in a properly ventilated area. (Further information on this subject may be found in section 14 of the "Institution of Electrical Engineers Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations", First Edition 1983). If fixed installations are required to have submerged recharging facilities, the charge should be limited to a level below the gassing voltage; as a result, extra cells may be required to attain the working voltage and the required battery capacity. Where devices are provided for the recombination of free hydrogen and oxygen, care should he taken to prevent overcharging which may lead to carry-over of the electrolyte and malfunction of the device. If water enters a battery compartment, an explosive or toxic gas mixture may be produced. Battery compartments should be completely watertight. Fuses should be fitted in the battery compartment as close as possible to the batteries and should be encapsulated to prevent a blown fuse from igniting the possible hydrogen atmosphere in the compartment. The state of batteries in battery-powered equipment should be checked before use. Batteries should be handled with caution. Since a battery cannot be turned off, it constitutes, even in a low charge state, a shock risk and also a burn risk, if accidentally short-circuited by metal tools. Electrolyte spillage or carry-over can also provide a leakage path from a high potential terminal. Where there is any possibility of relative movement between batteries, then flexible electrical connections should be used.

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2.1

Deck Compression Chamber (DCC) And Hyperbaric Evacuation System

Figure 4 shows a typical DCC electrical installation including a wide range of chamber equipment. In this particular installation only supplies of up to 24v d.c. are used in the chamber. Primary power is derived from a 440v 3 phase supply and is stepped down, isolated and rectified to give the protected 24v d.c. for use with the chamber. Instrumentation operates from a basic 220v a.c. supply. The excitation voltages for the various sensors are never greater than 10v d.c. at low currents, typically 15 mA. Regulation within the instrument associated with the sensor provides protection for these supplies.

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2.2

Transfer Chamber Submersible Compression Chamber (SCC) and Diver Lock-Out Submersible

Figure 5 shows the electrical arrangement of a typical mixed gas diving bell (SCC). The following points are worth noting: All primary power is supplied via a main isolation transformer on the surface. The centre point of the secondary winding of this transformer is earthed through a suitable impedance to limit the fault current to a maximum of 1A. An RCD is fitted to the circuit with an overall response time of 20 ms. Power inside the diving bell is entirely 24V dc with no active protection, and is supplied by transforming and rectifying the power supplied from the surface inside a pressure proof container mounted on the outside of the bell. The 220V a.c. which is used externally to power the underwater lights is protected by being fed from the isolation transfer with the fault current limited and active protection provided by the RCD.

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2.3

Diver Heating (Electrical)

As electrical diver heating is not presently used in commercial diving it is not possible to provide a typical example of a suitable circuit.


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2.4

Hand Held Equipment

Figure 6 shows the electrical installation of a typical hand held tool, in this case an NDT unit. Primary power is derived from a 440V 3 phase system stepped down through an isolating transformer to supply 110V a.c.. The secondary winding of the isolating transformer is earthed through an impedance which limits the fault current to 1A and an RCD is fitted with a response time of 20 ms.

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2.5

Habitat

Figure 7 shows the electrical arrangement of a typical welding habitat. The following points are worth noting: All primary power is supplied via a main isolation transformer on the surface. All voltages in the habitat are derived from additional isolation transformers on the habitat. Umbilical lines from the surface are continuously monitored for insulation breakdown using line insulation monitors. Critical power supplies within the habitat, such as preheat power and power for hand-held tools, are also monitored continuously using LIMs. The LIMs monitor leakage resistance by a d.c. injection method and can be arranged to provide shut-down of an affected circuit at any pre-set level of leakage. They also provide a continuous read-out to monitor progressive deterioration of circuit insulation.

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2.6

Wet Welding and Cutting

Figure 8 represents a typical electrical system for underwater welding and cutting. The following points are worth noting: a. The welding unit should be grounded to the vessel and the ground lead should be securely grounded to the work. b. No part of the torch or the submerged sections of the power cables should be left un-insulated. c. The power source for welding under water is a d.c. welding generator or rectifier of a least 300A capacity. d. Rubber gloves should be worn by the diver to provide additional protection. e. The return connection from the workpiece should be made as close to the work area as possible. f.

A high-quality two-pole knife switch or a contactor, rated for breaking d.c., should be included in the welding circuit as a means of positive disconnection in order to safeguard the diver. It is important that the switch: -

can be seen to be open if a contactor is used, a visual indication of the contact position should be provided.

-

should be fixed so as to be readily to the hand of the person controlling the welding equipment.

-

cannot be knocked or vibrated to the 'on' position (it should fall to the 'off' position) and that a slotted cover is used to prevent accidental contact with the fixed live terminals.

g. Welding cables of adequate cross section should be used, connected in parallel if necessary, particularly with longer lengths, to avoid excessive voltage drop in the cable. h. Lengths of cable should be kept to a minimum, consistent with operational requirements, to limit circuit inductance. i.

Cables should be arranged with positive and negative close together and tied at intervals to reduce the inductive effect.

j.

Cables connected in parallel should be arranged with leads of the same polarity diagonally opposite to reduce the inductive effect.

k. Welding cable should have a protective sleeve at the point of entry to connectors to reduce flexing and prevent cable damage. l.

The welding cable should comprise two fully-insulated conductors, one of which connects the negative terminal of the welding (or cutting) set to the torch, while the other bonds the positive terminal of the set to the workpiece. Welding cables of adequate cross-section should be used; guidance is contained in "BS 638:1979, Arc welding power sources, equipment and accessories, Part 4: Specifications for welding cables". All joints in the cable should be fully insulated.

m. The electrode holder for an oxy-arc cutting system should be so designed that the oxygen valve is at all times insulated from the electrode. n. The electrode should have an electrically insulating coating which is as resistant as possible to chipping and to deterioration caused by prolonged immersion in sea water.

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Note In the past a device known as an open circuit voltage reducer has been used to reduce to a safe level the voltage on the electrode when there is no arc. When used under water there have been many problems with these units and they should not be used as the primary means of ensuring safety. Since safety of the diver cannot always be guaranteed by electrical protection alone it is necessary to adhere to pre-arranged operational safety procedures, including: 1. Since the tip of the welding or cutting electrode cannot be insulated from the water, it should be at a safe distance from the diver's hand. The electrode should not be consumed beyond a safe minimum length such that the distance from the hand to the tip of the electrode is at least 100 mm. 2. All welding and cutting equipment (including cables and connectors) should be checked by a competent person before use to ensure that it is in a serviceable condition. 3. A clear command system with return confirmation should be established for switching supplies on and off. 4. Before lowering or raising of the workpiece, clamp or welding torch, a check should be made to ensure that the welding circuit is dead and that there is no welding rod in the welding torch. 5. An electrode with an unchipped insulating coating should be used. Electrodes which have been in the water for a long time should be rejected in case the coating has absorbed water. 6. Before welding or cutting begins, it is essential to check that there are no combustible solids, liquids or gases adjacent to, on or within the workpiece. 7. A check should be made that there are no gas entrapment spaces above the work area. Cutting or welding operations should never be carried out directly underneath a diving bell (SCC). 8. The welding circuit should only be switched on when: a. the diver is in position to start welding or cutting: this position must be as stable as possible. b. the workpiece clamp is securely fastened. c. the welding rod is fitted securely into the welding torch, is pointing away from the diver, and is as near to the workpiece as practicable. d. neither the diver nor any of the diver's equipment is between the welding torch and the workpiece. e. the diver confirms that he is ready. 9. Care should be taken with large loose metallic items carried by a diver (eg wrenches and backpacks) to prevent electrical contact with a live electrode. 10. Electrodes should not be changed with the supply switched on. 11. The welding torch should not be put down or carried with the power on. 12. Welding or cutting equipment should never be taken into a diving bell (SCC) or lock-out submersible. If any problem occurs with the welding/cutting gear, it should be returned to the surface for attention. 13. Whenever practical, a second diver, or the person controlling the diving operation, should be able to observe the diver carrying out the welding/cutting operation, either directly or via television. AODC 035 – September 1985

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14. The electrode must be inserted into the head of the torch so that it is seated firmly against the rubber seal within the torch head. 15. An additional coating of wax or tape should not be added to electrodes if welding in a habitat, as toxic or flammable gases may be produced. 16. A properly designed and maintained electrode holder should be used.

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2.7

Seabed Equipment

Figure 9 shows a typical electrical installation for a high power suction dredge pump. Power to the electric drive motor is provided via an umbilical from the surface on three-phase, 440 V a.c. Care should be taken to ensure that both the main umbilical and the electrical connections of the pump motor provide an unbroken, earthed shield around the live conductors. Power is supplied via a surface mounted isolation transformer, the secondary winding of which is continuously monitored for leakage to earth using LIM. If the leakage resistance drops below a pre-set level, power is automatically shut down.


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2.8

Impressed Current Systems

Figure 10 is a simplified version of a typical impressed current anode system. Although this only shows one anode, there may be up to 200 on a large structure. With regard to the electrical arrangements the following points are worth noting: The voltage at the anode is, in this case, 'safe' (based on the values contained within this document) and no active protection practices are necessary. Once the system has been polarized, the voltage at the anode is likely to be different from the voltage when the anode is first switched on.


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2.9

Remotely Operated Vehicle (ROV)

Figure 11 illustrates the electrical circuit for a typical remotely operated vehicle. Primary power is derived from the vessel's 440v 3 phase system stepped up to 1100V through an isolation transformer to provide 1000V via the umbilical to the hydraulic pump motor mounted on the vehicle. This supply is monitored by an LIM connected to an alarm. Lights and controls are also fed in a similar manner and monitored by a second LIM.


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2.10

Manned Submersible

Figure 12 shows the electrical circuit of a typical manned tethered submersible. In this particular installation only supplies up to 24V dc are used inside the pilot's compartment for life-support and control. Primary power is derived from a 440V 3 phase system stepped up through an isolating transformer to provide 1500V down the umbilical. This supply is monitored by an LIM set to trip at a pre-set fault level. On the outside of the submersible an equipment container houses a transformer and rectifier which produces the supplies used on the submersible and houses the control circuitry.

2.11

Umbilicals

No separate figure showing an umbilical is necessary as other figures already illustrate it fully. Particular reference is made to figures 5, 7 and 11. With regard to electrical arrangements, the umbilical is supplied from a main isolation transformer on the surface and this is the normal method of active protection.

2.12


No diagram has been included for high power equipment as it is outside the control of the diving contractor. It should be recognised that power cables exist which cannot readily be disconnected. Detailed investigation and assessment of possible hazards should be carried out before diving operations commence and the necessary safety protection provided. If the equipment cannot be made safe (even by the use of a physical barrier), then a request should be made to the Operator to isolate the equipment before diving work commences.

2.13

Surface Electrical Distribution

It is not within the scope of this Code to identify active protection practices for Surface Electrical Distribution as it is concerned only with the use of electricity under water. There are adequate guidelines available through national and international standards. In particular, the "Institution of Electrical Engineers Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations", 1st Edition 1983 and the "Regulations for Electrical and Electronic Equipment of Ships" 5th Edition with recommended practice for their implementation issued by the Institution of Electrical Engineers with amendments 1 to 4, give sound advice. Personnel involved in maintenance or modification of equipment used in connection with electricity under water should remember that the measures outlined in this Code are designed to protect man under water and may not, on their own, provide adequate protection for surface crew.


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The questions of surface back-up supplies and levels of redundancy are also not covered.

2.14

Fresh Water

No diagram has been included for fresh water applications as the only variation from seawater is that fresh water is usually a much poorer conductor of electricity. Careful consideration should therefore be given to the use of electricity under water in fresh water areas. The circuits for used in fresh water will be basically the same as for use in salt water. It is important when using electricity in fresh water that the necessary calculations are carried out to identify 'safe' voltages or that expert advice is sought. Fresh water requires greater safe working distances and adequate earthing systems.

2.15

Explosives

No diagram has been included for electrical devices intended to initiate explosives as divers will not be in the vicinity when they are energised.

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SECTION 3 - SYSTEM DESIGN 3.0

Introduction

This section is intended for the equipment designer or selector and gives more detailed information on certain aspects of equipment specification than are contained in Sections 1 and 2. It is assumed that before reading this section the design engineer has already read relevant paragraphs in Sections 1 and 2. Adequate records should be kept of the reasons for selection of electrical equipment, which should be available to a designer of future modifications.

3.1

New Techniques

This Code is based on equipment and practices which are in current use but it is not intended in any way to hamper development of new or improved techniques. Designers should use the basic guidance contained within this Code to evaluate new equipment and to establish levels of safety.

3.2

Reduction In Toxicity Hazard

3.2.1

Selection Of Insulating Material

All organic insulating materials decompose at elevated temperatures to produce toxic products, but the rate of decomposition and the degree of toxicity of the products vary. Particular care should be taken when selecting electrical insulation material which will be in contact with breathing gas circuits or the general working environment. Materials should be chosen which do not readily ignite and which emit the minimum of smoke and toxic gas when overheated. The definition of such performance is not strictly laid down, but Defence Standard 61-12, Part 18, provides useful guidance. A number of manufacturers provide materials to this standard and these are preferred. PVC is not recommended since it readily decomposes to hydrogen chloride when overheated. Cable manufacturers should be consulted about any particular application before a cable is selected. The following parameters should be determined: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦

normal current fault current and duration ambient temperature and pressure atmosphere and volume of environment possible contamination length of cable run supporting and termination methods identification required mechanical strength required.


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3.2.2

Terminal Blocks And Circuit Boards

The materials used for terminal blocks, circuit boards and cable markers should be chosen with toxicity in mind because these items may reach high temperatures, as contacts deteriorate. Terminals of higher thermal capacity and substantial thermal conductivity reduce the rate of temperature rise in the event of contact deterioration. 3.2.3

Quantity Limitation

The amount of potentially toxic material should be minimised by limiting the electrical equipment inside a chamber. Short cable runs should be used whenever possible. The thickness of the insulating material should be chosen to minimise the quantity of toxic material in the chamber, consistent with adequate electrical protection. 3.2.4

System Design

Where practicable, toxicity hazard should be reduced by keeping the electrical equipment separate from spaces containing breathing gas. The choice of position is influenced by the availability of the equipment to work in the surrounding environment. Electrical components used inside a chamber may need some specialised containment to withstand the working pressures and to protect against any toxic emissions which may occur. The wiring should be mechanically protected. The possibility of high humidity or even total immersion should also be considered as moisture could cause toxic fumes. Current ratings should be chosen to keep the normal operating temperature within accepted limits for a given insulation material. Acceptable voltage drop and the possible sustained fault current should be considered. Earth fault, overload and shortcircuit protection should be provided.

3.3

Protection Against Explosion And Fire Risk

In normal circumstances the gaseous environment of a diver or submersible pilot is not explosive, but there may be a fire hazard in certain circumstances as a result of oxygen enrichment (caused either by increase of oxygen or by increased pressure), particularly when using compressed air. Potential electrical sources of ignition include electrical arcs, sparks and hot surfaces. Type of Protection 'e' Increased Safety, as specified in BS 4683: Part 4: 1973 or in BS 5501 Part 6: 1977, incorporates supplementary protective measures to prevent the possible occurrence of excessive temperatures and arcs or sparks in apparatus which does not normally produce them. Apparatus which complies with its requirements may be applicable in the underwater case. Where apparatus contains components which may normally arc or spark, or hot surfaces capable of causing ignition, or where the possibility of potential ignition sources cannot be discounted, the apparatus should be of a protected type.

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Explosive gaseous mixtures can be produced during welding by the decomposition, by heat, of organic insulating materials. Similar operations can produce flammable and toxic gases and vapours which may accumulate in pockets or enclosures and increase the hazards. These gases may not disperse as in freely ventilated locations at the surface. On electrolysis, water produces hydrogen and oxygen in proportions which form a potentially explosive mixture. Installations should be arranged to minimise entrainment of hydrogen and oxygen as they significantly increase the risk of ignition and the consequential danger of explosion and fire. Precautions necessary to ensure electrical safety depend on the risks involved, the particular application, the nature of the gas mixtures present and the ambient pressure. Where, because of the nature of the application, it is considered that the area is not normally hazardous, due account should be taken of any possible unforeseen hazards. Particular account should be taken of the degree of control of gases present and their relative proportions, especially in manned environments. There are a number of British and other Standards and other publications which deal with the types of electrical equipment to be used in explosion or fire risk areas. However, great care should be used in applying them to underwater usage as all such standards refer to atmospheric pressure, and existing techniques for surface use are not necessarily applicable directly under water. Expert advice should be sought from manufacturers as to whether particular equipment is suitable for the environment. It may be necessary to carry out tests in some cases. Temperature classifications also vary under conditions of increased ambient pressure and they should also receive careful consideration. For applications where the absolute pressure does not exceed 1.1 bar, BS 5345: 1976 can be applied. Electrical safety in hazardous areas should be considered in a logical sequence. First, where reasonably practicable, electrical apparatus should be located outside the hazardous area. Second, if electrical apparatus has to be situated in the hazardous area then it should not contain arcing or sparking components, or hot surfaces capable of causing ignition during normal operation unless it is protected by suitable containment. Thirdly, electrical equipment intended for use in any area where moisture is likely to be present should be selected to eliminate moisture tracking and be otherwise suitable for the environment.


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APPENDICES

Page 72


APPENDIX A - BIBLIOGRAPHY SECTION 1 - Some Relevant Codes of Practice, Recommendations, Standards (Most of these documents are meant for use at atmospheric pressure and expert advice should be obtained on their applicability in the underwater environment.) BS 638: 1979, Arc welding power sources, equipment and accessories. BS 697: 1977, Specification for rubber gloves for electrical purposes. BS 3535: 1962, Safety isolating transformers for industrial and domestic purposes. BS 4137 1967, Guide to the selection of electrical equipment for use in Division 2 areas. BS 4533: Luminaires. Section 2.1: 1976, Luminaire with type of protection 'N'. BS 4683: Electrical apparatus for explosive atmospheres. BS 5000: Rotating electrical machines of particular types or for particular applications. Part 26: 1972, Type N electric motors. BS 5345: Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosive processing and manufacture). BS 5420: 1977, Specification for degrees of protection of enclosures of switchgear and control gear for voltages up to and including 1000V a.c. and 1200V d.c. BS 5490: 1977, Specification for degrees of protection provided by enclosures. BS 5501: Electrical apparatus for potentially explosive atmospheres. BS CP 1013: 1965, Earthing. IEC 79: Electrical apparatus for explosive gas atmospheres. IEC 479: 1974, Effects of current passing through the human body. IEC 502: 1978, Extruded solid-dielectric insulated power cables for rated voltages from 1 kV up to 30 kV. Defence Standard 61 - 12 (Issue 1) Part 18: Equipment wires, low toxicity, 13 October, 1978. Defence Standard 61 - 16 Differential current operated earth-leaked circuit-breakers, July 1982. Naval Engineering Standard 711 (Issue 2): Determination of the smoke index of the products of combustion from small specimens of materials, January, 1981.


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Naval Engineering Standard 713 (Issue): Determination of the toxicity index of the products of combustion from small specimens of materials, April 1981. Naval Engineering Standard 715 (Issue 2): Determination of the temperature index of small specimens of materials, March 1981. IEE Regulations for the electrical and electronic equipment of ships with recommended practice for their implementation, 5th Edition 1972. IEE Recommendations for the electrical and electronic equipment of Mobile and Fixed Offshore Installations - First Edition, 1983. The principles of safe diving practice, UR23, CIRIA Underwater Engineering Group, 1984. Underwater electrical safety - some guidance on protection against electric shock, UR14, CIRIA Underwater Engineering Group, 1979. Code of Practice for the safe use of electricity underwater, Department of Energy 1982. OT/0/8325.

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APPENDIX A - BIBLIOGRAPHY SECTION 2 - Some Technical Reference Papers 1. Protection of divers against electric shock - physiological criteria, ERA REPORT 771063, JUNE 1977, OT-R-8272. 2. Shock risk to swimmers and divers from an electrical field, Project ref 0367/TC/65, September 1980, OT-R-8273. 3. Calculation of safe distance from an electrically live object in water, Project ref 0367/TC/70, January 1981, OT-R-8274. 4. Methods of protection against electric shock under water - earth leakage circuitbreakers, Project ref 0367/TC/21, September 1979, OT-R-8275 . 5. Protection against electric shock under water - line-insulation circuit-breakers, Project ref 0367/ TC/21, September 1979, OT-R-8276. 6. Inherently safe systems, Project ref 0367/TC/23 (revised), December 1979, OT-R-8277. 7. Warning of electrical fields in water, project ref. 0367/TC/26B, December 1979, OT-R-8278. 8. Protection against shock from arc welding and cutting sets, Project ref 0367/TC/26A, December 1979, OT-R-8279. 9. An assessment of toxic gases from overheated electrical equipment, Project ref 0367/12, January 1979, OT-R-8280. 10. Emission of toxic gases from electrical equipment, Project ref. 0367/TC/5, July 1979, OT-R 8281. 11. Application of fatal concentrations and toxicity index from Defence Standard 61 12/18 and Naval Engineering Standard 713, Project ref 0367/TC/29, January 1980, OT-R-8282. 12. Polymeric materials commonly used in electrical equipment with calculations of their possible toxicity, Project ref 0367/TC/29 (appendix) January 198O, OT-R-8283. 13. Review of the requirements for explosion-protected electrical equipment under water: flammability characteristics of gases and vapours, Project ref 0367/TC/49. OTR-8284. 14. Requirements for explosion-protected electrical equipment under water: relevance of current standards for surface industry applications; Project ref 0367/TC/54, August 1980, OT-R8285. 15. Requirements for explosion-protected electrical equipment under water: assessment of flammability limits, Project ref 0367/TC/63, October 1980, OT-R-8286.


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16. An investigation into shock criteria for durations less than 10 milliseconds, Project ref 44/03/ 0894 OT-0-8327. 17. An investigation into the provision of passive protection by diving suits. Project ref 44/03/0896 OT-0-8329. 18. Danger levels of electrical shock at 50Hz in hyperbaric helium/oxygen gas mixtures, Project ref 44/03/3672 OT-0-8328. 19. Shock risk from d.c. arc welding sets - the effect of a.c. components in the electrode voltage. G. Mole and H.W. Turner. Project ref. OT/R/7976. Note All of the above reference papers were produced by ERA Technology Limited under contract to various government departments. Copies of these reference papers are available on request from: Membership Office, Service Enquiry Section, British Library Lending Division, Boston Spa, Wetherby, West Yorkshire, LS23 7BQ Tel: 0937 - 843434. 20. Dalziel C.F. Effects of electric shock on man, IRE Trans on medical electronics, May 1959. PGME - 5, pp. 44-62. 21. G. Biegelmeier and W.R. Lee, "New considerations on the threshold of ventricular fibrillation for a.c. shocks at 50-60Hz". IEE Proc. 127, 2Pt. A, (March 1980). 22. J. Jacobsen, S. Buntenkotter and H.J. Reinhard, "Experimentelle Untersuchungen an Schweinen zue Frage der Mortalitat durch sinustormige, phasenangeschmittene sowie gleichgerichtete elektrisch Strome". Biomedizinische Technik, 20. (1975) 99-107.

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APPENDIX B - USEFUL FORMULAE 1. Section 2 paragraph 2.0.2.1 states that there is a maximum limit which should be imposed on the length of cable connecting the transformer secondary winding to the load, due to its inherent capacitance. The maximum safe cable length in metres is given by the formula: cable length

=

I x 106 2 π fVC

where: I = safe current (mA) f = frequency (Hz) V = supply voltage (V) C = cable capacitance (nF/m) 2. Electrical safety can be ensured if the diver can be prevented from coming closer to any possible electrical fault than a specified safe distance. (See Section 2 Paragraph 2.0.2.2). Values of safe distance in water depend on the ratio of fault current (Io) to the safe body current (Ib). Guidance on levels of safe body current is given in the DISCUSSION (Page 8). For seawater, the approximate safe distance (in metres) is given by: 1+

10-4 IO Ib

½

-1

In fresh water, the safe distance (in metres) is much greater and is given approximately by: 1+

IO 40Ib

½

-1

A ripple-free d.c. source with voltage limitation will reduce the safe distance. If the equipment is supplied with direct current and the voltage does not exceed 30V, then the safe distance is zero and a barrier is unnecessary. NOTE - The references for derivation of these two equations and the simplifying assumptions made are in Appendix A, Section 2, numbers 2 and 3. A table of typical safe distances in salt water and fresh water indicates the substantial differences in the two environments. AC Fault Current Safe Body Current Io(A) Ib(A)

Safe Distance (m) Salt Water Fresh Water

10,000

0.01

9.05

157

1,000

0.01

2.32

49

100

0.01

0.41

15


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APPENDIX C - ALLOWABLE A.C. CURRENT IN THE BODY The graph (Figure 13) coordinates the results of a number of research workers. The IEC 479 curve was published in 1974, before the work of Dr Green was carried out. His work allows an extension of the graph to 0.1 millisecond and gives rise to the dashed "provisional extension" line. The graph gives a reliable overall view, and shows broadly that shock energy is the main criterion. The shorter shocks are slices of a sine wave and the r.m.s. equivalent has been derived from the energy content in one millisecond samples. Other work (not shown) on electrical discharges supports the short duration shock limit. It must be noted however that, in some work, the shock was synchronised with the vulnerable part of the heart-beat whereas, in other work, it was randomly applied. Apart from the 'let-go' level, the work was carried out on animals and there is an unavoidable small doubt in applying the results to humans. A 'let-go' level of 0.5% means that only one in 200 volunteers was able (or willing) to release his grasp of the live bar. The references on which the graph is based are listed in Appendix A Section 2.


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APPENDIX D - EARTH FAULT CURRENT RESTRICTION Differential current operated RCDs offer several major advantages over other earthfault protection devices, eg. reliability, availability and circuit discrimination. The latter is particularly important in the diving situation since it is undesirable to switch off certain items of equipment whose circuitry is 'healthy' in the event of an earth-fault occurring on another circuit connected to the same isolating transformer. However, in order that differential current RCDs may operate, a secondary return path must exist. This is conveniently achieved using an earthing impedance (Zn) between the star-point, or central tapping on the secondary winding of the isolating transformer, and 'earth'. Ignoring the effect of cable capacitance, this will allow a maximum current, Ie., to flow through the earth-return circuit (the water) where Ie

=

V Zn

V being the open circuit voltage between the power supply system and 'earth' at the point of occurrence of the earth-fault. Usually V will be the phase voltage of the system. The question remains as to the choice of Zn and hence Ie. From the point of view of risk to the diver from through-water electric currents, the lower Ie the better. However, from the point of view of RCD operation, the higher the Ie the better. In the event of an earth-fault it is important to ensure adequate tripping margin over the RCD tripping level. National Coal Board experience with similar systems suggests that Ie should be at least 10 x RCD trip level. This factor of 10 allows for the reduced tripping current available if a phase-phase fault occurs simultaneously with an earth-fault. Although satisfactory performance has been experienced with RCD tripping levels of 2030 mA, such a level may lead to spurious tripping problems, particularly with longer cable lengths than are usual at present. However, making 'worst' assumptions about cable lengths and capacitance, a tripping level of less than 100 mA (NCB level is 80 mA) will be practicable. Thus, Earth Fault Current Restriction at 1 amp is acceptable and has been used in this document.


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AODC035 - Safe use of electricity underwater.pdf

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