Oil -filled electrical distribution and other switchgear

OC 483/27

This OC and attached Information Document (ID) updates and replaces FIM 1989/128 Performance of oil circuit breakers under fault conditions. The ID, which covers oil- filled electrical distribution and other switchgear that is about 30 or more years old, gives guidance to users regarding assessment of the risks and precautions to be taken. The ID may be given to interested persons outside HSE (see para 4).


1 The incoming electrical supply into any premises requiring a high or low-voltage supply usually passes through switchgear, whose purpose is to interrupt, without danger, electrical faults which develop on the system and safely switch electric current "on" or "off".

2 There are risks of failure of switchgear due to its age, condition, installation conditions, design (eg manually operated), which could result in death or serious injury.

3 Distribution switchgear is normally located in substations or switchrooms which are locked and to which access is restricted to authorised persons. Inspectors will rarely see it and may not be aware of its presence or age and condition.

4 Consequently an ID has been produced with the intention of helping managers identify and control electrical risks from this equipment. The ID can be given/sent to occupiers in older premises (30+ years) who, because they have a high voltage supply, are likely to have oil-filled distribution switchgear. Due to the high numbers of premises involved, additional copies of the ID will be sent to all areas shortly.

National project

5 OM 1995/2 Supplement 10 gives details of the national project which will be undertaken in 1995/1996 and may continue into the following year. This project is designed to obtain more information about approximately how much oil-filled switchgear remains in use, its age and general condition and whether it is more likely to be used by particular sectors. As part of this project a standard letter and brief proforma will be sent to selected premises, together with the ID.

7 Where the age, condition or method or operation of the switchgear results in a significant risk (see ID para 5), appropriate enforcement action may be necessary. Under such circumstances FCGs should be consulted at an early stage. When enforcement action is taken, the NIG covering that sector and FOD C2, Room 708 Daniel House, Bootle, should be informed.

8 Where high-voltage equipment is in use, inspectors should check the user's procedures for management of the electrical risks.

9 The appendix, which gives suggested time-scales for action and precautions to be taken, may be useful to inspectors, but the actual time-scales will vary according to the risk and the circumstances at any particular premises or company.

11 OM 1995/2 Supplement 10 para 6 -note "See OC 483/27 for Information Document".

29 March 1995

(FOD/ 447/95)

Disc Ref: FODF1\H:\CPOC\48327\DH


ASI headings

Electric(ity): national projects: switchgear and switches.

Appendix: Suggested time-scales for action and precautions to be taken

(para 9)

These suggested time-scales for action to be taken are based upon practical experience and advice from switchgear manufacturers and have been found to give sufficient time to both plan and execute the necessary work whilst limiting the potential risks.

Assess switchgear rating :

(1) as soon as possible .

Overstressed switchgear :

(1) prevent access to live switchgear immediately .

(2) Prohibit all live operation and automatic tripping as soon as possible .

(3) Take actions as soon as possible to reduce fault levels.

(4) Maintain switchgear as soon as possible .

(5) Arrange to remove under-rated switchgear from the system and replace with suitable rated switchgear within a period not exceeding 12 months .

Dependent manual operation:

(1) prohibit manual operation of live hv dmo switchgear immediately unless circuits have been tested before operation

(2) Restrict operation to trained staff immediately .

(3) Fit power closing mechanisms where appropriate within 12 months .

(4) Replace all switchgear manufactured prior to 1960 and those which cannot be fitted with power closing mechanisms within 3 years .

(5) Replace all modified switchgear within 8 years .

(6) Maintain all DMO switchgear that has not been maintained in the last 3 years immediately. Thereafter, maintain every 12 months.


(1) check with manufacturers regarding recommended modifications and update as necessary as soon as possible.

Anti-reflex operating handles :

(1) fit necessary handles within a period not exceeding 6 months from date problem identified.

Health and safety executive

Information document

HSE 483/27


1 This document contains internal guidance which has been made available to the public. The guidance is considered good practice but is not compulsory. You may find it useful in deciding what you need to do. However, the guidance may not be directly applicable in all circumstances and any queries should be directed to the appropriate enforcing authority.

2 This document contains information for the attention of both managers and technical staff concerning the electrical risks (see para 5) that can arise from the use of high-voltage and low-voltage oil-filled electrical distribution and other switchgear which was manufactured prior to 1970. The term 'switchgear' includes : oil-switches, oil-isolators, oil- switch fuses and, in particular, oil circuit-breakers (see Appendix 1 for definitions). Advice is given on the precautions which should be taken to eliminate or control these risks. (Because of the nature of this subject many of the terms used will be more easily understood by electrical engineers than by managers).

3 The document is based upon accident/incident investigations, the testing of some types of equipment, users' experience, advice from manufacturers and information from British and international standards.

4 The guidance does not address direct current (dc) systems or single-phase alternating current (ac) traction systems of any voltage, but users of such systems may still find the document useful.


5 In general, oil-filled switchgear has a proven record of reliability and performance. Failures are rare but, where they occur, the results may be catastrophic. Tanks may rupture, resulting in the ejection of burning oil and gas clouds, causing death or serious injury to persons and major damage to plant and buildings in the vicinity of the failed equipment. Accident experience has shown that failure usually occurs at, or shortly after, operation of the equipment. Thus, the way switchgear is operated, its condition and the circumstances existing in the system at the time of operation, to a large extent, determines whether the equipment will safely perform its duty.

6 Switchgear of all types and ratings has been manufactured in accordance with British and international standards for a period in excess of 60 years. As with most equipment, however, current specifications bear little resemblance to those of earlier years in that the previous specifications have been shown, by subsequent experience and by technical developments, to be deficient. Examples of differing requirements between earlier and current standards are those relating to operating mechanisms and fault test sequences.


7 The Health and Safety at Work etc Act 1974 (HSW Act), the Management of Health and Safety at Work Regulations 1992 (Management Regulations) and the Electricity at Work Regulations 1989 (EAW Regulations) apply. The HSW Act contains requirements to the effect that anyone employing people should ensure their safety so far as is reasonably practicable. The Management Regulations require an employer (or self-employed person) to make an assessment of risks to employees or others, taking specialist advice where necessary. The level of detail in the assessment should be broadly proportionate to the risk, which means it needs to be fairly detailed in the case of oil-filled switchgear as the risk is one of death. The EAW Regulations require electrical equipment for use at work to be constructed, maintained and operated in such a way as to prevent danger so far as is reasonably practicable. Equipment should also not be used where its strength and capability may be exceeded and it should be protected from excess current.

Equipment and its location

8 The incoming electrical supply to any premises which requires a high voltage supply often passes through switchgear. Switchgear varies in size, age and appearance. Typical examples are shown at Appendix 2 , Figures 1 and 2.

9 The purpose of oil-filled circuit breakers is to interrupt, without danger, electrical faults which develop on the system and switch electric current ON or OFF safely, particularly when operated by hand.

10 Such switchgear tanks contain insulating oil and the contacts, which open and close the electrical circuits, are immersed in the oil. The oil provides for insulation and acts as an aid to arc extinction when the contacts open.

11 Switchgear is generally located in substations and switchrooms, ie areas that are separated from the day-to-day activity of the premises and which, in many instances, are visited on a very infrequent basis. Such rooms are generally locked and access is usually restricted to authorised persons.

12 In some premises low-voltage switchgear will be found located in work activity areas. There will also be cases where high-voltage switchgear and starters are found adjacent to the machinery which it controls, a typical example being a large rubber mill. Occasionally switchgear is located outdoors.

13 Much of the equipment still in service is some 20 years old or more. Equipment that is 30 or more years old is usually of particularly robust construction. As a result it is often assumed, incorrectly, that the equipment has an unlimited life in service. Much of the older equipment has a limited operation capability and these limitations are not always understood by present-day operating staff.

Potential problems with switchgear

14 There are a number of potential problems that may be encountered with oil-filled electrical switchgear that is more than 20 years old. These include:

(1) lack of knowledge of the equipment;

(2) being overstressed;

(3) not being modified as per manufacturer's advice;

(4) having dependent manual operating switchgear, ie where the movement of the contacts is directly dependent on the movement of the handle by the operator;

(5) not being maintained properly; and

(6) being fitted with operating handles which are not anti-reflex type (high-voltage oil-switches and isolators only).

20 This is usually the result of oversight, lack of knowledge of the equipment, or pressures to keep the equipment, and hence the plant, in operation. In many cases the expertise in handling and maintenance techniques for insulating oil is lacking. Where oil-filled switchgear has been neglected, it is difficult to assess the actual fault capability of the switchgear in the state in which it is found.

Action to be taken

22 Before deciding upon the precautions to be taken, all such switchgear that is in service should be identified. The potential risks, eg overstressing, dependent manual operation, should be assessed so that any necessary remedial action can be identified to ensure that the equipment and systems are being operated safely, and that work is put in hand to eliminate or reduce the risks (see paragraph 24 onwards).

23 In some cases, sufficient technical expertise may not be available in-house to carry out an assessment of risk and to decide on the appropriate precautions. However, having identified that a problem exists or may exist, switchgear users should be able to reach decisions about seeking further help from suitably competent organisations. Such organisations include:

(1) regional electricity companies;

(2) switchgear manufacturers (see Appendix 3 for list of medium and high-voltage switchgear manufacturers and their contact points);

(3) switchgear maintenance companies with particular expertise in older types of switchgear; and

(4) consulting organisations specialising in switchgear.

Note The Trade Association, British Electrotechnical and Allied Manufacturers Association (BEAMA) may also be able to provide help and guidance as to other sources of information and expertise (see Appendix 3 for their address).

(1) location;

(2) manufacturer and type reference for each item of equipment and type of equipment;

(3) serial number and year of manufacture;

(4) date of installation;

(5) voltage rating;

(6) current rating;

(7) fault rating and whether it is a certified or assessed rating;

(8) type of operating mechanism (dependent manual, independent manual, dependent power, independent power and stored energy);

(9) details of any modifications or repairs, eg fitted anti-reflex handles;

(10) date equipment last maintained/serviced;

(11) if the equipment is an oil circuit-breaker whether it is a plain break equipment (ie equipment without arc control devices) or not; and

(12) type of electrical protection fitted and details of the settings, eg HRC fuse 30amps.

(1) identify the British or other standards relevant to the individual switchgear;

(2) calculate fault energy levels at the output terminals of each item of switchgear. In some cases it will be necessary to include the fault energy contribution from rotating plant on site. The electricity supply company can provide, on request, the maximum short-circuit current at the incoming supply terminals. (Electricity Supply Regulations 1988 regulation 32);

(3) determine the switchgear rating. Where necessary, seek re-assessment of the rating by manufacturers or specialists of those items of switchgear designed to obsolete British standards (see paragraph 46), taking account of those standards identified at (1) above; and

(4) compare the fault energy levels calculated at (2) above with those certified or assigned switchgear fault energy ratings determined at (3) above, to establish whether the equipment is overstressed. Through-fault ratings for the switchgear and any assigned ratings given by the manufacturer or specialist resulting from re-assessment of previous fault energy ratings should be included.

26 It is recommended that the procedures are applied to both high-voltage (ie 3.3 kV, 6.6 kV, 11 kV, 22 kV, and 33 kV) systems and low-voltage (230/415 volt) systems as separate exercises. It may be easier to deal with each individual system at each voltage separately as the individual systems may have very different problems.

Precautions to reduce the risk

27 The need for precautions and how quickly they should be instigated will depend on whether the equipment is overstressed, whether it has been modified in accordance with manufacturer's instructions, the type of operating mechanism, the maintenance condition etc.

(1) Prohibit all live operation and automatic tripping of the switchgear. This action will necessitate re-adjustment of electrical protection further back towards the source of supply in order that the electrical protection at the switchgear can be made non-operative. The re-adjustment is needed to ensure adequate levels of electrical protection for the system.

(2) Prevent access by persons to the switchgear whilst it is live.

(3) The switchgear should be maintained in accordance with manufacturer's advice by trained personnel. Particular attention should be paid to insulating oil, solid insulation, contact assemblies, operating mechanisms, seals and gaskets (see paragraph 37).

(4) Reduce the fault energy levels. In some cases changing system operating conditions will achieve this, for example operating transformers as single feeders to switchboards and not in parallel with other transformers. These changes should be made as soon as possible to reduce the fault energy level to as low a value as practicable.

(5) Longer-term measures that can be taken to reduce fault energy levels include fitting reactors or re-cabling of systems. These measures may be used to reduce fault energy levels to values less than the fault energy ratings of switchgear. They will not overcome the problems associated with switchgear that has no fault energy rating. This action is normally only a solution for high-voltage switchgear.

Note: The cabling might be as old as the switchgear, hence other problems such as cable failure could occur as a result of disturbing it.

29 Where the action in paragraph 28 reduces the fault energy levels below the ratings of the switchgear then electrical protection and live operation can be restored, after interlocks or other measures have been provided to prevent the rating being exceeded at any time, eg to prevent the paralleling of transformers onto the switchgear busbars.

30 If this action (in paragraph 28) does not reduce the fault energy levels below the ratings of the switchgear, and it is sited in open workshop areas, the provision of blast protection should be considered. This may take the form of suitable walls, or sand-bagged enclosures. The purpose of these is to contain any failure of the switchgear whilst it is energised and prevent injury to persons. However, this is a complex matter and it is often more practical to make the switchgear dead and provide alternative electrical supplies.

31 Where high-voltage and low-voltage switchgear share the same switchroom and only one set of switchgear is under-rated, it will be necessary to either:

(1) keep personnel out until the under-rated switchgear is made dead; or

(2) where space permits, erect a suitable blast wall around that switchgear, so permitting personnel access to the other switchgear. ( See paragraph 30)

32 Arrangements should be made to replace the under-rated switchgear as soon as possible. This will often be made easier by re-design of the electrical system. Priority should be given to replacing such switchgear that is both overstressed and DMO.

Dependent manually-operated switchgear

33 All operation and maintenance of dependent manually-operated switchgear (DMOS) on site should be restricted to those personnel trained in the operation of the switchgear concerned and who are conversant with the dangers of mal-operation, the construction of the switchgear and the manufacturer's maintenance requirement s. These personnel will need to be conversant with the safe system of work outlined in paragraph 34(4). Where this switchgear is also overstressed the precautions in paragraph 28(1) and (2) are particularly important in the short term.

34 Where the switchgear is not overstressed (ie ratings are greater than the actual fault energy levels), the following precautions are needed to reduce the risks that result from the fact that it has dependent manual operation:

(1) All DMOS should be maintained in accordance with manufacturer's advice. This should include the checking of seals and gaskets which should be properly installed and in good condition. An annual maintenance schedule for this equipment should be prepared and be implemented, see paragraph 37.

(2) Power closing mechanisms should be fitted as a matter of urgency to all high- voltage DMOS (ie 3.3 kV, 6.6 kV, 11 kV). However, this should only be carried out in accordance with the manufacturer's advice. It may not be possible to obtain the necessary guidance and advice where the original manufacturer no longer exists and there are no agents. In these cases it is not advisable to fit power closing mechanisms.

(3) A phased replacement programme for all the switchgear should be prepared and implemented for all DMOS manufactured prior to 1960 and for those high-voltage systems manufactured and installed after 1960, which cannot be fitted with power closing mechanisms. In some cases it is possible to obtain replacement circuit breakers of modern (airbreak) design (often called cassettes) which can be used to replace old high and low-voltage units and which can use the existing switchgear busbar housings and support arrangements. This approach can mitigate the costs of replacement. Advice should be sought from the manufacturer regarding this approach.

(4) When a DMO circuit-breaker is to be closed the preferred method of operation is as follows (in order to achieve this preferred method of operation it may be necessary to change system running conditions and adjust the electrical protection accordingly):

(a) make the system dead upstream using a suitably rated independent operated switch or circuit-breaker;

(b) check, where practicable, the system beyond the DMO circuit-breaker to ensure that it is fault-free. This will mean applying various electrical tests to the system;

(c) If the system is healthy, close the DMO circuit-breaker to ON; and

(d) energise the system from the remote point ensuring that no personnel are in the vicinity of the DMO circuit-breaker.

(5) The following can however, be operated with the system live:

(a) bus section couplers on a fully energised system; and

(b) circuits that have been tested immediately before closure.

(6) Where the circuit-breaker has recently been operated for the purpose of routine isolation, it may be reclosed manually providing the electrical circuit it feeds has not been disturbed.

35 Where work has been undertaken on the electrical system normally made live by a DMO circuit-breaker, the circuit should be comprehensively tested prior to the operation of the circuit-breaker.

(1) cleaning of the internal mechanism, contacts and the oil tank;

(2) replacement of the existing insulating oil with new or reconditioned insulating oil;

(3) dressing, refurbishing or replacing contacts as appropriate;

(4) replace seals and gaskets;

(5) adjusting mechanisms; and

(6) testing electrical protection periodically and, where necessary, after fault operation.

Thereafter it should receive maintenance at a frequency appropriate to the equipment. The manufacturer, or others, may be able to give advice on this.

Oil isolators

40 These have limited capability and should only be operated when dead. They should be labelled "Dead Operation Only". They should be fitted with non-standard operational locks whose key should be kept in a secure place under the control of a responsible person. They should only be operated by trained competent staff who understand the limitations of the equipment.

42 Short-circuit protection is afforded by other switchgear such as circuit-breakers or fuses. For high-voltage systems, the usual device is a circuit-breaker.

43 It is important that the incoming device is fully capable of dealing with the energy levels present. These devices should be assessed in the same manner as all other switchgear. Where deficiencies or short-comings are found appropriate remedial action should be taken.

44 Due to the more frequent operation of these units compared to other electrical switchgear, suitable maintenance procedures are particularly important.


45 It is important that programmes of work are developed in order to ensure that action to eliminate or reduce the risks will take place. In most instances it should be possible to put in hand modifications to operating procedures immediately. However, replacement of overstressed switchgear or DMO mechanisms will take time.

46 Appendix 4 contains a list of useful references, including obsolete British standards which may now only be available through the archives of switchgear manufacturers and specialists.

47 Further advice or information can be obtained from the local HSE area office.

March 1995

Technical definitions

Low voltage normally exceeding 50v ac or 120v dc but not exceeding 1000v ac or 1500v dc between conductors, or 600v ac or 900v dc between conductors and earth.

High voltage normally exceeding 1000v ac or 1500v dc between conductors, or 600v ac or 900v dc between conductors and earth.

Note : Some companies and persons use the term "medium voltage" to describe distribution voltages in range 3.3 kv to 72.5 kv to distinguish these from the higher values of voltage associated with transmission systems. There is no international electrotechnical vocabulary (iev) meaning which specifies values; all that is stated is that the upper value lies between 30 kv and 100 kv. The term has not been used in the uk to prevent confusion with the widely understood use of the term for 415 v 3 phase systems.

Oil-isolator this is a switching device which is used to open (or close) a circuit in oil either when negligible current is interrupted (or established) or when no significant change in the voltage across the terminals of each pole or phase of the isolator will result from the operation.

Oil-switch this is a switching device suitable for making or closing a circuit under normal and abnormal conditions such as those of short-circuit, and capable of breaking or opening a circuit in oil under normal conditions.

Oil circuit-breaker this is a switching device capable of making and breaking, or closing and opening, a circuit in oil under normal conditions and under abnormal conditions such as those of short-circuit.

Oil-switch fuse this is a switching device in which the fuses, which may be mounted in air or be immersed in oil, are connected in series with an oil-switch in a combined assembly.

Note: all of these types of switching devices can be found in service on high voltage systems. The devices found in service on low-voltage systems will be oil circuit-breakers.

Dependent manual operation (of a mechanical switching device)

Independent manual operation (of a mechanical switching device)

Dependent power operation (of a mechanical switching device)

Independent power operation (of a mechanical switching device)

Stored energy operation (of a mechanical switching device)

Figure 3. Typical anti-reflex operating handle

APPENDIX 3 : Medium and high voltage switchgear manufacturers

(para 23(2))

Contact points

GEC Alsthom T & D Switchgear Ltd A Kinder
Higher Openshaw Technical Manager
Manchester M11 1FL
Tel: 0161 370 4000
Fax: 0161 370 3925
Hawker Siddeley Switchgear Ltd D Steadman
PO Box 19 Customer Services Manager
Loughborough (Direct Line 01509 612930)
Leics LE11 1HL
Tel: 01509 611311
Fax: 01509 610404
Long & Crawford Ltd A Wyatt
Gorton Road Manager - Technical Support
West Gorton and After Sales
Manchester (Direct Line 0161 230 2208)
M12 5DA
Tel: 0161 223 3235
Fax: 0161 223 5379
W Lucy & Co Ltd J Rix
Eagle ironworks Sales Manager
Walton Well Road
Oxford OX2 6EE
Tel: 01685 311411
Fax: 01865 310504
Merlin Gerin Ltd D Hudson
Medium Voltage Division Customer Service
Meanwood Road QA Manager
Leeds LS6 2BN
Tel: 01532 757121
Fax: 01532 743698
Reyrolle Switchgear Ltd A Hartridge
Hebburn Manager
Tyne & Wear Special Services Department
NE31 1UP (Direct Line 0191 401 5488)
Tel: 0191 401 5555
Fax: 0191 401 5353
Whipp & Bourne Ltd D Taylor
Switchgear Works Business Development Manager
Lancs OL11 2SS


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