Keeping electrical switchgear safe

Keeping electrical switchgear safe

Health and Safety Executive Failure of electrical switchgear can cause death, serious injury and major damage. If you own or operate this type of equipment in industrial or commercial Keeping electrical switchgear safe organisations, this book is mainly aimed at you. It should help you to select, use and maintain switchgear safely and reduce the risk of accidents.

HSG230 (2nd edition) Published 2015

HSE Books

© Crown copyright 2015

First published 2002 Second edition 2015

ISBN 978 0 7176 6557 0

You may reuse this information (excluding logos) free of charge in any format or medium, under the terms of the Open Government Licence. To view the licence visit www.nationalarchives.gov.uk/doc/open-government-licence/, write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email psi@nationalarchives.gsi.gov.uk.

Some images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to copyright@hse.gsi.gov.uk.

This guidance is issued by the Health and Safety Executive. Following the guidance is not compulsory, unless specifically stated, and you are free to take other action. But if you do follow the guidance you will normally be doing enough to comply with the law. Health and safety inspectors seek to secure compliance with the law and may refer to this guidance.

Contents Introduction 5 Scope 5 Background 5 What does the law require? 5

Switchgear safety 6 Understand your system 6 Maintenance 7 Design rating and system modifications 8 Design modifications and limitations on use 8 Dependent manual operating mechanisms 8 Anti-reflex handles 9

Management of switchgear 9 Management systems 9 Training and competence 10 Operational safety documents 11 Network diagram 12 Maintenance systems and asset registers 12 Maintenance records 13

Operation of switchgear 13 Fault level and ratings 13 Effect of on-site generation and other large rotating machines 14

Reducing the risk of switchgear failure 14 Overstressed switchgear 14 Dependent manually operated (DMO) switchgear 15 Fault clearance 16

Care and maintenance of HV switchgear: Common requirements 16 Time-based preventive maintenance 16 Condition-based maintenance 17 Reliability-centred maintenance (RCM) 18 Switchgear availability 18 Trip-testing 18 Protection testing 19 Tests to be undertaken following maintenance 19 Routine inspection of substations and switch rooms 19 Signs of abnormal conditions 19 Equipment environment 20 Visual examination of switchgear 20 General services 20 Circuit-breakers subject to special duty 21 Disposal issues, post-maintenance 21

Care and maintenance of oil-filled switchgear 21 Maintenance procedures 22 Frequency of maintenance 23

Cleaning and inspection of oil-filled switchgear 23 Tank cleaning techniques 24 Insulating oil 24 Post-fault maintenance of oil-filled circuit-breakers 25

Care and maintenance of non-oil switchgear 25 General guidance 25 Maintenance procedures: Vacuum switchgear 26 Maintenance procedures: Air-brake switchgear 27 Maintenance procedures: SF6 switchgear 27 SF6 gas handling and safety precautions 28 Release of SF6 28 Hazards 28 Sampling 29

Care and maintenance of ancillary equipment 29 Test probes 29 Earthing equipment 30

Assessment of aged switchgear 31

Protection 34 Protection relay schemes 34 Fuse protection 35 Batteries and chargers 35

Selection of new, replacement or refurbished switchgear 36 General advice 36 Retrofit circuit-breakers for withdrawable switchgear 37 Second-hand equipment 37

Measures to limit fires 37 Compartmentation 37 Control and extinction 37 Detection 37 Safety issues 38

Appendix 1: Examples of switchgear configurations 39

References and further reading 47

Glossary 49

Further information 50

Introduction

1 This guidance is aimed at owners and operators of electrical switchgear in industrial and commercial organisations. It may also be useful to others. It will help managers, engineers and others to understand their responsibilities and duties in the selection, use, operation and maintenance of high-voltage switchgear. Some knowledge of electrical switchgear and distribution systems is necessary to gain most benefit from this document. Owners and operators of electrical switchgear with little electrical knowledge or expertise should refer to Electrical switchgear safety: A guide for owners and users.1

Scope

2 Guidance is given on the selection, use, operation and maintenance of three- phase electrical switchgear with voltage ratings from 1 kV alternating current (AC) up to and including 33 kV AC. This includes circuit-breakers, switches, switch fuses, isolators and high-voltage (HV) contactors that use oil, air, sulphur hexafluoride (SF6) or vacuum as the interrupting medium. Guidance is also provided on assessing risks associated with aged switchgear and the actions necessary when considering replacement or refurbishment of switchgear. Low-voltage (ie below 1 kV) equipment is out of the scope of this guidance, although some references are made to low-voltage (LV) equipment where it is an essential part of an HV system.

3 This guidance does not address direct current (DC) switchgear, switchgear used on single-phase AC traction systems, or power electronic switching devices used at high voltage such as inverters, cyclo-converters and rectifiers.

Background

4 Switchgear failures are rare, but when they do occur, the results can be catastrophic. In the case of oil-filled switchgear, burning oil and gas can be ejected, causing death or serious injury to people who may be nearby, and major damage to equipment and buildings. Switchgear using SF6 gas or vacuum as the insulating medium presents other risks that need to be controlled and managed.

5 Failures are most likely to occur during, or shortly after, switchgear operation. The way switchgear is operated, its condition and the conditions in the electrical system at the time it operates will largely determine whether it will function safely.

What does the law require?

6 The Health and Safety at Work etc Act 1974 (the HSW Act),2 the Management of Health and Safety at Work Regulations 1999 (the Management Regulations)3 and the Electricity at Work Regulations 1989 (EAWR)4 are applicable to the selection, use, operation and maintenance of high-voltage switchgear. The HSW Act also places duties on the manufacturers of switchgear.

7 Older switchgear may contain parts that were manufactured from asbestos or asbestos-containing materials (ACMs). The Control of Asbestos Regulations 20125 place a duty on those who have responsibility for the maintenance and repair of equipment to manage the risks from the potential for exposure to asbestos. This includes the responsibility to determine if asbestos is present so that others can be made aware of the hazard and take appropriate action. Advice should be sought from the original equipment supplier or a specialist maintenance provider on the likelihood of asbestos or ACMs being present in switchgear.

8 The HSW Act requires employers to ensure, so far as is reasonably practicable (see paragraph 11), the safety of employees and other people who may be affected by their activities. Similarly, self-employed people while at work must ensure that they do not expose themselves to risks to their own health and safety, and that they do not affect the health and safety of others.

9 The Management Regulations require an employer or self-employed person to make an assessment of the risks to employees and others. To do this, they should consider what might cause harm to people and decide whether they are doing enough to prevent that. For more information on risk assessment, see Risk assessment: A brief guide to controlling risks in the workplace.6

10 The EAWR requires electrical equipment, which includes switchgear, for use at work to be constructed, maintained and operated in such a way as to prevent, so far as is reasonably practicable, danger. Electrical equipment, including switchgear, must not be used where its strength or capability may be exceeded, unless it is used in such a way that nobody could be exposed to danger. This includes protection against the effects of excess current and exposure to the physical environment in which the switchgear is located. People who work on or near to electrical equipment, and those responsible for managing such work activities, must be suitably competent for the activity to be undertaken (ie they must have the appropriate level of technical knowledge or experience). Alternatively, those working on switchgear must be suitably supervised.

11 ‘So far as is reasonably practicable’ means balancing the level of risk against the measures needed to control the real risk in terms of money, time or trouble. However, you do not need to take action if it would be grossly disproportionate to the level of risk. In the context of HV switchgear, the procedures to select and maintain suitable equipment and to ensure the safety of those working on or near it are well established. These are supported by existing standards, guidance and training qualifications. Many organisations successfully apply these measures to the management of their switchgear. For certain work activities on switchgear, there can be a risk of death, and this must be taken into account when considering the measures needed to ensure safety when such tasks are completed. For more information, see Risk assessment: A brief guide to controlling risks in the workplace.

Switchgear safety

12 The use, maintenance and operation of high-voltage switchgear must be managed to prevent both the equipment giving rise to danger and to ensure the safety of the people who use it. Allowing equipment to become unsafe and, as a result, exposing people to danger during its use is likely to breach the law. Oil-filled switchgear presents particular issues not encountered with other types of equipment.

Understand your system

13 Switchgear varies in size, age and appearance. Examples are shown in Appendix 1. Different types of switchgear such as switches, isolators, switch fuses, contactors and circuit-breakers have different switching and fault-handling capabilities. It is important to be able to recognise the different types of switchgear and understand the differences in their capability. Failure to do so can result in switchgear being used incorrectly, which may be dangerous for the operator.

14 Some switchgear may appear to be of robust construction due to its size. Despite appearances, it may have limited operational capability and be restricted in

the way it should be used. Any limitations or restrictions must be understood by those who operate or maintain the equipment.

15 HV switchgear may be located close to machinery controlled by it. People who have access to the switchgear may not be electrically competent, but must be made aware of its presence and the potential hazards associated with it. They should know what to do if they notice anything unusual or wrong with the switchgear.

16 In certain industries, the location of switchgear in relation to the process activity is important. For example, for explosives manufacture and storage, HV equipment should be sited outdoors, see Explosives Regulations 2014: Safety provisions. Guidance on Regulations.7 It is important that the location of HV switchgear is considered if the use of a premises changes.

17 Switchgear must only be operated by people who are competent to do so. Being competent means having sufficient knowledge or experience to prevent danger or injury; or being under such degree of supervision as may be appropriate for the nature of the work. Switchgear operated incorrectly can result in danger both to those operating it and others. Experience will include an understanding of the operation of the switchgear and knowledge of the arrangement of the system in which it is installed. People who are required to operate older switchgear must be specifically trained and experienced in its use. They must be aware of its limitations. People trained only on more modern equipment may not have such knowledge.

18 People with the necessary competence to operate switchgear are often referred to as ‘authorised persons’. A system should be in place to assess and appoint the people who can operate switchgear (authorised persons), record who they are, what training they have received, what experience they have, what items of switchgear they are permitted to operate, and what duties they are authorised to undertake. If no one within an organisation has the necessary competence to make the assessments to appoint people or to operate switchgear, someone should be developed to fulfil this role, or it must be given to an independent organisation with the necessary competence. It may be necessary to consider arrangements for support outside normal working hours. Even if the operation and maintenance of switchgear is contracted to an external provider, the owner of the switchgear retains a legal responsibility for its safe operation and maintenance.

19 A number of organisations (eg electricity distribution companies or specialist training organisations) can provide training. Attending a training course alone is unlikely to demonstrate that an individual has adequate technical knowledge or experience to satisfy the legal requirement for competence. Depending on the work to be undertaken, it is likely that specific experience of the switchgear to be operated and an understanding of the system in which it is installed will be necessary to prevent danger.

Maintenance

20 A system must be in place to ensure that switchgear is maintained appropriately by people competent to do so, at an appropriate frequency to provide assurance that it remains in a safe condition. Oversights, lack of knowledge, lack of the necessary skills, or concerns over the impact of a loss of power are common reasons for failing to maintain HV switchgear. Failure to carry out maintenance may result in the switchgear not operating when required to do so. This can place unnecessary stress on switchgear elsewhere within a system and result in more extensive damage if faults do occur. The switchgear may also become dangerous to operate.

Design rating and system modifications

21 Switchgear is described as being ‘overstressed’ when it is operated beyond its design rating. This could be due to excess load or where the fault energy (the energy that the switchgear would need to carry, make or interrupt in the event of an electrical fault) exceeds the capability of the switchgear. If required to operate under these conditions, overstressed switchgear may not cope with the electrical, magnetic and thermal stresses imposed upon it. This can result in it failing catastrophically with the potential to cause injury. Switchgear must not be used where its strength and capability may be exceeded, unless it is used in such a way that nobody could be exposed to danger.

22 When originally installed, switchgear should have been adequately rated for the duty it was intended to perform. Changes to an electrical system can result in switchgear becoming overstressed. Careful consideration must be given to system modifications to ensure that dangerous conditions are not created. Changing the configuration of a system through switching operations that put transformers into parallel operation can result in an increase in fault energy. Such arrangements may result from a response to abnormal system conditions or be necessary during planned maintenance work. Interlock systems should be used to prevent overstressing in such circumstances. Where it is not possible to use interlocks, clear operating instructions must be provided to prevent configurations that have the potential to overstress switchgear.

Design modifications and limitations on use

23 During the life of switchgear, defects with the original design or manufacturing process may become apparent. Manufacturers may recommend modifications, some of which may be required for the continued safe operation of the switchgear. While manufacturers must inform the original purchaser of modifications required to prevent danger, it is the current owner and operator of the switchgear who has the legal duty to ensure it remains safe. Audits at appropriate intervals can be helpful to identify switchgear that may require modification. These could involve either the switchgear manufacturer or specialist maintenance providers. Modifications essential for safety must be completed. If defects relating to safety are identified and result in restrictions on the use of switchgear, the people who operate or maintain the equipment must be made aware of the restrictions so that they can perform their work safely.

Dependent manual operating mechanisms

24 Dependent manual operating (DMO) mechanisms used to be fitted to both HV and LV switchgear. Although no longer made, they may still be in use on some systems. In switchgear fitted with DMO mechanisms, the operator opens and closes the contacts within the switchgear solely by manual effort. Movement of the contacts is dependent on the speed and actions of the operator. Hesitancy can lead to failure of the switchgear with potentially fatal consequences. Under some circumstances, such as operation during fault conditions, it may be physically impossible to close a DMO switch due to the electromagnetic forces involved. Attempting to operate switchgear under fault conditions can cause it to fail.

25 Where DMO switchgear is in service, it must only be used by people who have been trained specifically in its use and are aware of the potential danger associated with its operation. In general, the switchgear should only be operated after first being made dead by the operation of other switchgear which is suitably rated and does not have a DMO mechanism. Similarly, it should only be put back into service through the operation of such switchgear. See paragraphs 56−59 for further information on DMO switchgear.

26 There is a foreseeable risk associated with the use of DMO switchgear. It is foreseeable that such equipment present within an electrical system may at some time be used. It is a legal requirement that all systems shall at all times be of such construction as to prevent, so far as is reasonably practicable, danger. It is highly recommended that DMO switchgear is replaced.

Anti-reflex handles

27 Oil switches, such as those incorporated within ring main units, may be rated to close onto a fault but not to interrupt fault current (sometimes referred to as fault make, load break switches). Where switchgear using oil switches is also fitted with integral earthing, incidents have resulted when a failure to check the position of the selector mechanism has led to operators switching from OFF to EARTH instead of from OFF to ON. This action has the potential to put an earth fault onto the HV system. This is not necessarily a dangerous situation as oil switches are designed to close onto a fault, but danger arises if the operator, realising that a mistake has been made, instinctively reacts and attempts to open the switch. Oil switches are not rated to interrupt fault