GEC Type VMX High Voltage Switchgear (including GEC Alsthom, Alstom or Areva T&D)

Health and Safety Executive - Safety notice

Issue

An HSE investigation into a dangerous occurrence that resulted in electrical arcing and fire, due to failure of a GEC type VMX HV vacuum circuit breaker (VCB) inside an electrical substation at a Control of Major Accident Hazards (COMAH) Regulations 2015 lower tier establishment, identified a number wider concerns.

Despite the upstream electrical protection systems disconnecting the associated electrical short circuit within 700 milli-seconds, there was considerable damage. At the time of the equipment failure the site was not carrying out any switching activities and the associated VCB was connected in the service position but open.

Approximately 24 months prior to the incident, partial discharge activity had been detected on the failed VMX VCB using ultrasonic testing which measured 6 to 8db and relative humidity measured at 71% at 24ºC.

Approximately 14 months prior to the incident, partial discharge activity was again detected on the failed VMX VCB using ultrasonic testing which measured 6 to 8db and relative humidity measured at 81% at 24ºC.

No corrective action was taken to investigate the cause of the partial discharge activity as it was believed to be background noise (such as high frequency lighting).

Intrusive maintenance was carried out approximately 2 months prior to the incident which did not identify any evidence of partial discharge activity or loss of vacuum interrupter integrity. The vacuum interrupter integrity was checked using a voltage withstand test (11kV). Partial discharge measurements were not checked immediately before or after completion of the intrusive maintenance.

Investigation highlighted that the most probable cause of the failure was partial discharge activity.

Following the incident the HV switchgear required replacement causing significant disruption to site operations.

HSE investigations revealed that the failed VMX VCB was a replacement which was bought second hand in 2004. The original VMX VCB had been replaced after it had suffered a severe level of partial discharge activity on the centre phase front bushing (busbar side), detected before disruptive failure occurred.

HSE is aware of several failures of GEC type VMX HV Switchgear due to partial discharge issues.

In addition, HSE is aware that Northern Powergrid have taken the decision to restricted normal access to their substations containing GEC type VMX HV Switchgear following a disruptive failure (see the suspension of operational practice; SOP 385 on SP Energy Networks website ) and continued partial discharge issues. Confirmation that substations are free from the smell of ozone and audible partial discharge activity are sought before entry. If clear, Northern Powergrid then tests each panel for partial discharge activity before allowing others to access the substation (as described in Northern Powergrid Operational Restriction 33 (OR33) GEC VMX 11kV Switchgear, 24 August 2024).

Failure to identify partial discharge issues or a leaking vacuum interrupter and take the necessary corrective action in a timely manner has the potential to lead to disruptive failure of the equipment and give rise to danger (such as risk of injury or death if site personnel or contractors are present).

Outline of the problem

VMX HV Switchgear was originally manufactured by GEC which later became GEC Alsthom then Alstom and later Areva T&D.

GEC (including GEC Alsthom, Alstom and Areva T&D) manufactured 3 forms of VMX HV VCBs in the UK between 1985 and January 2003 all of which are susceptible to partial discharge issues brought on by inadequate environmental conditions in the associated substations.

Schneider Electric currently manufacture an updated version of the VMX VCB (Form C) in addition partial discharge testing was added as a routine factory test.

HSE understands that Areva T&D and Schneider Electric type VMX VCBs (Form C) manufactured after January 2003 included the necessary design modifications during the assembly and are not susceptible to partial discharge issues if the required environmental conditions are maintained.

HSE is aware that between 1988 and 1997 several incidents were raised on National Equipment Defect Reporting Scheme (NEDeRS) on the Energy Network Association (ENA) website. These incidents led to the issuing of several Suspension of Operational Practices (SOPs), which you can read on SP Energy Networks website, including SOP 165, SOP 182,and SOP 187, and SOP 67 and SOP 184 on ENA NEDeRS.

In response to the SOP’s GEC Alsthom / Alstom / Areva and later Schnieder Electric proactively sent out documents, ‘Table of Potential Problems (Version 1 to 5)’ to all owners. This document identified equipment type, form, and serial number affected by the SOPs and cross referenced the serial number to additional maintenance requirements set out in a range of documents titled ‘Improved Maintenance Practices (IMP)’ that should have been carried out to resolve the issues with the effected VMX VCB’s. Refer the latest version of the Table of Potential Problems relating to VMX HV Switchgear provided by Schneider Electric.

Post 1997 a further 3 SOPs were issued which also related to partial discharge issues. You can read these on SP Energy Networks website; SOP 322, SOP 385 and SOP 393.

In all cases the recommended operational and access restrictions and necessary remedial actions are described on the SOPs and in some cases the need to improve the substation environmental conditions.
It is well documented that there is strong relationship between the environmental conditions in a substation (i.e. relative humidity and ambient temperature) and the level of partial discharge activity. More detail is available in Evaluation of lifetime of air-insulated switchgear versus service conditions in MV substations | CIRED in the Journal of The Institution of Engineering and Humidity Effects in Substations Report on EA Technology’s website.
The British Standard BS 5311-1:1976 on BSI’s website did not include requirements for operating under specified humidity conditions or degrees of allowable condensation and referenced that specification of humidity conditions and degrees of allowable condensation were under review at the time of issue.

BS 5311:1988 on BSI’s website defined the service conditions for indoor switchgear as having average relative humidity not exceeding 95% and vapour pressure of 22mbar in any 24-hour period as set out in the International Standard IEC 694:1980, available from the International Electrotechnical Commission. It recognised that under these conditions. condensation will occasionally occur. As such, HV Switchgear designed to BS 5311:1988 and later standards should be designed and tested to cope with the effects of humidity and occasional condensation, such as breakdown of insulation and corrosion of metallic parts or be prevented from such effects by special design of the substation i.e. suitable ventilation and heating or using dehumidifying equipment.
HSE is aware that the VMX HV Switchgear Installation, Operation and Maintenance (IOM) instructions stipulate that the substation should, as far as possible, be free from damp and widely varying temperatures. Relative humidity must not exceed 80% and the combination of humidity and temperature must be such that condensation in or on the equipment must not occur. Special precautions must be taken where the humidity exceeds 80%.

HSE has concerns that duty holders that are not members of ENA NEDeRS, or who have not recently experienced problems due to partial discharge issues with GEC type VMX HV Switchgear, may not be aware of legacy design issues or limitations on use associated with GEC type VMX HV Switchgear manufactured between 1985 and January 2003 (including units manufactured by GEC Alsthom, Alstom and Areva T&D) .

HSE advises operators of GEC type VMX HV Switchgear manufactured between 1985 and January 2003 to familiarise themselves with this safety notice and seek competent advice.

Action required

Duty holders should:

1. Establish if your VMX type switchgear was manufactured between 1985 and January 2003.
   If so, carry out the following actions:
   
   • identify which modifications and limitations are relevant, the Table of Potential Problems provides details
   • you may also need to refer to SOPs issued via NEDeRS (competent maintenance contractors may be able to assist with this)
   • check installed serial numbers against those listed in the Table of Potential Problems
   • identify the relevant design modifications and limitations on use
2. Review your maintenance records for your GEC type VMX HV Switchgear and verify that the required design modifications and improved maintenance practices identified in item 1 have been implemented. If this is not the case, it will be necessary to contact Schneider Electric for detailed advice on how to complete the design modification and improved maintenance instructions (IMPs). Schneider’s email address is [email protected]
   
   Note in some instances it may be necessary to periodically repeat the improved maintenance practices.
3. Determine if the environmental conditions of your substation containing GEC type VMX HV switchgear meets with the minimum requirements specified in the manufacturer’s instructions.
Refer to the British Standard BS EN IEC 61936-1 available on BSI’s website for the design of electrical substation building including requirements for heating, ventilation and air conditioning (HVAC).

Refer to: Evaluation of lifetime of air-insulated switchgear versus service conditions in MV substations | CIRED in the Journal of The Institution of Engineering, Humidity Effects in Substations Report on EA Technology’s website and Annex C of IEC TS 62271‑304, available from the International Electrotechnical Commission for guidance on controlling substation environments.
4. Undertake an initial partial discharge survey to assess the condition of the high voltage insulation on installed GEC type VMX HV Switchgear ensuring that you also measure the relative humidity and ambient temperature in the substation.
5. Review if the vacuum interrupter integrity is within limits prescribed in the manufacturer’s instructions considering requirements for checking vacuum interrupter integrity defined in BS 6626:2010 available on BSI’s website and HSG 230, Keeping electrical switchgear safe.
   
   Note Magnetron Atmosphere Condition (MAC) testing is available to verify vacuum interrupter integrity and to predict end of life.
6. Review and update your risk assessment for accessing substations containing GEC type VMX HV Switchgear or undertaking operation and maintenance activities on GEC type VMX HV Switchgear. This is likely to lead to the need for access and operational restrictions until such time as all the applicable IMPs have been implemented.
7. Review and update your maintenance arrangements to ensure that periodic partial discharge monitoring and periodic checking of vacuum interrupter integrity of GEC type VMX HV Switchgear are carried out by a competent person (as defined in clause 34.4 of BS 6626:2010 available on BSI’s website and paragraphs 116-119 of HSG 230, Keeping electrical switchgear safe). The frequency of the periodic partial discharge monitoring should be determined through a risk assessment. However, HSE would recommend that partial discharge testing is conducted on each entry to the substation.

Duty holders are advised to consult with Schneider Electric for further information, [email protected]

Guidance

HSG 230, Keeping electrical switchgear safe states:

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.”

Maintenance procedures: Vacuum switchgear

“116 Vacuum bottles may need re-certification or replacement after a specified operating life. Typically, the design life expectancy quoted by manufacturers is 20 years, although vacuum equipment of this age and older can be found in use. Testing during maintenance may identify a failed vacuum bottle but is unlikely to provide information about the continued fitness for purpose of a bottle that does not fail the test. It is possible for a bottle which has lost vacuum while in use to remain in service under normal load conditions with no obvious signs of defect. Failure is likely only if the device is switched or required to break a heavy load or fault current. Failure is likely to be catastrophic.

117 Owners of vacuum switchgear should be aware of the design life expectancy of their equipment. While the equipment does have a high level of reliability, it cannot be assumed that this level of performance will be maintained indefinitely, or that extension to life is justifiable solely on failure rates experienced while operating the equipment within the manufacturer’s design life expectancy. There must be a strategy in place to manage the risks associated with the potential for loss of vacuum when equipment reaches the limit of its design life expectancy.

118 X-rays may be generated when the open contact gap of vacuum switchgear is stressed at high voltage. High-voltage tests can be used to verify that vacuum is present, there being no other indication of the presence of vacuum within a bottle. There are no harmful emissions at normal service voltage. If a high-voltage pressure test is carried out with the switchgear in an open position, X-rays may be generated. Guidance should be sought from the applicable standards and the manufacturer.

119 Maintenance activities for vacuum switchgear should be based on manufacturers’ recommendations, but are likely to include:

• inspection of the external condition;
• verification of the design life expectancy of the vacuum bottles and a check to determine the age of the equipment being maintained (note design life expectancy);
• measurement of contact wear where a measurement method is available; measurement of contact resistance when closed;
• a check on the vacuum integrity, e.g. by a high-voltage pressure test (X-ray risk);
• inspection, adjustment and lubrication of mechanisms, including shutters, where appropriate;
• on withdrawable equipment, examination of primary isolating contacts for damage, burning or corrosion – cleaning and refurbishing (as necessary);
• on withdrawable equipment, checking and lubrication of circuit-breaker isolating mechanism; checking correct function of position indicators and interlocks;
• examining the inside of cable termination chambers and other chambers as appropriate – removal of surface contamination from accessible solid insulation (where applicable);
• checking the operation and integrity of any automatic earthing facility, where applicable;
• examining and checking the voltage transformer, as required;
• secondary injection testing on circuit-breaker protection systems or the use of proprietary electronic simulation devices (or, if this is not scheduled, carry out manual trip-test); and
• examination of secondary contacts, wiring and auxiliary switches.”

HSR 25, Guidance on the Electricity at Work Regulations 1989 states:

Guidance on regulation 4

“66 Regulation 4(2) is concerned with the need for maintenance to be done to ensure safety of the system, rather than with the activity of doing the maintenance in a safe manner (which is required by regulation 4(3)).

67 The obligation to maintain arises only if danger would otherwise result. The maintenance should be sufficient to prevent danger so far as is reasonably practicable.

68 Inspection and, where necessary, testing of equipment is an essential part of any preventive maintenance programme. Practical experience of use may indicate an adjustment to the frequency at which preventive maintenance needs to be carried out. This is a matter for the judgement of the duty holder, who should seek all the information they need to make this judgement including reference to the equipment manufacturer’s guidance.

69 Records can aid demonstration of compliance and allow useful analysis of equipment condition, although keeping records is not a legal requirement. Maintenance records (including test results), preferably kept throughout the working life of an electrical system, will allow the condition of the equipment and the effectiveness of maintenance policies to be monitored. Without effective monitoring, duty holders cannot be certain that the requirement for maintenance has been complied with.”

Relevant legal documents

Check legislation.gov.uk for details of:

• the Health and Safety at Work Etc. Act 1974, section 2(1) and section 3(1).
• the Electricity at Work Regulations 1989, regulation 4(2)