Standards that apply to MV switchgear rated for arc flash protection
By B Johnson, ABB
Published in the October 2014 issue of Electricity+Control (pages 34-39)
Enquiries: Bryan Johnson. Email email@example.com
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Typically arc faults occur when:
o There is an ingress of foreign material, water, insects or rodents
o Mislaid or forgotten materials, tools, loose wires, test connections
o Faulty insulation, derogation of insulation
o Insufficient over voltage protection
o Incorrect operation, incorrect interlocks, or disregard for operating rules
Any one of these can trigger the internal arc. Once the internal arc is ignited the surrounding air is ionised so the internal arc will continue to burn at a high temperature until interrupted. The internal arc can be divided into four phases. During phases 3 – 4 hot plasma (gases, particles, molten metal and any other material damaged by the internal arc) will be released from the switchgear to the surrounding atmosphere endangering people in the vicinity. The danger comes from hot plasma being released and a shock wave that is released from the faulted cubicle.
The improved level of manufacturing techniques has led to improved, safer and more reliable switchgear systems for Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS). An internal arc can occur at any time, usually caused by factors outside the control of the manufacturer, and may involve any people within the vicinity of the switchgear system whether they were operating the gear or not. The IEC 62271-200 standard provides a clear definition of classification for internal arc, and covers a broad range of switchgear systems.
Users who specify this standard and enforce compliance to this standard by verifying type test certification from reputable manufacturers are assured of peace of mind as they are using the latest available standards for operator safety. Preventing the arc from occurring by specifying switchgear compliant to the relevant IEC standards is the first line of defence. The next line of defence is providing a safe environment, taking into account the fault level, the network, the appropriate switchgear design together with the building design and the protection systems available. The last line of defence should be operating procedures, and the appropriate PPE.
- The occurrence of an internal arc can never be totally prevented or predicted.
- Arc faults are usually caused by external factors outside the control of the manufacturer.
- Preventing the internal arc from occurring by specifying switchgear compliant to the relevant IEC standards is the first line of defence.