Operators and manufacturers of high-voltage and medium-voltage switchgear should replace oil switchgear with circuit-breakers that have passed an internal arc type test in every HV/MV compartment. Using mechanical cable sealing systems helps prevent common causes of internal arc. This is what arc-flash specialist Ian Naylor states in the brand new white paper “Internal Arc & Arc-flash in HV/MV Switchgear” from Threepwood Consulting.
The white paper highlights typical types of switchgear, explains the catastrophic effects of internal arc and arc-flash faults and describes internal arc testing as well as related worldwide standards. It also provides an insight into design standards as well as the hierarchy of design measures to prevent internal arc and flashover/arc flash.
Prevent fireball inferno
The main purpose of the document is to save lives. Too many oil switchgear failures have caused fatalities, downtime and cost. Therefore, Ian Naylor recommends the replacement of all oil switchgear. It is crucial to replace it with switchgear having passed internal arc type testing, which is prescribed within the North American standard IEEE C37.20.7 and optional within the widely used standard IEC 62271-200.
Oil-filled switchgear is not internal arc rated – and neither is new oil interruption/insulated switchgear. In order to mitigate switchgear failure, and avoid catastrophic explosions and pressure rise, all operators and manufacturers should make sure their equipment is internal arc rated. It is the only way to know that the switchgear will contain the arc and blast and thereby fail safely in case of an internal flashover.
Right seals for testing
The white paper describes a successful internal arc testing performed by SATS, Scandinavian Association for Testing of Electric Power Equipment, in Norway. On this occasion, ABB achieved internal arc classification in the MV connection compartment of their switchgear. As part of their design, ABB used the Roxtec cable sealing system. The seals withstand blast load and peak pressure, provide certified fire performance and have appropriate IP and NEMA ratings for water and dust. In addition, the seals ensure resistance against vermin. The sealing material is halogen-free and does not fuel a possible fire.
Safety in operation, protection in failure
Ian Naylor writes that every opportunity should be taken to reduce the risk of exploding switchgear, and that remote operation naturally is number one in the hierarchy of control measures. He sees internal arc type testing as very important to reduce risk, and states that tested and approved cable sealing systems can help switchgear pass internal arc type tests. The seals both prevent causes of arc, such as water and dust ingress and vermin, and mitigate blast load or fire spreading in case of an arc-flash.
It is also cost-efficient to standardize with the same cable sealing system for both switchgear and buildings. Reliable HV/MV switchgear is critical for safe and stable power distribution, and it is important to ensure protection against relative humidity causing PD, partial discharge activity and eventual arc-flash. The objective of sealing is to ensure the equipment operates in a warm, dry and dust-free environment.
Learn from the author
Ian Naylor is a principal consultant of Threepwood Consulting. He has a long experience from working with internal arc type testing and HV/MV switchgear projects in Europe, the Middle East, Asia, Australia and New Zealand. He represents Great Britain in the IEC Maintenance Team (MT) 14, which is responsible for updating IEC 62271-200, including internal arc type testing.
Take this great opportunity to learn more by reading the entire Threepwood Consulting white paper.
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