The impact of under sized cable on MV vacuum metal clad switchgear during fault

Failure of vacuum metal clad switchgears often results in major load loss and long restoration time. Failure analysis is conducted with the aim to prevent future recurrence, thus an improved life of the equipment. This paper presents two failure cases of vacuum metal clad switchgears which had suffered from flashover. Both switchgears were connected to a local transformer and had exhibited similar post-fault damage characteristics. Evaluations techniques used to assist the analysis process include visual examination, photo documentation, tear down examination, electrical data analysis and computer simulation. The significant effect caused by the use of under sized cable is discussed. In the first case, a fault had occurred downstream of the switchgear. The cable had melted, causing significant damage inside the switchgear. From calculation, it was deduced that the minimum cable size needed to cater the fault current was 135 mm². In this case, it was found that the existing cable was grossly under sized. For the second case, the minimum cable size calculated was 74 mm². A computer simulation was carried out, in order to further verify the short circuit capacity and corresponding melting temperature of the 70 mm² cable. From the simulation, it was found that the maximum permissible current before melting was 6.3 kA. The actual fault current of 8.8 kA had far exceeded this value. As a result of under sizing, both the cables could have melted and vapourised during fault current flow and subsequently caused explosion inside the switchgear.