Internal faults in distribution switchgear-risk reduction and design considerations

One of the primary functions of switchgear in the distribution network is to assist in the management of faults (short circuits). Circuit breakers and fuses are used to interrupt the flow of current into a fault, and other items of equipment are used to re-configure the network to allow supply to continue following a fault. An “internal fault” is a short-circuit which occurs within the switchgear itself. It gives rise to electrical arcing which heats the medium within which it occurs. The resulting pressure-rise is likely to provoke the disruptive relief of enclosures followed by the exhaust of hot gases. Whilst such faults are not commonplace, they require special consideration, as presented in this paper. The internal fault qualification of equipment to IEC 298 leads to an understanding of how the enclosure responds to pressure rise and other effects related to arcing. Following sensible policies for design, manufacture and installation reduces the probability that a fault will occur. Performing tests under realistic conditions with respect to the substation environment and fault duration provides confidence that the test results can be used to predict the consequences of a fault in service