The physics of high frequency parasitic arcs produced during switching

It has been long recognised that direct experimental information about the behaviour of the electric arc plasma in a power circuit breaker is important for understanding the current interruption process in such equipment. More recently, it has become apparent that, under certain operating conditions when reactive load currents are being switched, circuit breaker failures can occur well within the specified current levels for successful circuit breaker operation. These failures are believed to be associated with arcing in the interrupter at frequencies up to several MHz and currents of several hundred amperes. Optical investigations of arcing within such interrupters cannot be undertaken with transparent nozzles without prejudicing the role that the PTFE nozzle material may have in the current interruption process. To overcome this difficulty, optical investigation of the arc column has been undertaken using a minimal number of optical fibres embedded in the PTFE nozzle to gain optical access to the arc within the nozzle and also simultaneously to observe a significant part of the interrupter volume. Experimental results are presented that provide an insight into the behaviour of such high frequency arcs and in particular the probability that some of these arcs become parasitic through being located outside the PTFE nozzle of the interrupter.