Non-sustained disruptive discharges: test experiences, standardization status and network consequences

In the test practice of KEMA, more than 30% of the vacuum circuit breakers, offered for testing in 1999 have shown nonsustained disruptive discharges (NSDD), mostly within 300 ms after interruption. Although the vast majority of the observed NSDDs occurred after high current arcing, several cases of NSDD after small current were observed. It is demonstrated from short-circuit current test data, that during the occurrence of NSDD, a much larger part of the circuit is contributing to transient phenomena than just the `local´ parasitics in the immediate vicinity of the breaker. This is derived from the measured duration of the conducting period during NSDD. The new standard IEC 60056 for circuit breakers makes no distinction between: (a) capacitive restrike (late breakdown leading to polarity reversal of the load capacitor); and (b) NSDD in the case of a vacuum switching device breaking down more than half a power frequency cycle after capacitive current interruption. It is demonstrated that system overvoltages that accompany such `capacitive NSDDs´ can not always be neglected