Dielectric Recovery and Shield-Currents in Vacuum-Arc Interrupters

Abstract
Dielectric recovery data is presented following forced current interruption of 930 A dc copper vapor arcs in a vacuum interrupter. At given free recovery periods during the first 6 to 25 , us following interruption, the instantaneous dielectric strength is determined by the application of high voltage step function pulses. This instantaneous dielectric strength is defined as the maximum voltage which can be consistently reapplied without causing reignition. The influence of electrode spacing (0.6 or 1.3 cm), shield potential and the polarity of the reapplied voltage on the recovery of dielectric strength have been determined. The more rapid rate of recovery initially observed at long spacings is attributed to the lower mean inter-electrode vapor density immediately following interruption. Recovery is also more rapid for "reverse" rather than "same" polarity reapplication indicating surface effects, in particular the roughening of the cathode surface during arcing. Post arc currents lead to an initial reduction of dielectric strength for "reverse" reapplication with the vapor shield tied to the "arcing-anode". The mechanism of these post arc currents has been investigated by observations of the high ion currents which flow to a negatively biased shield during and immediately following arcing.

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