Long-delayed breakdown in vacuum gaps under DC pulses

Concerning HV insulation in vacuum, there is still some uncertainty about the processes leading to breakdown, especially in the case of long time lags and large contact spacing. The present measurements show that mechanical shocks cause breakdown of the gap. Breakdown probability is highest for virgin contacts and decreases after heavy arcing. The role of nonmetallic inclusions and organic impurities is highlighted. In comparison with a demountable vacuum device industrial vacuum interrupters do not show any sensitivity to mechanical shocks. Besides breakdown probabilities and time lags, the temporal development of voltage and current during breakdown was measured. For a contact spacing of >14 mm and for breakdown sites located on the side face of the contacts, HV discharges were observed lasting for ~500 μs. In this case, current is limited by a negative space charge layer in front of the anode