Investigation of prebreakdown currents in vacuum interrupters

The authors attempted to measure the current growth prior to delayed discharges with high temporal resolution and high sensitivity. The quantity of charge within a microdischarge was found to vary. Depending on the charge, it was possible for the shield potential to be affected, or for a disruptive discharge to be initiated between the contacts. The probability of delayed discharges was very low at low currents but increased at higher currents. With the majority of the recorded delayed discharges, the observation can be explained on the basis of a trigger discharge occurring between a charged particle and the side of the contact. In several cases cathode spots carrying currents on the order of 10 A with no observed reduction in voltage over a period of ⩽300 μs were observed. Some delayed discharges were preceded by increasing predischarge currents. The breakdown discharge took place after a critical emission current had been reached. The desorption of gas molecules on the anode surface is regarded as the cause