Triggered discharges with high arc voltages in a vacuum interrupter

Studies of nonsustained disruptive discharges (NSDDs), isolated cases of which can occur in vacuum interrupters, indicate lateral discharges between the cathode and shield, which can initiate a brief discharge between the contacts. To facilitate the study of such discharges, the sample discharges were triggered by a surface discharge induced by a spark gap, built into the side of the cathode, and observed with a high-speed film camera and image-converter camera. The tests showed a cathode spot after igniting. The emitted electrons first charge the shield negatively and then are directed toward the anode. The discharge burns at a high voltage, with current ranging from 10 to 100 A. After a period of up to 400 mu s, the current demand increases abruptly; an arc discharge occurs between the contacts and discharges the capacitances near the switch. The contact gap undergoes a rapid dielectric recovery, and the restored voltage is maintained. These types of discharge were also observed with NSDDs; thus it can be assumed that the triggered discharges studied correspond to the NSDD type.>