The motion of vacuum arcs between spiral contacts with a fixed-polarity arc shield [in vacuum interrupters]

A framing camera was used to study drawn vacuum arcs between spiral-type contacts in vacuum interrupters. This design imposes a transverse (radially directed) magnetic field on the arc column. The fixed electrode was brazed to a cylindrical arc shield. With the fixed electrode and shield as cathode, cathode spots were observed to be produced on the shield by two mechanisms. One of these occurred only for the intense columnar arc modes which had active erosion at both the anode and cathode arc roots (I_ARC>~25 kA peak). In this range, the arc column interacted with the shield for brief intervals as it was driven along the edge of the contacts. With the fixed electrode and shield as anode, there was less interaction of the arc column with the shield. In this case, there was no formation of an anodic arc root on the shield at least up to ~38 kA peak