Concurrent optical and electrical observations of the `spontaneous' surface flashover of solid insulators in vacuum

Abstract

The performance of high-voltage solid insulators that are used to bridge vacuum gaps is frequently limited by surface flashover. An important characteristic of the flashover process under DC conditions is that it occurs `spontaneously', namely without warning and any obvious external triggering agents. Significantly, this feature is not readily explained by the basic assumptions of the theoretical models that are currently used to explain the surface flashover mechanism. In an attempt to further our understanding of this observation, concurrent optical - electrical measurements were performed on standard truncated alumina ceramic cones. These studies have revealed that there are two important physical processes associated with spontaneous surface flashover events. The first has a clear origin at the cathode triple junction, whereas the second is associated with a micro-plasma that is initiated at two radially opposite points of the cathode and anode triple junctions. Two alternative physical mechanisms are proposed to explain the initiation of these two types of flashover event. Lastly, a novel physical description of the final phase of both types of flashover event is given.