Optical analysis of an electric discharge evolution on an insulating polluted surface materializing a polluted insulator

We observed the extension of a discharge on an electrolyte surface either with an electronic camera or by using an original set-up permitting simultaneous measurements of optical and electrical parameters on the same flashover. The corresponding results are presented here. They show two zones of different luminosity, the development of which are very different. Zone 1, the darker of the two, is nearer the ground electrode, whereas the zone 2, the brighter one, is behind it starting from the HV electrode and has a better defined outline. At the onset of the flashover in its first phase, the zone 1 spreads faster than the zone 2. Later, in a shorter second phase, the zone 2 develops more quickly and progressively overruns the zone 1. Thereafter, suddenly the flashover is completed. We comment these results and suggest an explanation.