Partial Arcs above a Damp Contaminated Surface

Arcs extending part way across a thin layer of damp powder have been studied. The powder consisted of alumina with 0.05% sodium chloride, and formed a thin layer on a glass strip. These partial arcs were formed if the applied voltage was above a certain minimum and below the voltage for complete flashover. They appeared to be initiated by localized low-current surface breakdowns (scintillations) which could develop into partial arcs having currents of several milli-amperes. It was found that, using Warrington´s expression for the voltage drop along an arc, a good correlation was found with the measured arc parameters. However this left a current in the remaining part of the thin layer of damp alumina which could not be explained by ohmic conduction, being too high by more than two orders of magnitude. It appears that ´ionization conduction´ must be invoked to explain the very much higher apparent conduction occurring in the thin alumina layer. This ´ionization conduction´ is a phenomenon in which multiple series-parallel micro-flashovers occur between and over the granules in a granulated material.