Partial arcs over wet contaminated surfaces

Abstract

Partial arcs over a damp mixture of alumina with 0.05% sodium chloride have been studied. The mixture was laid in a thin layer on a glass strip. It was found that the arcs only reached part way if the applied voltage was below the flashover voltage and above a certain minimum. These partial arcs appear to originate in low-current flashovers which may or may not develop into arcs having currents of several milli-amperes. It was found that, if the expression for the voltage drop along an arc was applied, a good correlation was found with the measured applied voltage. 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 over two orders of magnitude. It is proposed that ionisation conduction be invoked to explain the very much higher apparent conduction which occurs in the thin layer of damp alumina. Ionisation conduction is a phenomenon well-known in respect of the anomalous conduction between the earth rod of a lightning conductor and the surrounding soil.