The effects of electrical discharges between electrodes across insulation surfaces: III. Discharges occurring in nitrogen and in flowing gas

Abstract

In earlier phases of this program, which have been reported1,2, a study was made of the effects of electrical discharges in static air on the electrical resistance of different insulators2. The surface power density to which the insulators were subjected was set at different levels (for the initial period of discharge) in the range of 4 to 360 watts/cm2. Two interesting observations made in that investigation were; minima in some of the electrical resistance-time curves with recovery of good insulation characteristics after prolonged periods of discharge, (Class 1 and 2 materials), and greater discharge resistance at 360 watts/cm2 than at lower power densities for certain insulators. The initial decrease in electrical resistance and low discharge resistance at relatively low power densities were associated with carbonization of resin at temperatures below 800°C. Sustained combustion at 800°C and above, and also gas blast effects were the mechanisms suggested for increases in discharge resistance during prolonged periods of discharge and at 360 watts/cm respectively. Therefore, it was desirable to study the effects of discharges also in the presence of a non-oxidizing atmosphere, i.e. nitrogen. This was also prompted by the observed greater discharge resistance in nitrogen than in air below 4 watts/cm (1). Flowing gas was also used, to eliminate from the discharge column organic vapors evolved from the insulators. These vapors, in the static air experiments, altered the conditions of discharge and were decomposed to carbon in the discharge.