“Degradation of insulating materials by electrical discharges”

When discharges occur on the surface of, or in internal voids in, solid dielectrics, chemical changes in the solid and the gas take place and the solid breaks down. In some cases the part played by chemical deterioration is clear. For example, a strained vulcanised natural rubber subjected to discharges in air, cracks due to ozone attack and being cracked will no longer support voltage. More commonly it is not clear if, or how, the chemical and physical processes are related. The uncertainty is apparent in the methods suggested for the measurement of discharge resistance of materials. In a British method(1) the time to breakdown of a sheet placed between rod and plate electrodes is used as a measure of discharge resistance. On the other hand in other European methods(2,3,4) a large area of sample is exposed uniformly to discharge and the weight change or change in another property used to compare one material with another. Artbauer(5,6) thinks that there are objections to both methods but favours a method based on time to breakdown. At an International Conference to mark the official opening of our Laboratories last year, Leroy, Lacoste and Bui-Ai(7) described the apparatus set up for a series of experiments to determine the relationship between discharge parameters and chemical degradation of the solid. Only a few preliminary results were reported. As in previous French work the system is designed to expose a maximum area of sample to discharges so that large amounts of deterioration products are formed to facilitate analysis. The system does not lend itself to measurement of the time to electrical breakdown. At the same Conference the present writers reported(8) some preliminary results using a cell designed primarily for the latter measurement, but arranged so that the chemical changes could also be measured. The object was to try to decide the importance of chemical deterioration in electrical breakdown. This paper describes some of the further work done on this investigation from which tentative conclusions can be drawn.