The detection of initial failure in high-voltage insulation

In a series of accelerated life tests¹ on laboratory samples of impregnated-paper insulation of the type commonly used in high-voltage cables, certain methods have been employed for limiting the breakdown current and thereby the attendant burning of the insulation. In this way it has been found possible to obtain partial failures in which the breakdown is limited to only a portion of the insulation wall, say one-half or one-third of the total number of layers, the remaining layers being apparently unaffected. In these partial failures, the burning is very slight and it is possible to obtain a very good idea as to the location of the initial point of failure and the nature of its subsequent progress. With one of the methods used it has been possible on occasion to limit the breakdown to only two layers of paper and to study the extension of the failure through adjacent layers. The method gives great promise for the study of the mechanism of breakdown under laboratory conditions. In this paper we are reporting some of the preliminary results of a more extended program of study still under way.