Experiments on paper insulation in a corona environment

Insertion of an insulating paper between point-plane electrodes normally increases the total breakdown potential threshold, except when its position is next to the plane. A series of DC (both (+) and (-)) experiments was designed and conducted to examine this latter distinction. It was found that visual ionic spots on the paper surface are a dominant prebreakdown phenomenon. Local failure at one such discrete spot ultimately brings about a streamer sparkover in air at a sufficiently high potential. The variability and other characteristics of the pattern in a 2D spot distribution are described. Related experiments reveal that, with double paper layers in place, the pattern of spots becomes less dense visually and the breakdown potential depletes further, always with some mechanical agitation in accompaniment. Also noted was the existence of a sharp, well-defined dark band which clearly bridges the separate domains of single-layer and double-layer spot distributions. From this band, glow spots are excluded.<>