Experiments on paper insulation in a corona environment

Insertion of an insulating paper between point-plane electrodes normally in creases the total breakdown potential threshold, except when its position i s next to the plane. A series of de [both (+) and (-)] experiments was desi gned and conducted to examine this latter distinction. It was found that vi sual ionic spots on the paper surface are a dominant prebreakdown phenomeno n. Local failure at one of such discrete spots ultimately brings about a st reamer sparkover in air at a sufficiently high potential. This paper descri bes the variability and other characteristics of the pattern in a two-dimen sional spot distribution. Related experiments reveal that, with double pape r layers already in the barrier, the pattern of spots becomes less dense vi sually but the breakdown potential would increase for the case of (+) point hut would decrease further for the case of (-) point if a third sheet is i ntroduced. A sharply defined "dark" band on the double-layer side in which glow spots are absent was always observed to clearly bridge the separate do mains of, respectively, single-layer and double-layer spot distributions. A lso related were the mechanical agitations of the paper accompanying all ph ases of the different discharge activities.