Direct-voltage instantaneous breakdown of oil-impregnated paper capacitors as a function of area

Theories relating to the decrease of breakdown strength of dielectrics with increasing area are reviewed. The breakdown-voltage distribution of 2- and 3-layer capacitors of various areas and dielectric thicknesses are examined and it is shown that these are usually skewed towards lower values and may be double humped. The lower hump increases in importance with increasing area and is due to the coincidence of conducting particles in different layers. It is shown that the variation of mean breakdown voltage with increasing area is represented adequately by Milnor's equation and that the breakdown voltage between spheres may be taken as the limiting value. Edge effect prevents such values from being approached with normal foil-type capacitors. The errors involved in the assumption of a normal distribution are assessed. The coefficient of variation appears to be independent of area but decreases with increasing thickness of dielectric. This information permits prediction of the voltage below which a given fraction of the sample will fail.