Breakdown of Solid Dielectrics in Divergent Fields

Breakdown channels are shown to propagate from steel needle-point electrodes embedded in polythene and polyisobutylene when the maximum effective stress at the end of the point reaches the intrinsic electric strength of the material at the test temperature. The average breakdown stress shows no significant variation with the radius of curvature of the point (for radii of 1¿20 micron), but, at room temperature, about 25% greater stress is required with a negative than with a positive point. These results are explained by the effect of enhanced conductivity and space-charge accumulation increasing the effective point radius to some 25 microns for positive and about 45 microns for negative points. The effect of polarity decreases with increasing temperature and is negligible at 100°C. The decrease of the average stress required for breakdown with increasing electrode separation is explained, and the same concept is used to predict the variation of the industrial electric strength of materials with specimen thickness. Factors affecting the industrial electric strength of materials are discussed and illustrated by tests on polythene, polystyrene and cellulose acetate, using British Standard electrodes in air and in clean and contaminated transformer oil.