Detection of Joule heating before dielectric breakdown in polyethylene films

Localized Joule heating in polyethylene films was detected at room temperature before dielectric breakdown under DC field. When a high-temperature spot appeared, the conduction current increased with time. The final breakdown occurred at the point of highest temperature. This correlation between the breakdown point and the Joule heating spot shows that a thermal process takes part in the DC electrical breakdown of polyethylene films at room temperature and that there are weak points for the breakdown in the film where the current density is higher than average. Almost the same results were observed for ethylene-vinyl-acetate copolymer films, although the appearance of the localized Joule heating and the electric breakdown occurred at lower electric fields than those for polyethylene films