Detection of Joule heating before dielectric breakdown in polyethylene films

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