In the electrode system without epoxy molding, it is supposed that dielectric breakdown of polymeric insulating films under dc voltage in a room temperature region is caused by the positive feedback process of localized electro-mechanical deformation and Joule heating at weak points distributed in films. Under ac voltage application, localized heat generation of low-density polyethylene (LDPE) film leading to dielectric breakdown was scarcely detected. However, considering that (1) the ac breakdown strength is lower than the dc one, where high temperature point leading to dielectric breakdown was detected and (2) ac breakdown strength was affected by rise rate of applied voltage, ambient medium and type of applied voltage, it is suspected that thermal process must be involved in ac dielectric breakdown of LDPE film at a weak point distributed in the film in a very short time range within the time resolution of thermograph.
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