This paper summarizes the progress of an extensive study of the ac voltage endurance, extending from a single cycle to more than a year, for cast silica-filled epoxy resin, with recessed silver paint electrodes and cast-in aluminum electrodes, giving a nearly uniform electric field. Although the specimens were prepared to avoid gas cavities, and partial discharges were usually undetected, the ac strength declines regularly with time in a manner similar to that due to partial discharges. Therefore, it is inferred that undetected partial discharges are responsible. A derivation is given for a suggested new analytic relation for the ac volt-failure time curve. It is suggested that the distribution of ac dielectric strength values for a given test time is correlated with the distribution in failure times at a given test voltage level.
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