Voltage endurance of epoxy resins with microcavity type defects

This paper presents a progress report on a continuing investigation into the voltage endurance of cast and molded resins, particularly cast epoxy resins. The relation of gas cavity size to the observable discharge magnitude and the stress level on the resin at which discharges can be expected to occur within internal cavities is estimated. This illustrates that, with small cavities, the discharge picocoulomb magnitudes can be well below the usual present limits of detection. Experiments with cast resins in which low level discharges exist indicate the trend of discharge magnitude with voltage and time. Continuing voltage endurance tests of specimens with low level discharges indicate a decreasing time to failure with increasing time of ac stress similar to those with surface discharges of much greater magnitude.