The mechanism of the breakdown occurring when a dc ramp voltage is applied to low-density polyethylene (LDPE) and silane cross-linked PE at high temperatures is discussed, using experimental results on the Young's modulus Y, high-field conduction and pre-breakdown currents. The electric strength F B of PE was found to be a function of Y, irrespective of cross-linking. It was also found that, over a wide temperature range, a sharp pre-breakdown current increase could not be detected till a time before breakdown of 100 ns, the resolution limit of the measuring system. The results showed that, under the present experimental conditions, thermal breakdown is not applicable to PE over a wide temperature range and that breakdown is initiated by an electronic process which has a very short time lag to breakdown of the order of some tens of ns or less.

A technique of measuring current prior to breakdown was introduced. The breakdown mechanism was discussed on the basis of results obtained for polythylene (PE) and polymide (PI) by applying dc ramp voltage. At 90°C the pre-breakdown current of PI sharply increased from about 1 msec before breakdown. For PE, the sharp current increase could not be detected till about 100 ns before breakdown, which is the resolution limit of the measuring system. If this sharp increase is due to the temperature rise of the sample, thermal breakdown may be dominant in the case of PI but not of PE.

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