The degradation of epoxy resin-coated ZnO varistors at elevated temperatures and ambient humidity conditions

Li, J., Li, S., Alim, M. A. and Chen, G. (2003) The degradation of epoxy resin-coated ZnO varistors at elevated temperatures and ambient humidity conditions. Active and Passive Electronic Components, Vol. 2 (No. 4), 235-243.

Abstract

The degradation of the epoxy resin-coated commercial ZnO varistors at elevated temperatures and ambient humid conditions has been investigated experimentally. It has been observed that the leakage current of the ZnO varistors increases under the voltage stress at elevated temperatures with ambient humidity content. The change in the leakage current corresponding to a fixed electric field with respect to the initial current is taken as the dimensionless degradation index. This index is monitored at various experimental conditions in conjunction with the curing conditions of the epoxy resin powder. The results are carefully evaluated, and it has been found that the diffusion process of the moisture into the ZnO varistors plays a key role in the degradation process provided that these varistors had excellent property to begin with. The ionisation of the moisture at the interface between the ZnO block and the epoxy resin coating leads to the increase of the leakage current. Furthermore, the role of the ambient pressure corresponding to the elevated temperatures is considered as the variable to the degradation process. Those data are also monitored and analysed as a function of time.

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