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Active and Passive Electronic Components
Volume 26 (2003), Issue 4, Pages 235-243

The Degradation of Epoxy Resin-Coated ZnO Varistors at Elevated Temperatures and Ambient Humidity Conditions

1State Key Laboratory of Electrical Insulation for Power Equipment, Xi'an Jiaotong University, Xi'an 710049, China
2Department of Electrical Engineering, Alabama A & M University, P.O. Box 297, 202 Engineering and Technology Building, Normal, Alabama 35762, USA
3Department of Electronics and Computer Engineering, University of Southampton, Southampton SO17 1BJ, United Kingdom

Received 18 December 2002; Revised 21 March 2003

Copyright © 2003 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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 condition 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. These data are also monitored and analysed as a function of time.