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Behaviour of metal-oxide-based varistors subjected to partial discharges in air

The behaviour of zinc oxide varistors subjected to partial discharges in air is studied with respect to their protective coating (glass and epoxy resin). The decomposition of the medium with residual impurities can considerably influence the varistor current-voltage characteristics. Degradation is due to a conducting deposit around the varistors and a physical corrosion of the coating. The conducting deposit is analysed by physico-chemical methods. The authors show the influence of the partial discharge energy, humidity and surrounding gases on the varistor degradation. Moreover they point out a significant modification of the l(U) characteristic in the ohmic region, with a large increase of leakage current following degradation.