The metal oxide varistors are used as voltage dependent switching devices to protect high voltage equipment and also low voltage electronic devices against transient overvoltages. The performance of a ZnO surge arrester (SA) is determined by the electrical and thermal properties of metal oxide elements, insulating materials used and design features of SA. The primary consideration in applying metal oxide SA is the thermal consideration of the environment. Ambient temperature and environmental conditions directly affect the performance of internal current drawn by and energy input to the metal oxide SA. The interaction between the polluted porcelain housing and the inner varistor columns due to capacitive coupling is responsible for the temperature rise of the varistor element. Metal oxide SAs are exposed to heavy thermal and electrical stresses if the porcelain housings of the units are polluted by industrial dusts, coastal salts, etc. The first part of this paper proposes an electrical model of SA that is computer simulated for leakage current investigation and the second part presents the effect of dry band on the performance of SA and damage due to this dry band.
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