I. Introduction

Metal Oxide surge arrester (MOSA) is an equipment which provides protection to electrical apparatus and insulators from external and internal over-voltages. Arrester bypasses the excess energy associated with over-stress to ground. MOSA consists of mainly two parts, non-linear Zinc Oxide (ZnO) column and housing over the column. This ZnO column of surge arrester is made of a large number of microvaristor units. Such a single unit contains a very thin disordered layer, called intergranular region, made of several additive oxides of Bismuth (Bi), Cobalt(Co), Manganese(Mn) etc. sandwiched between two ZnO grains [9]. This inter-granular zone around ZnO grains is responsible for non-linearity of metal oxide surge arrester. At the two junctions between ZnO grains and intergranular zone back to back schottky junction is formed which provides the voltage dependent non-linearity of arrester [8].