Degradation of metal oxide surge arresters under simulated natural conditions

The degradation of metal oxide surge arresters (MOSA) under continous operating voltage (C.O.V) with time, is a well known fact. But the contribution of transient overvoltages in the form of lightning surges, switching surges etc.-against which the arrester is used as a protection device, to the arrester degradation has not been studied much. The simulation of a situation, wherein the arrester is kept uder C.O.V and different surges are applied on it, is somewhat complex. A method for simulation of field operating conditions of an arrester in the laboratory is presented in this paper. Results of two seperate degradation studies on different samples under A.C, and D.C. excitations with superimposed transient stresses are given. A comprison of observed data with calculated ones is also made.