Mean time to failure of metal oxide varistors under lightning stress

The metal oxide varistors (MOVs) are the most used surge protective devices (SPDs) in order to limit the maximum voltage on protected circuits. Since many of the outages experienced on such circuits are due to overvoltages originated by direct and nearby lightning flashes, it is important to evaluate the reliability of such devices when stressed by lightning pulses. Actual MOVs are affected by ageing, due essentially to number and amplitude of stresses, but also to other factors such as overheating, pollution and humidity. Aim of this paper is to present a method, based on probabilistic arguments, to evaluate such ageing process of MOVs. The expected life of the device, so obtained, can be used to decide when the MOV must be changed before its failure occurs (reliability estimation), since national and international standards do not give definitive indications. The estimated expected life can also be directly used in the design of logistic/maintenance procedures.