Reliability of surge protective devices stressed by lightning

Surge protective devices (SPDs) are used to limit the maximum overvoltage value on protected circuits. The great part of such overvoltages are originated by direct and nearby lightning flashes to structures and to lines, it is then important to evaluate the reliability of the protective device. Actual SPDs are affected by ageing, due essentially to number and amplitude of stresses, but also to other factors such as overheating, pollution and humidity. A method, based on probabilistic arguments, to evaluate the ageing process of SPDs, when stressed by lightning pulses, is proposed and analysed. Its relevance stands in the fact that National and International standards do not give complete indications to decide when a SPD must be changed before its failure occurs (reliability estimation), instead the estimated expected life, obtained with the method proposed, can be directly used in the design of logistic/maintenance procedures.