Estimation techniques for risk of failure

There has been a growing interest in approaches based on reliability techniques using the concept of the risk of failure (e.g., in the IEC 60071 standard). The idea is to define the average probability of failure of apparatus per year, consider the lightning reliability of the system as a design parameter, and design the system to meet that objective in terms of acceptable risk of failure. The risk is calculated, using: the probability distribution of the basic random variables (point of impact of the strokes, crest value of the lightning current etc.) and the EMTP simulation results, e.g. overvoltages due to the lightning strokes. One major difficulty is to evaluate precisely the risk with a limited number of EMTP simulations (the simulations are time consuming). A partnership involving the University of Clermont Ferrand (France), Electricite de France) and Ontario Hydro Services Company (Canada), has been formed to work on existing probabilistic methods and adapt them to direct and induced lightning studies. In this session the probabilistic methods to calculate the risk of failure are presented. They are applied to practical test cases related to direct and induced lightning