The effect of a certain class of potential common mode failures on the reliability of redundant systems

This is a theoretical investigation of the importance of common mode failures (cmfs) on the reliability of redundant systems. These failures are assumed to be the result of fatal shocks e.g. from earthquakes and explosions, which occur at a constant rate. This formulation makes it possible to predict analytically results obtained in the past which showed that the probability of a cmf of the redundant channels of the protection system of a typical nuclear power plant was orders of magnitude larger than the probability of failure from chance failures alone. Furthermore, since most reliability analyses of redundant systems do not include potential cmfs in the probabilistic calculations, criteria are established which can be used to decide either that the cmf effects are indeed insignificant or that such calculations are meaningless, and more sophisticated methods of analysis are required, because cmfs cannot be ignored. (Auth.)