Abstract
The optimal logical structure is developed for safety monitoring systems, which have two types of contradictory failures; a fail-dangerous failure and a fail-safe failure. The optimal structure which minimizes an expected damage caused by the two types of contradictory failures is analytically shown to be k*-ont-of-n: G system among all the coherent structures composed of n identical components. A simple formula to find the optimal k* is obtained. We discuss how the optimal k* varies, depending on the two failure probabilities of the sensor, the probability of the plant failure and damages caused by the two types of contradictory failures of the safety monitoring system. Illustrative examples are given.
