THE IMPACT OF DIVERSITY UPON COMMON-MODE FAILURES

Recent models for the failure behaviour of systems involving redundanc y and diversity have shown that common mode failures can be accounted for in terms of the variability of the failure probability of componen ts over operational environments. Whenever such variability is present , we can expect that the overall system reliability will be less than we could have expected if the components could have been assumed to fa il independently. We generalise a model of hardware redundancy due to Hughes, [Hughes, R. P., A new approach to common cause failure. Reliab . Engng, 17 (1987) 211-236] and show that with forced diversity, this unwelcome result no longer applies: in fact it becomes theoretically p ossible to do better than would be the case under independence of fail ures. An example shows how the new model can be used to estimate redun dant system reliability from component data.