Methodology for the Common Mode Analysis

The assumption usually made in the safety analysis of most systems is that the failure of any one component is independent of the failure of any other. If this assumption is not valid due to the system design and implementation, the estimated Fault Tree Analysis (FTA) event probabilities will be more optimistic than what is found in practice. One of the most important modes of failure and one, which can severely degrade the actual safety, is a common mode failure. This type of failure involves the simultaneous outage of two or more components due to a common cause. Common Mode Analysis (CMA) provides evidence that the failures assumed to be independent are truly independent. In reality, this analysis is extremely complex due to the large number of common mode failures that may be related to the different common mode types such as design, operation, manufacturing, installation and others. This paper provides a methodology that can be applied to perform the CMA in order to minimize the amount of work related to this analysis. Proposed methodology is based on the classification of all AND gates, which have to be analyzed into four categories and it establishes the depth of required analysis that varies from one category to another.