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DEFENCE IN DEPTH
Defence in depth

Common cause failures

 

Consideration of common cause failures addresses events where maybe only one component failed, but on further inspection, others were found to have a similar problem developing. For example, one pump seized, and others were found to have worn bearings.

To rate the event as if all components had failed would exaggerate the event, but equally, to rate the event as if only one component had failed would underplay the event. The approach here is to consider increasing the rating by one level to account for the degradation of the other components. Under this heading we may also increase the basic rating of events where there is a difficulty in system operation caused by absent or misleading information.