Particularly if different combinations of failures do not have significantly different effects on the plant, it may be desirable to group redundant components or trains under a single top event. This is shown in Figure 6, where pumps 1, 2, and 3 are all grouped within the AFW top event. If the three trains of the AFW system were instead each assigned to a different top event, the corresponding top events would not be conditionally independent of each other, due to the potential for common cause failure (i.e., simultaneous failure of two or more AFW pumps due to the same cause). Modeling common cause failure between different top events would be complicated, since, for example, the failure probability of pump 2 might depend on whether pump 1 had failed due to common cause or not.
