A common cause is an event or mechanism that can cause two or more failures (basic events) to occur simultaneously. The failures resulting from the common cause are called common cause failures (CCFs). Because common causes can induce the failure of multiple components, they have the potential to increase system failure probabilities. Thus, the elimination of common causes can appreciably improve system reliability. To eliminate common causes, analysts must be able to recognize the failure sources that are responsible for CCFs and implement specific solutions to deal with them. The following table lists examples for types of common causes that are frequently encountered.

Several models are available for quantifying systems subject to common cause failures. Descriptions of these models, which are all supported in the FTA module, are found in Common Cause Failure Models.