Fuzzy-Based Approach to Solve Classical Risk Priority Number Drawbacks for Railway Signaling Systems

Failure modes, effects, and criticality analysis (FMECA) is a powerful tool used to quantify the risk related to safety-critical systems and to develop the proper corrective actions to minimize the associated risk. However, traditional FMECA has been widely criticized for its many drawbacks and weaknesses. Different combinations of occurrence, severity, and detection may produce the same value of risk priority number (RPN), and the relative importance among the parameters is not taken into consideration. Furthermore, the three factors—occurrence, severity, and detection—are difficult to precisely estimate. Trying to solve these needs, this article proposes a simple and effective fuzzy-based approach that uses fuzzy linguistic terms to assess the parameters and then evaluates the RPN of each failure mode as the fuzzy multiplication of the indexes. Fuzzy weights are also taken into account to assess differences in importance among the parameters. The proposed approach solves all the RPN drawbacks while maintaining intact the concept of the traditional RPN. To test and validate the performance of the proposed fuzzy-based approach, the method has been applied to a safety-critical system used as a train protection system in railway signaling applications. The results prove the ability of the procedure to overcome every RPN drawback.