Failure mode prioritization by improved RPN calculation method

Failure mode and effects analysis (FMEA) is an essential ingredient of reliability engineering and is a powerful technique used for improvement in design. However, the commonly used approach of calculating of a risk priority number (RPN) as the product of three ranking factors has been criticized for several reasons. This paper presents an improved calculation method for failure mode prioritization introducing casualty, cost, downtime, probability of occurrence and detectability into the consideration for risk priority number (RPN). The value of each factor is based on the relative importance of a failure mode, obtained by comparing the failure mode with the worst situation in the concerned area, instead of ranks determined by experts. The final RPN value is a number between 0 and 1, which means the total importance compared with the worst acceptable situation. This improved FMEA is illustrated with an application to a door protection system of an elevator. Using the proposed method, the prioritizations of four failure modes are different from traditional method results, facilitating proper ordering of corrective actions. This conclusion aligns with engineering logic.