The FMEDA approach to improve the safety assessment according to the IEC61508

According to the Standard IEC61508, the paper presents a case study concerning the evaluation of both the safe failure fraction (SFF) and the probability of failure on demand (PFD) for a complex system. After a preliminary presentation of the criteria for the safety integrity level (SIL) verification, the work focuses the attention on the method to achieve the PFD. In particular, an approach based on failure modes, effects and diagnostic analysis (FMEDA) is proposed and then a comparison with the approach described in the Standard. The paper aims to clarify both the knowledge and the application of the IEC61508 and proposes a technique to satisfy the hardware safety integrity requirements.