Evaluation of the Safe Failure Fraction for an electromechanical complex system: remarks about the standard IEC61508

The paper discusses a case study concerning the evaluation of the Safe Failure Fraction (SFF) for a complex system. Being such evaluation made according to IEC61508 standard, the paper focused on some related ambiguities. The paper is structured in two phases. First, a reliability prediction of each component of the system is implemented and then a Failure Mode and Effect, Diagnostic Analysis (FMEDA) is applied, we selected FMEDA for more information about failure condition of each component (modules or systems) under examination. Using this method it is possible to study SFF of different complex systems in compliance with qualitative requirements described in the standard. The main purposes of this paper are to facilitate the application of the standard and to clarify some of related ambiguities and to propose ways to satisfy the requirements related to how hardware architecture may be configured also referred to the architectural constraints.