What is Safe Failure Fraction (SFF)?

Safe Failure Fraction (SFF) is the proportion of failures that are either safe (process goes to safe state) or detected (diagnostic coverage). SFF = (safe failures + dangerous detected failures) / total failures. Higher SFF allows higher SIL achievement with simpler architectures. Type A devices (well-understood, all failure modes known): SIL 1 requires SFF >60%, SIL 2 >90%, SIL 3 >99% for 1oo1 architecture. Type B devices: more stringent requirements. SFF is specified by device manufacturers in safety manuals. High SFF combined with diagnostics enables higher SIL with less hardware redundancy.