Definition:
Safe Failure Fraction (SFF) is a math term that recognizes dangerous undetected as the worst case type of failure. It is the ratio of safe (SD and SU) and detected dangerous (DD) failures to total failures (SD+SU+DD+DU). This is part of the analysis and intertwines with route 1H.
It is important to note that SFF ONLY applies to things with diagnostics. If it doesn’t, SFF would be so low that it doesn’t matter.
SFF is very tied to route 1H. If 2H is used, SFF does not need to be calculated.
Key Points:
- Higher SFF reduces required hardware fault tolerance.
- Important for achieving higher SIL levels.
- Do not confuse with Diagnostic Coverage (Cpt)
- Do not confuse with Diagnostic Coverage (DC)
Example:
If a device has 90 safe or detected failures out of 100 total, its SFF is 90%.
See also: DC, Routes, Hardware Fault Tolerance (HFT), Cpt
Cited Source:
- IEC 61508-2:2010, Clause 7.4.4.