Definition:
Safe Failure Fraction (SFF) represents how good a device is at avoiding undetected dangerous failures by showing the percentage of failures that are either safe or detected. 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. Typical ranges for SFF are <60% for lower values, and >90% for those with high SFF.
SFF is very tied to route 1H. If 2H is used, SFF does not need to be calculated.
This can also be confusing as SFF is similar to DCdang.
- DC focuses on the dangerous portion of failures
- SFF focuses on the entire portion of failures
- SFF is always >= DCdang
- DCdang Used in route 2H – / SFF used in route 1H
Key Points:
- Higher SFF reduces the required hardware fault tolerance.
- Important for achieving higher SIL levels.
- Do not confuse with diagnostic coverage (DC). They are closely related but distinct topics.
- Do not confuse with proof test coverage (Cpt)
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.