Safe Failure Fraction and HFT Calculator
Find the safe failure fraction (SFF) and diagnostic coverage of a device from its FMEDA failure rates, and the highest SIL its architecture can claim under IEC 61508 Route 1H.
- SFF = (λS + λDD) ÷ (λS + λDD + λDU) = 92.11 %
- DC = λDD ÷ (λDD + λDU) = 86.96 %
- IEC 61508 Route 1H table for Type B, HFT 0: SIL 2
Maximum SIL, Type B elements (IEC 61508-2 Route 1H)
| SFF | HFT 0 | HFT 1 | HFT 2 |
|---|---|---|---|
| < 60 % | Not allowed | SIL 1 | SIL 2 |
| 60 to < 90 % | SIL 1 | SIL 2 | SIL 3 |
| 90 to < 99 % | SIL 2 | SIL 3 | SIL 4 |
| ≥ 99 % | SIL 3 | SIL 4 | SIL 4 |
SFF and Diagnostic Coverage
DC = λDD ÷ (λDD + λDU)
Failure rates come from the device FMEDA or certificate. IEC 61511 also allows an alternative route based on prior use and HFT requirements per SIL.
Worked example
λS = 300, λDD = 400, λDU = 60 FIT give SFF = 92.1 % and DC = 87 %. A Type B element with HFT 0 is limited to SIL 2.
Frequently Asked Questions
What is safe failure fraction?
The fraction of all failures that are either safe or dangerous but detected by diagnostics.
What is the difference between Type A and Type B?
Type A elements have well defined failure modes and behaviour (for example relays, simple valves); Type B elements are complex, such as microprocessor based transmitters.
What is hardware fault tolerance?
The number of faults a subsystem can tolerate while still performing its safety function. 1oo2 has HFT 1.
Learn more on the blog
What is SIL (safety integrity level)
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article