How it's calculated (ISO 26262-5, Annex C)
- Per failure mode: safe faults = λ × safe %. If there is no safety mechanism (DC = 0), the rest are single-point faults. Otherwise residual faults = λnon-safe × (1 − DC) and multiple-point faults = λnon-safe × DC, of which latent = λMPF × (1 − DClatent).
- SPFM = 1 − Σ(λSPF + λRF) ÷ Σλ. LFM = 1 − ΣλMPF,latent ÷ Σ(λ − λSPF − λRF).
- Targets: ASIL B: SPFM ≥ 90%, LFM ≥ 60%, PMHF < 100 FIT. ASIL C: ≥ 97%, ≥ 80%, < 100 FIT. ASIL D: ≥ 99%, ≥ 90%, < 10 FIT.
- PMHF here is the single-point and residual fault rate. The full PMHF also adds dual-point faults, which depend on lifetime and the fault detection time, so treat the figure as a lower bound.
For early architecture decisions. The FMEDA for a safety case must use your failure-rate sources (for example IEC 62380 or SN 29500) and reviewed DC values.
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Questions
Which elements do I include?
Only hardware elements that can contribute to violating the safety goal. Enter each failure mode separately, for example a resistor open and short, because they often have different safe fractions and coverage.
Where do diagnostic coverage values come from?
ISO 26262-5 Annex D gives typical coverage of common safety mechanisms (low 60%, medium 90%, high 99%). For a safety case, justify each value with analysis or fault-injection results.
Why is my PMHF lower than the supplier's?
This tool counts single-point and residual faults only. The full PMHF also includes dual-point faults over the vehicle lifetime, which a complete FMEDA tool or quantitative FTA adds.
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