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ISO 26262 hardware metrics: SPFM, LFM and PMHF target references
The tables in ISO 26262-5 provide references for SPFM, LFM and PMHF rather than one combined score. The three metrics address different failure contributions and are not interchangeable.
Target values and applicability paraphrase ISO 26262-5:2018 Clauses 8.4.5, 8.4.6 and 9.4.2, and Tables 4–6. Table values are possible sources for deriving targets; they do not replace project-specific applicability, full calculations or safety assessment.
Separate Clause 8 from Clause 9
ISO 26262-5 Clause 8 covers the SPFM and LFM hardware architectural metrics. Clause 9 evaluates the probability of safety-goal violation due to random hardware failures; PMHF is addressed in 9.4.2. The three metrics are related, but their scope and meaning differ.
The captions of Tables 4, 5 and 6 identify their values as possible sources for deriving target values. Read the numbers together with the applicable ASIL, clause requirements and item structure.
| Metric | Part 5 clause and table | Reference targets shown in the table |
|---|---|---|
| SPFM | Clause 8.4.5; Table 4 | ASIL B ≥ 90%; ASIL C ≥ 97%; ASIL D ≥ 99% |
| LFM | Clause 8.4.6; Table 5 | ASIL B ≥ 60%; ASIL C ≥ 80%; ASIL D ≥ 90% |
| PMHF | Clause 9.4.2; Table 6 | ASIL B and C < 10⁻⁷ h⁻¹; ASIL D < 10⁻⁸ h⁻¹ |
Apply reference values within their scope
Tables 4 and 5 are possible sources for target derivation, not context-free acceptance criteria for every project. Clause 8.4.5 applies to ASIL (B), C and D; Clause 8.4.6 applies to ASIL (B), (C) and D. Under the standard's parenthesized-ASIL convention, parentheses indicate a recommendation rather than a requirement for that ASIL.
Table 6 is also a possible target-derivation source, and its note addresses items made up of a single system. For an item with multiple systems, analyse target allocation under the applicable Clause 9.4.2 requirements instead of mechanically assigning the table value to every system.
FIT and PMHF can share units without being the same quantity
FIT is a unit for failure rate; 1 FIT equals 10⁻⁹ h⁻¹. PMHF is expressed as an average probability per hour over the operational lifetime of the item. Part 5 Clause 9.4.2.1 explicitly distinguishes PMHF from failure rate even when the units match.
A single component datasheet FIT value therefore cannot be treated directly as the item PMHF. PMHF assessment follows Clause 9 for relevant failure contributions and combinations; a simplified exercise should be labelled as a teaching approximation.
Common mix-ups and assessment limits
SPFM and LFM are both Clause 8 architectural metrics, but they cover different fault categories; PMHF belongs to Clause 9. Do not transfer one metric’s target value to another.
Meeting a table reference value alone does not establish ISO 26262 conformity for an entire item. Target derivation, analysis scope, failure-data assumptions and project confirmation still matter.
Common questions
- Does ISO 26262 define one overall hardware score?
- This guide covers three separate hardware metrics: SPFM, LFM and PMHF. They have different definitions and clause locations, so they should not be combined or substituted for one another.
- Can Tables 4–6 be used as limits for every project?
- Not without considering applicability. The table captions identify possible sources for target derivation; the clause, ASIL, item structure and project basis also matter.
- Does a component FIT value below the Table 6 number mean PMHF is met?
- No. FIT expresses failure rate, while PMHF is an item-level metric. Unit conversion cannot replace the system-level assessment in Clause 9.
Sources and scope
- ISO 26262-5:2018
Target values and applicability paraphrase ISO 26262-5:2018 Clauses 8.4.5, 8.4.6 and 9.4.2, Tables 4–6 and related notes. No standard text is reproduced.