Glossary · Requirements and interfaces
Safety integrity requirement
Also known as: safety integrity requirements
German: Anforderung an die Sicherheitsintegrität
In functional safety under IEC 61508, a safety integrity requirement specifies how reliably a safety function must be performed, expressed as a safety integrity level with its target failure measure. It complements the functional requirement, which states what the safety function does.
- Functional safety
- Systems engineering
In one sentence
A safety integrity requirement states how reliably a safety function must work, usually as a SIL with a target failure measure.
Example
For an overspeed trip, the functional requirement is to stop the turbine above 110 percent speed; the safety integrity requirement is SIL 2 in low-demand mode.
How it applies
- Requirements: Every safety function has two sides: the functional requirement and the safety integrity requirement. Both belong in the Safety requirements specification (SRS).
- Derivation: The integrity requirement comes from SIL allocation, which in turn rests on the hazard and risk analysis. It must not be chosen simply to match an available device.
- Verification: Hardware failure measures, architectural constraints and systematic capability are checked against the integrity requirement during SIL verification.
- Documentation: Write integrity requirements as separate, traceable requirement items with the mode of operation (low demand, high demand or continuous). The documentation team should use the same wording and identifiers in specifications, test reports and manuals.
Safety integrity requirement vs. functional requirement
A Functional requirement describes the behavior, for example the trip threshold and the safe state. The safety integrity requirement describes the probability with which that behavior must occur on demand. A function can be correct but insufficiently reliable, or reliable but specified wrongly.