Glossary · Requirements and interfaces
Safety requirements specification (SRS)
Also known as: SRS, SIS safety requirements specification
German: Spezifikation der Sicherheitsanforderungen (SRS)
In functional safety under IEC 61508 and IEC 61511, the safety requirements specification (SRS) is the document that states the functional and safety integrity requirements of each safety function, including trip conditions, safe states, response times, required SIL and test requirements.
- Functional safety
- Technical documentation
In one sentence
The safety requirements specification (SRS) states what each safety function must do and how reliably, as the basis for design and validation.
Example
The SRS of a furnace SIS lists each SIF with its sensors, trip setpoints, voting, safe state, SIL, proof-test interval and reset behavior, and it serves as the reference for the site acceptance test.
How it applies
- Engineering: The SRS is the bridge between analysis and design. Its inputs are the hazard and risk analysis and SIL allocation; its outputs guide hardware design, application programming and testing.
- Content: Typical items are the safe state, process conditions and setpoints, response time, SIL and demand mode, bypass and reset requirements, manual shutdown, and requirements for proof testing and diagnostics.
- Validation: SIL validation checks the installed system against the SRS, so every requirement must be testable.
- Documentation: Write the SRS as atomic, uniquely identified requirements with Traceability to the analysis and to test cases. The documentation team should align operating and maintenance instructions with the SRS, especially for reset, bypass and proof-test procedures.
SRS vs. software requirements specification
In software engineering, SRS usually means software requirements specification. In functional safety it means safety requirements specification. IEC 61511 also knows an application program safety requirements specification derived from it, so spell out the term at first use.