Functional Safety
What is a Safety Requirements Specification (SRS)?
Vinit Pandey · Published 4 August 2026
In short: A Safety Requirements Specification (SRS) is the document defining the functional and integrity requirements a Safety Instrumented Function must meet, produced after SIL determination and before detailed Safety Instrumented System design begins.
Key takeaways
- SRS sits between SIL target-setting (LOPA) and detailed Safety Instrumented System design
- Functional requirements describe what the SIF does, independent of specific hardware
- Integrity requirements specify target SIL, architecture constraints, and proof test assumptions
- A documented SRS is what makes later SIL verification an objective, checkable exercise
Once LOPA or an equivalent method sets a target SIL for a safety instrumented function, the SRS translates that target into concrete engineering requirements: what the function must sense, what action it must take, its response time, and its required proof test interval.
The functional requirements section of an SRS describes what the SIF does in plain engineering terms — for example, closing a specific valve within a defined time when a pressure transmitter exceeds a setpoint — independent of which specific hardware will implement it.
The integrity requirements section specifies the target SIL, the required architecture constraints, and any assumptions about proof test interval and diagnostic coverage that the eventual hardware selection and SIL verification calculation must satisfy.
A well-written SRS is what allows SIL verification to be checked objectively later: without a clear, documented requirement, there is no fixed target against which an as-designed system's calculated PFDavg can be judged adequate or inadequate.