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Requirements · Core workflow

Writing a SIF Safety Requirements Specification

The SRS is the controlled definition of what the safety function must do and the integrity it must achieve. It should be specific enough for an independent person to design and validate the function.

Engineering guide11 min readUpdated 10 October 2026Published by SILProof Engineering

In brief

What to carry into the engineering work

  • Write requirements that are specific, testable and traceable to the hazardous scenario.
  • Keep functional behaviour, integrity targets and lifecycle controls distinct but connected.
  • Control document identity, assumptions, review status and changes as part of the engineering record.

Document identity and basis

Start with the project, unit, SIF tag, document number, revision, status, preparer, checker and approver. Link the hazardous scenario, risk-study reference, required SIL and assumptions that created the requirement.

Functional requirements

  • Initiating condition and trip setpoint.
  • Safe state and required final action.
  • Operating modes in which the function is required.
  • Process safety time and maximum SIF response time.
  • Reset, restart, latching and manual-action philosophy.
  • Interfaces with BPCS, utilities and other protection layers.

Integrity and architecture

State the demand mode, required SIL, quantitative target, applicable architectural route, systematic-capability requirement and any independence constraints. Reference the verification method rather than embedding a changing calculation result as a requirement.

Testing, bypass and maintenance

Define proof-test interval and required coverage, diagnostic response, maximum restoration time, bypass authorization, compensating measures, alarm handling and records. A requirement should be testable and have a clear acceptance criterion.

Review and issue

Resolve assumptions or mark them as controlled actions before issue. Check consistency with the cause-and-effect, logic diagrams, instrument data sheets, proof-test procedure and verification record. Validation should trace back to the issued requirements.

Useful split: a concise minimum SRS can carry the mandatory function and integrity requirements; an audit-ready issue can add rationale, interfaces, evidence and lifecycle records. Both should remain unambiguous.

Primary references

  1. IEC 61511-1 — process-industry SIS framework and requirements
  2. UK HSE — Functional safety
  3. ISA — ISA-84 series of standards

Use the applicable edition and national adoption. Project specifications and controlled procedures may add requirements.

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Related guides

Determining the required SILBuild the risk-reduction basis with a defined scenario, tolerable-risk criterion and reviewable assumptions.Designing a Safety Instrumented FunctionTranslate the safety requirement into sensors, logic solver interfaces, final elements and a testable safe action.Proof testing and restoration assumptionsConnect proof-test interval, coverage, diagnostics and repair assumptions to a procedure that can actually be executed.