Many safety system failures trace back not to broken hardware but to a document that asked for the wrong thing. A clear, complete specification tells designers exactly what each protective function must do and how well.
Between hazard analysis and detailed design sits one document that shapes every safety function in a plant. This guide explains what it must contain and how to check response time against process safety time.
What Is a Safety Requirements Specification?
A safety requirements specification, or SRS, is the IEC 61511 document that defines, for every safety instrumented function, what it must do and how reliably it must do it. It is written after hazard and risk assessment and before design in the SIS safety lifecycle.
Think of it as the contract between the process safety team and the instrument engineers. If a requirement is missing here, it will usually be missing in the finished system.
exida highlights a UK HSE study from 2003 which found that 44 percent of control system accident causes were specification issues. No other lifecycle phase contributed more.
Where the SRS Fits
Inputs come from the HAZOP study and from LOPA or other SIL determination methods. The output feeds design and later validation.
exida notes that IEC 61511 calls for both a process safety SRS and a design SRS. The first states the need from the process view, while the second adds engineering detail for each device.
Two Kinds of Requirements
- What process condition triggers action.
- Trip point and measurement.
- Safe state and final elements.
- Response time and sequence.
- Target SIL or risk reduction.
- Proof test interval.
- Allowed spurious trip rate.
- Architecture and diagnostics.
Every safety instrumented function needs both columns filled. A function with a SIL but no clear trip logic cannot be designed properly, and vice versa.
9 Vital SRS Contents
The SIL target for each item must match the result in SIL assessment. Cause and effect logic is often attached as a cause and effect matrix.
Proof test intervals written here drive the numbers in later calculations, see proof test interval and coverage.
Good and Bad Specification Habits
| Aspect | Good SRS | Bad SRS |
|---|---|---|
| Trip logic | Clear setpoints and voting | Vague phrases like high pressure |
| Traceability | Each SIF linked to a hazard | No link to HAZOP or LOPA |
| Timing | Response time stated | Timing left to the vendor |
| Testing | Proof test method defined | Testing not mentioned |
| Ownership | Reviewed and approved | Copied from an old project |
Copying a previous project SRS is a common shortcut that carries old assumptions into a new plant. Each function should be written from its own hazard scenario.
Process Safety Time Check
Pass if Total response ≤ 50 percent of process safety time
Example:
Sensor 1 s, logic 0.5 s, valve stroke 6 s
Total response = 7.5 s
Process safety time = 20 s, allowed = 10 s
Pass with 2.5 s margin
The 50 percent rule is a common engineering practice, not a fixed rule of the standard. Some companies use other margins, so follow your own design basis.
Response Time Checker
Valve stroke time usually dominates, so large valves may need faster actuators or quick exhaust solenoids. Record the tested stroke time during validation.
- Fewer design errors and rework.
- Clear basis for validation tests.
- Consistent bypass and reset rules.
- Easier audits and assessments.
- Missing response times.
- Vague or copied text.
- No spurious trip target.
- Not updated after changes.
The specification is the benchmark for SIL verification and validation. Assessors also check it during a functional safety assessment.
ABB SRS Factsheet PDF
Writing the Specification Video
Safety Requirements Specification FAQ
Related Articles
- SIS Safety Lifecycle Stages
- SIF Design
- LOPA for SIL Assessment
- SIL Verification vs SIL Validation
- What Is Functional Safety Assessment
External References
What We Learn Today
- A safety requirements specification defines function and integrity for every SIF.
- Include safe state, trip points, timing, bypass, reset and testing.
- Check response time against process safety time with a clear margin.