Safety Requirements Specification
In functional safety projects, the Safety Requirements Specification (SRS) is a cornerstone document. For elements and subsystems, such as a gas detector forming part of a larger safety system, the SRS defines exactly what the product must do to meet its intended Safety Integrity Level (SIL) and ensure safe operation throughout its lifecycle.
An SRS provides a clear, unambiguous set of requirements that engineers, suppliers, and assessors can follow — avoiding assumptions that could compromise safety. If you’re developing an element to IEC 61508 (or sector-specific standards such as IEC 61511), the SRS is essential.
Below, we’ll walk through how to write an SRS for a gas detector used in a hazardous area monitoring application.
1. Define the Scope and Purpose
Begin by stating:
What the element is — e.g. “Point-type flammable gas detector for use in onshore process plant.”
Its role in the overall safety instrumented function (SIF) — e.g. “Detects methane gas concentration above 20% LEL and sends an alarm signal to the Safety PLC.”
Applicable standards and regulations — IEC 61508, IEC 60079 (for Ex compliance), and any customer-specific requirements.
Example entry:
This SRS defines the functional and safety requirements for the GD200 gas detector, a SIL 2-capable point sensor designed to detect methane in air, intended for use in IEC Zone 1 hazardous areas. The device forms part of a safety instrumented function that initiates process shutdown upon detection of a hazardous gas concentration.
2. Reference Key Inputs
List the reference documents and information sources used to generate the SRS:
Hazard and Risk Assessment results (HAZOP, LOPA)
Overall SRS for the complete safety system
Regulatory requirements
Customer specifications
Example entry:
This document is based on the safety function definition in the plant’s overall SRS (Document XYZ) and risk reduction requirements derived from LOPA Study Ref. LOPA-01.
3. Functional Requirements
Clearly define the intended functional behaviour:
Gas types and concentration ranges
Detection principles (e.g. catalytic bead, IR sensor)
Required accuracy, resolution, and response time
Environmental operating conditions
Example entries:
The detector shall measure methane concentration from 0–100% LEL.
The detector shall provide an alarm signal to the Safety PLC within ≤ 3 s of detecting 20% LEL.
The detector shall maintain accuracy of ±3% LEL over the operating temperature range of –20 °C to +55 °C.
4. Safety Requirements
Define how the element ensures safety and meets its SIL target:
Safety Integrity Level capability (e.g. SIL 2, type B, hardware fault tolerance = 0)
Safe failure fraction (SFF) and diagnostic coverage targets
Proof test interval and methods
Reaction to detected faults
Example entries:
The detector shall meet SIL 2 requirements per IEC 61508-2, with SFF ≥ 90% and diagnostic coverage ≥ 60%.
In the event of a detected internal fault, the detector shall signal a fault state within ≤ 1 s via 4–20 mA output (≤ 3.6 mA).
The maximum proof test interval shall be 1 year.
5. Interface Requirements
Document all hardware, software, and communication interfaces:
Signal types (analogue, digital, relay)
Protocols (e.g. HART)
Connector types and pin assignments
Example entry:
The detector shall output a 4–20 mA signal proportional to gas concentration, with 3.6 mA indicating fault and 21 mA indicating overrange.
6. Environmental and Mechanical Requirements
Include:
Temperature, humidity, vibration limits
IP rating
Hazardous area certification requirements
Example entry:
The detector shall operate in ambient temperatures –20 °C to +55 °C, with 0–95% RH non-condensing, and withstand vibration per IEC 60068-2-6.
The detector shall be IP66 rated and certified for IECEx Zone 1, IIC T4.
7. Verification Requirements
Define how compliance will be verified:
Type tests, factory acceptance tests, on-site tests
Proof test procedures
Documentation deliverables
Example entry:
SIL capability will be verified by FMEDA analysis, validation testing, and assessment to IEC 61508 by an independent third party.
8. Maintenance and End-of-Life Requirements
Set requirements for:
Maintenance intervals
Calibration needs
End-of-life replacement indicators
Example entry:
The detector shall include an end-of-life indication when remaining sensor life is ≤ 6 months.
9. Document Control
Include:
Version history
Approval signatures
Change control process
Final Tips
Be specific — avoid vague language such as “fast response” or “good accuracy”.
Link each requirement to a hazard or safety function to demonstrate necessity.
Separate functional performance from safety integrity so engineers and assessors can trace compliance easily.
Maintain traceability from hazard analysis → overall SRS → element SRS → design → verification.
A well-written SRS for a subsystem like a gas detector not only supports compliance with IEC 61508, but also reduces misunderstandings during design, manufacture, and certification. It becomes the “single source of truth” for safety performance expectations — vital for delivering a safe and reliable product.