Safety Requirement Specification (SRS)

SRS (Safety Requirement Specification) is a description of the requirements and operation of a SIF(Safety Instrumented Function). This document contains all the essential information for designing the SIF in detail:

  • Purpose of the SIF
  • General requirements for the SIF
  • The required SIL level, determined from a HAZOP/LOPA
  • De vereiste mode van de SIF (Low Demand < 1/jaar, High Demand of Continuous Demand)
  • The requirements for a manual shutdown.
  • The required response time of the total SIF (Sensor - Logic Solver - Final Element), determined from a HAZOP/LOPA study, often in combination with a process engineering calculation (Process Safety Time (PST)). For example, how quickly does the pressure in a distillation column increase when the cooling water pump fails. The reaction time of the SIF must therefore be within the PST.
  • The location and function of the sensors and actuators
  • Definition of the safe state of the SIF (e.g. feed valve closed or pump stopped)
  • Redundancy requirements to meet the SIL level or to increase availability
  • Requirements for defining Common Cause Failures.
  • Maintenance override functions (MOS, Maintenance Override Switches) or Operational override functions (OOS, Operational Override Switches) to allow, for example, start-up. These are preferably automatic and fall off automatically after a (preferably) short time.
  • Leakage class of valves
  • Communication with other systems
  • Any preferred brands and models of components, for example to stock common spares of critical instrumentation.
  • Requirements for the Test Intervals and the required Test Coverage Factors (TCF)
  • Requirements related to Energize or De-Energize to trip
  • Requirements for resetting after a shutdown (e.g. requirements for manual, semi-automatic, or automatic resetting of the final element after a trip).


The SIL verification program aeShield can generate an SRS in various formats (depending on the detailing). Format D of the aeShield SRS is fully compliant with IEC 61511 ed.2.


"Focus on occupational safety in the prevention of process safety incidents is misleading at best and catastrophic at worst".

- – Enrico Lammers