The requirement is quite clear in the IEC 61511-1 standard.
The SIS safety requirements specification shall include:
a description of all the safety instrumented functions necessary to achieve the required functional safety [IEC 61511-1];
The IEC 61511-1 standard does not give precise instructions for the design of the SRS other that the SRS shall be expressed in a clear, precise, verifiable and maintainable way.
The functional requirements for the SIF shall be described. The SRS input requirement documentation is used to give detailed information regarding functional requirements.
The functional requirement describes, “How it should work”:
- definition of safe state
- process inputs and their trip points
- process parameters normal operating range
- process outputs and their actions
- relationship between inputs and outputs
- selection of energize-to-trip or de-energize-to-trip
- consideration for manual shutdown
- consideration for bypasses
- action on loss of power
- response time requirements for the SIS to bring the process to a safe state
- response actions for overt fault
- operator interface requirements
- operator actions
- reset functions
- response time requirements
The integrity requirements is also described.
The SRS includes:
- the requested SIL for each SIF
- requirements for diagnostics coverage to achieve the required SIL
- requirements for maintenance and testing to achieve the required SIL
- reliability requirements if spurious trips may be hazardous
- high or low demand mode
- requirements for proof testing
- environmental stress