Safety Requirement Specification (SRS) documentation is extremely important, as it contains all the information necessary for the implementation of a Safety Instrumented System (SIS). This information must obviously be exhaustive and transmitted in a clear and detailed way, without any ambiguity, as also IEC 61511 Standard requires.
SRS writing must follow the identification of the SIL (Safety Integrity Level) within the Safety Life Cycle. Information must be acquired in close correlation with stakeholders and suppliers who will be responsible for the Safety Instrumented Function (SIF). In this phase, part of the main information is obtained from the Process and Hazard Report (PHA), a structured analysis that allows the evaluation of risks requiring specific measures to be implemented within the SIS.
As Safety Requirements Specifications must be transmitted to the Project Manager, or someone covering analogous role, it is essential that the document has no margin of interpretation. It must be written in a clear and concise manner, limiting (or avoiding) acronyms and abbreviations. If the use of both is necessary, an explanatory glossary may be included.
The SRS must also cover the requirements of each single system functionality.
To be best used by people operating on the plant, SRS must have another main feature: modularity. In other words, it has to be written in distinct units that can be extracted from the whole document, to focus on SIF’s single functional elements.
SRS format should be divided into 3 components:
Regarding contents, it must include:
The second and latest edition of IEC 61511 Standard requires that Safety Requirement Specification lists 29 elements (they were 27 in the first version), ranging from SIF and SIS description to operating procedures in each single circumstance, up to the definition of security measures to prevent malicious or unauthorized actions.