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.
Writing SRS: information
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.
Writing SRS: clarity and completeness
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.
Writing SRS: modularity
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.
Writing SRS: format and content
SRS format should be divided into 3 components:
- General requirements;
- Functional requirements;
- Integrated security requirements.
Regarding contents, it must include:
- Design and architecture;
- Reliability;
- SIL (Safety Integrity Level);
- Support systems;
- Installation, test and maintenance;
- Hardware specifications;
- Software specifications;
- Human machine interface.
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.
