The IEC61511 standard is a performance-based standard that is all about the performance of the SIS and reducing risk, and defines what the SRS should contain all the requirements for the Safety Instrumented System and its associated Safety Instrumented Functions (SIFs).
The SIS consists of one or more Safety Instrumented Functions (SIFs), which can consist of a combination of sensor(s), logic solver(s) and final element(s), including all interfaces and sources of power. Therefore, the SRS needs to define a number of criteria for each SIF: a set of Functional Requirements and a set of Integrity Requirements (Safety Integrity Level target), indicating the amount of risk reduction required to be achieved (i.e. a measure of its dependability), as well as failure modes, operations and maintenance related information. The SRS templates account for these functional and integrity requirements.
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