Safety Instrumented Systems (SIS) are a specific layer of protection that requires detailed knowledge and criteria for proper definition and installation based on functional safety principles and associated standard requirements. This paper focuses on providing guidance and criteria for conducting the following tasks:
Reference  provides an overview of layers of protection suitable to reduce the risk level of a process facility, i.e., measures intended to prevent and/or mitigate the identified hazardous scenarios. The cited paper  explains that based on the results of a risk-based quantitative assessment, zones or locations and their associated hazardous scenarios having the most significant intolerable risk level can be identified. . As a result, sensitivity and cost-benefit analyses can be performed with the aim to decide which safeguards achieve to reduce the risk to an acceptable level at the most reasonable cost.
From all layers of protection considered in reference , the Safety Instrumented Systems (SIS) and performance-based Fire and Gas Detectors Systems (FGS) are safeguards that should comply with very specific requirements based on the following standards; i.e., IEC 61508 , IEC 61511  and ISA 84.00.TR.07 , respectively. While performance-based FGS selection, verification and mapping guidance can be found in reference , the main purpose of this paper is to address SIS intended to comply with standards  and . A SIS consists of at least three subsystems:
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