Clause 9-Allocation of safety functions to protection layers

9 Clause 9 - Allocation of safety functions to protection layers 

Safety functions are allocated to protection layers. For each safety function, the owner/operator should evaluate potential common-cause failures that could disable multiple safety functions. Annex G provides an additional explanation concerning common-cause failures. 

When safety functions are allocated to the safety instrumented system layer, they are known as Safety Instrumented Functions (SIF). The differences and/or relationships between safety functions, safety instrumented functions, safety instrumented systems, interlocks, and permissives are important. Annex H provides a discussion including important historical references to assist in the understanding of the new terms. 

Safety instrumented functions (SIF) are assigned a safety integrity level (SIL) and a mode of operation (continuous or demand) during the H RA. These are recent requirements and application guidance is limited at best. 

Some of the new requirements in this ciause inciude the following  

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Clause 8 - Hazard and risk assessment (Layers of Protection Analysis, Layered Risk Analysis, etc,) Clause 9 - Allocation of safety functions to protection layers L R A R R P P A... 

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