November 2004

Process Safety/Environment

'Proven-in-use' criteria for safety instrumented systems

Use these guidelines to qualify equipment

The functional safety standards: IEC 61508,1 IEC 615112 and ANSI/ISA 84.013 each specify the safety integrity level (SIL) performance parameter for safety instrumented functions (SIFs). For an SIF to meet a specific SIL, the sum of the average probability of failure on demand ( PFDavg) of all components, part of that SIF, needs to fall in the PFDavg bandwidth related to that SIL. Besides the PFDavg requirement for a specific SIL, the IEC 61508 and IEC 61511 standards define the concept of architectural constraints. This concept puts additional requirements on equipment items that are part of the SIF. The architectural constraints are expressed in the required minimum level of hardware

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