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By Starr Tze, TÜV FSEng. Net Safety Monitoring Certification and Quality Engineer

When selecting a detection device to achieve a Safety Instrumented Function (SIF) within a Fire and Gas System (FGS), there are many options in the marketplace. Various statements are made by device manufacturers as to the suitability of their product including "Certified to SIL X", "SIL X Rated", "FIT for Use in SIL X", "Suitability Level SIL X" or "Compliant to SIL X". Understandably, this has caused a certain amount of confusion for end users.

Certified devices have passed an assessment on the management system, hardware and embedded software that shows adherence to the requirements of IEC 61508 Parts 1, 2 and 3. A rated, suitable for use, fit for use or compliant device meets IEC 61508 Part 2 requirements for hardware architectural constraints in terms of Safe Failure Fraction (SFF) and Probability of Failure on Demand or per Hour (PFD or PFH) requirements quantified by a Failure Mode Effects and Diagnostic Analysis (FMEDA).

ISA84.01-2004 and IEC 61511:2004 - Functional safety – Safety Instrumented System (SIS) for the Process Industry Sector provides two means for selecting a device to be used in a Safety Instrumented Function. First, the selection of a device can be based on prior use/proven-in-use which shows evidence of suitability by consideration of the manufacturers quality management system, adequate identification and specification, demonstration of performance in similar operating profiles and physical environments and volume of operating experience. Secondly, the selection can be based on the device being designed for compliance to IEC 61508. The system designer can make this determination themselves, or use evidence provided by the vendor or a third party certification body. It should be made clear that in order to achieve a SIL certified system, the devices themselves do not have to be certified.