November 2005

Special Report: Plant Safety and Environment

Selecting instrumentation for safety instrumented system applications

Follow these guidelines for the most effective and economical installations

Goble, W., exida LLC

Safety instrumented system (SIS) design requires care and expertise. This expertise includes not only a good knowledge of what instrument best meets the needs of the application, but also knowledge of functional safety standards. The ANSI/ ISA 84.00.01/ IEC 61511, Functional Safety for the Process Industries, requires careful justification of instrumentation. The equipment used in SISs must be chosen based on either IEC 61508 certification to the appropriate safety integrity level (SIL) or justification based on "prior use" criteria (Part 1, Section 11.5.3). Unfortunately, the IEC 61511 standard does not give specific details as to what prior use really means. Most agree, however, that

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