A Safety Instrumented System (SIS) is composed of a sensor, logic solver and final control element. These three components form a Safety Instrumented Function (SIF) that brings a process to a safe state during a hazardous event. Safety Instrumented Systems contain ten to hundreds of SIFs that make up the entire system and protect people, processes, equipment, environment and company image.
AANSI/ISA 84.00.01-2004 (IEC 61511) - Functional Safety: Safety Instrumented Systems for the Process Industry Sector standard lists the specifications, design, installation, operation and maintenance required in a SIS. This standard applies to applications in chemical and oil refining, oil and gas production, pulp and paper, and electric power generation facilities. IEC 61511 is intended as a guide for engineers that design safety instrumented sytems.
IEC 61508 - Functional Safety: Safety Related Systems is a standard that certification agencies, such as Exida and TUV, use as a basis of approval for vendor-certified products. These certification agencies analyze the failure rates of products and provide data in the form of FITs (failure in time) in product certification reports, required to calculate Safety Integrity Levels (SILs). Failure rate data is necessary to determine whether a product meets the performance level requirement of the SIF. IEC 61508 is intended as a guide for device manufacturers that design equipment intended for use in safety instrumented systems.
The U.S. Occupational Safety and Health Administration (OSHA) requested a grandfather clause be included in ANSI/ISA 84.00.01-2004 covering safety systems designed prior to the standard's release. The grandfather clause (Part 1, Clause 1Y) states: "for existing SIS designed and constructed in accordance with codes, standards, or practices prior to the issue of this standard, the owner/operator shall determine that the equipment is designed, maintained, inspected, tested and operating in a safe manner."
In order to meet this clause, owner/operators shall demonstrate that they have conducted a hazard and risk analysis (HAZOP), performed a quantitative or qualitative examination identifying the required performance level of each SIF, and prove that all target SILs are achieved. OSHA requires owner/operators of legacy systems to follow best engineering and manufacturing practices.