What Regulations and Standards Apply to Safety Instrumented Systems?
User companies are responsible to define a framework that facilitates compliance with applicable safety standards.
T he International Electrotechnical Commission (IEC, Geneva, Switzerland) draft standard IEC 61508 has been developed to support companies that use Safety Instrumented Systems (SISs) to protect against hazardous events. SISs are composed of sensors, logic solvers, and final control elements assembled for the purpose of taking the process to a safe state when predetermined conditions are violated. Other terms commonly used to describe SISs include emergency shutdown systems, safety shutdown systems, and safety interlock systems.
IEC 61508 is a performance-based draft standard developed as an umbrella standard to be applied to any industrial process that uses Electrical, Electronic and Programmable Electronic (E/E/PE) Safety Related Systems (SRS) or SISs. This umbrella standard allows development of industry sector specific standards provided they follow a safety life cycle model similar to the one defined in IEC 61508. The life cycle model provides a structured framework to identify and provide guidance for all process activities that affect the functional safety of an SIS and relies on performance-based metrics such as process risk and SIS Safety Integrity Level (SIL). Therefore, it can be objectively and systematically applied by industry, manufacturers of systems, industry regulators, and approval agencies.
Parts 1, 3, 4 and 5 of IEC 61508 have been published. Parts 2, 6 and 7 are Final Draft International Standards (FDIS). Final voting, without comments, occurred in December of 1999.
Scope and technologies
IEC 61508 provides guidance to evaluate what activities should be performed throughout the life cycle of the SIS to ensure functional safety is part of overall safety, or freedom from risk, relating to SISs.
Suitable for defining functional safety related aspects in any industry, IEC 61508 specifically addresses:
Correct functioning of the SIS;
Other technology safety systems; and
External risk-reduction facilities.
IEC 61508 covers SISs in any industrial application comprised of E/E/PE equipment including sensors, logic solvers, final elements, and interfaces.
Technologies used in various SIS industrial applications include electromagnetic relays, solid-state logic, programmable electronics, motor-drive relays and timers, hard-wired logic and combinations of the above. Pneumatic technologies are excluded.
Other standards and regulations
IEC 61508 provides requirements for:
Safety applications in a particular environment; and
Safety related sub-systems (e.g., safety programmable logic controllers, sensors, etc.) that will be a part of the safety system solution.
The standard requires compliance with all national and international regulations that are application specific. For example, using an SIS on a machine to provide protection requires compliance with the European Union Machinery Directive. Similarly, using an SIS as a burner management system requires compliance to National Fire Prevention Association standards.
Safety related sub-systems
For a safety-related sub-system to comply with IEC 61508, other relevant standards must be met. IEC 61508 does not make a direct reference to many of these standards. However, good engineering practice and regulations do specify compliance to such things as environmental conditions, electromagnetic compatibility, etc. The following list provides standards most commonly used to supplement IEC 61508 compliance for safety related sub-systems.
'IEC d61508; Functional Safety of Electric/Electronic/Programmable Electronic Systems,' International Electrotechnical Commission, Draft Report, 1997.