What Regulations and Standards Apply to Safety Instrumented Systems?

User companies are responsible to define a framework that facilitates compliance with applicable safety standards.

03/01/2000


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.


Applicable Standards for Safety Related Sub-Systems


Standard


Specification


IEC 61508, Parts 1 to 7 (inclusive)


Functional Safety for Safety Related Systems


ANSI / ISA S84.01


Application of Safety Instrument Systems for the Process Industries


IEC 68 Parts 1, 3,2,14, 26, 30


Environmental Testing


IEC 801Parts 3,4,.5,6


Electromagnetic Compatibility for Industrial Process Measurement and Control


IEC 1000 Parts 4-4 and 4-6


Electromagnetic Compatibility (EMC)


IEC 1131


Programmable Controllers


EN 50081


Electromagnetic Compatibility -Emission Standard


EN 55011


Electromagnetic Compatibility -Emission Power Lines


ANSI / IEEE C62.41


Immunity, Power Line Surge


ANSI / IEEE C37.90


Immunity, Electrical Fast Transients


EMC Directive


EMC European standard

References

'IEC d61508; Functional Safety of Electric/Electronic/Programmable Electronic Systems,' International Electrotechnical Commission, Draft Report, 1997.




Paris Stavrianidis is the director of risk engineering methodologies for Factory Mutual Research. He is a recognized expert in the area of appliedstatistics, system reliability methodologies, and quantitative risk assessment techniques. He has contributed to the development of risk based inspection guidelines for the electric powergeneration industry, wasthe Vice-Chairman of the ISA SP84.02 subcommittee developing a technicalreport on reliability modeling techniques for programmable electronic systems used for safety; and participated in the development of IEC 61508. Currently he is a technical consultant to IEC 61511 and chairman of ANSI/ISA TR84, Part 4, entitled 'Reliability of Safety Instrumented Systems using Markov Modeling Techniques.' He is also the chairman of the Safety Engineering and Risk Analysis Division of American Society of Mechanical Engineers and a member of the CCPS Technical Steering Committee. He received his B.S and M.S. in Mechanical Engineering from Northeastern University. He is currently a Ph.D. candidate at Eindhoven Technical University, Eindhoven, The Netherlands.

For additional reading visit:
www.controleng.com and www.factorymutual.com

Comments? E-mail dharrold@cahners.com





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