Do You Really Need SIL 3?

Process plants rarely require that high a safety integrity level for instrumented protection.

By Alan G. King, ABB Engineering Services

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A Safe Approach
IEC 61508 and IEC 61511 represent current good practice in the management of SIF at process plants across the world. It's important for those managing the operation of such plants to be aware not only of these standards and what they contain but also of the benefits that adopting them can bring in demonstrating suitable management of risks. Indeed, unless operators of high-hazard process plants adopt such standards, they are open to the criticism that they don't adhere to current good practice for management of instrumented protective measures and can't demonstrate that operating risks are being properly managed to an appropriate level.

SIL determination requires care. Any prospective SIL-3 SIF demands reassessment. Process plants rarely require SIF with that high a level. Those claiming the need for SIL-3 SIF face a major challenge demonstrating that SIL-3 performance is achieved by the combination of hardware and human interactions — one that may not stand up well to close scrutiny, either by company stakeholders or external regulatory authorities.

Dr. Alan G. King is a hazard and reliability specialist with ABB Engineering Services, Billingham, U.K. E-mail him at

1. "Functional Safety of Electrical, Electronic and Programmable Electronic Systems," IEC 61508, Intl. Electrotechnical Comm., Geneva, Switz. (1998, 2000).
2. "Functional Safety: Safety Instrumented Systems for the Process Industry Sector," IEC 61511, Intl. Electrotechnical Comm., Geneva, Switz. (2003).
3. "Functional Safety: Safety Instrumented Systems for the Process Industry Sector," ANSI/ISA-84.00.01-2004 (IEC 61511 MOD), Intl. Soc. of Automation, Research Triangle Park, N.C. (2004).
4. King, Alan G., "Inclusion of Human Failure in Risk Assessment," Proceedings, 12th Intl. Loss Prevent. Symp. (Edinburgh, U.K., May 2007), I.Chem.E. Symp. Ser. No. 153, Institution of Chem. Eng's., Rugby, U.K. (2007).
5. King, Alan G., "SIL Determination: Common Cause Concerns," presented at SIL Determination: Principles and Practical Experience seminar (London, Mar. 2006), Institution of Eng. And Tech., Stevenage, U.K.
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