PT Notes
Common Cause Failure in SIL Determination
PTNotes is a series of topical technical notes provided periodically by Primatech for your benefit. Please feel free to provide feedback.
The IEC 61511 / ISA 84 standard requires that safety integrity levels (SILs) be determined for safety instrumented functions (SIFs) that make up safety instrumented systems (SISs). Many practitioners perform such determinations using risk analysis techniques such as layers of protection analysis (LOPA) with hazard scenarios taken from process hazard analysis studies. The risk of each scenario is evaluated.
Common cause failures (CCFs) in which simultaneous (or near-simultaneous) multiple failures result from a single shared cause impact the evaluation of risk. A fundamental assumption in using LOPA is that safeguards credited with risk reduction be independent of one another and of other elements in the scenarios they protect (they must be Independent Protection Layers (IPLs)). CCFs invalidate many safeguards as IPLs but, unfortunately, novice LOPA teams do not always recognize the potential for CCFs and, therefore, underestimate scenario risk and judge it to be tolerable when it is not.
CCFs can be subtle and even experienced analysts may miss them. Some possible CCF causes such as common locations and common people can be difficult to eliminate which exacerbates the problem. Other sources of CCF may go unrecognized or be dismissed as incredible. Requirements for preventing CCFs are addressed in IEC 61511 / ISA 84. They are important for both SIS and non-SIS safety functions and must be addressed in SIL determination by careful consideration of possible sources.
For more information, you can contact Primatech or consult the article:
The Interface of Functional Safety with Process Safety and Risk Analysis, by Paul Baybutt, Process Safety Progress, Volume 32, Issue 4, pages 346–350, December 2013.
The article is available at: http://onlinelibrary.wiley.com/doi/10.1002/prs.11640/abstract.
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