SIL rating is a measure of the risk reduction capability and probability of failure-on-demand. It measures only the "Fail Safe" nature of the device, and should not be the primary or sole measurement considered when selecting a safety system.
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"When a SIL Rating is Not Enough" by Robin McCrea-Steele, Invensys - Premier Consulting Services.
Abstract: A common misconception is that the SIL claim limit of a safety PLC is the main, or sometimes the only, criteria for selection in a process safety application. Although it is true that the SIL rating is a measure of the risk reduction capability and probability of failure on demand, it only measures the "Fail Safe" nature of the device. There is no doubt that a plant that is "shut down" is relatively safe. However, apart from the economical / quality / lost production and other negative implications, it is important to consider the higher risks associated with an unplanned shutdown and subsequent re-start of the process. So the question begs, what measures the process up-time capability of a subsystem?