A Safety Instrumented System SIS consists of one or more Safety Instrumented Functions SIF.
Safety Instrumented System
A Safety Instrumented Function SIF is defined as a „Function to be implemented by a SIS, which is intended to achieve or maintain a safety state for the process with respect to a specific hazardous event"
Safety Instrumented Function
Safety Integrity Level SIL is a measure of risk reduction provided by a SIF based on four levels. Each level represents an order of magnitude of risk reduction. Every SIF has a SIL assigned to it, the SIS and equipment does not have a SIL assigned to it.
SIL Safety Integrity Level | RRF Risk Reduction Factor | PFD Probability of Failure on Demand pro year = 1/RRF |
SIL 4 | 100,000 to 10,000 | >= 10-5 to < 10-4 |
SIL 3 | 10,000 to 1,000 | >= 10-4 to < 10-3 |
SIL 2 | 1,000 to 100 | >= 10-3 to < 10-2 |
SIL 1 | 100 to 10 | >= 10-2 to < 10-1 |
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