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Safety Instrumented Systems
What is a Safety Instrumented System?

A Safety Instrumented System SIS is a new term used in standards like IEC 61511 or IEC 61508 for what used to be called Emergency Shutdown System ESD, Safety Shutdown System, Interlock System, Permissive Systems, etc. ...

A Safety Instrumented System SIS consists of one or more Safety Instrumented Functions SIF.



























What is a Safety Instrumented Function SIF?

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"



















What is a Safety Integrity Level SIL?

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.
Safety Instrumented Systems
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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