Safety and SIS · Glossary

What is a Safety Instrumented Function?

Symbol: SIFone protective functionSafety and SIS

A safety instrumented function is one protective action of a SIS, from sensing a hazard to the final element reaching the safe state.

Safety Instrumented Function Explained

Each SIF has its own sensors, logic and final elements, for example high pressure on a vessel closing the inlet valve. Each SIF gets its own SIL target.

A SIS may contain many SIFs sharing the same logic solver.

Calculators for Safety Instrumented Function

CalculatorPFDavg CalculatorFree online calculator with formula and worked example.CalculatorLOPA SIL CalculatorFree online calculator with formula and worked example.CalculatorRisk Reduction Factor CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadCause and Effect Matrix Template in ExcelFree download in Excel and PDF.Free downloadSIL PFDavg Calculation Excel SheetFree download in Excel and PDF.Free downloadSIL Proof Test Record Template in ExcelFree download in Excel and PDF.

Frequently Asked Questions

What is a Safety Instrumented Function?

A safety instrumented function is one protective action of a SIS, from sensing a hazard to the final element reaching the safe state.

Is SIL given to the SIS or the SIF?

To each SIF. Saying a SIS is SIL 2 is shorthand for its SIFs meeting SIL 2.

How is a SIF documented?

In the safety requirements specification, with its trip point, response time, safe state and test interval.

Learn more on the blog

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on safety instrumented function and how it is applied on real plants.

Read Full Article

More Safety and SIS Terms

Safety termPFDavgPFDavg is the average probability that a safety function fails to act when a demand occurs, over the…Safety termPFHPFH is the average frequency of dangerous failures per hour, used for safety functions in high demand or…Safety termRisk Reduction FactorRisk reduction factor is how many times a safety function reduces the frequency of a hazardous event; it…Safety termFailure RateFailure rate is the number of failures per unit time of a device; in safety it is split into safe and…Safety termSafe Failure FractionSafe failure fraction is the share of all failures that are either safe or dangerous but detected.Safety termDiagnostic CoverageDiagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.

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