Safety and SIS · Glossary

What is a Safety Instrumented System?

Symbol: SISsensors, logic solver, final elementsSafety and SIS

A safety instrumented system is an independent set of sensors, logic solver and final elements that takes a process to a safe state on a hazard.

Safety Instrumented System Explained

A SIS is separate from the basic process control system, so a control failure cannot also disable the protection. It carries one or more safety instrumented functions.

In the process industry SIS design, operation and testing follow IEC 61511 (ANSI/ISA 61511 in the US).

Calculators for Safety Instrumented System

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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.

Frequently Asked Questions

What is a Safety Instrumented System?

A safety instrumented system is an independent set of sensors, logic solver and final elements that takes a process to a safe state on a hazard.

Is an ESD system a SIS?

Usually yes. Emergency shutdown, burner management and HIPPS are common types of SIS.

Can the DCS do safety functions?

IEC 61511 limits how much risk reduction the BPCS can claim; separate SIS hardware is normally used for SIL rated functions.

Learn more on the blog

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on safety instrumented system 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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