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Safety and SIS · Glossary

What is Safe Failure Fraction?

Symbol: SFF%Safety and SIS

Safe failure fraction is the share of all failures that are either safe or dangerous but detected.

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Safe Failure Fraction Explained

A high SFF means most failures either trip safely or are caught by diagnostics. IEC 61508 uses SFF with hardware fault tolerance to limit the SIL a subsystem can claim.

IEC 61511 Edition 2 relies mainly on hardware fault tolerance and prior use instead.

Safe Failure Fraction Formula

SFF = (λS + λDD) ÷ λtotal
λSsafe failure rate
λDDdangerous detected rate
λtotaltotal failure rate
Example: λS = 300, λDD = 600, λDU = 100 FIT: SFF = 900 ÷ 1000 = 90 %.
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Calculators for Safe Failure Fraction

CalculatorSafe Failure Fraction CalculatorFree online calculator with formula and worked example.CalculatorPFDavg CalculatorFree online calculator with formula and worked example.

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Frequently Asked Questions

What is Safe Failure Fraction?

Safe failure fraction is the share of all failures that are either safe or dangerous but detected.

Is high SFF always good?

It shows good diagnostics, but many safe failures also mean more spurious trips.

Where do I find SFF?

In the device safety manual or FMEDA report.

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Related Terms

Safety termHardware Fault ToleranceHardware fault tolerance is the number of hardware faults a subsystem can have while still performing its…Safety termDiagnostic CoverageDiagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.Safety termFailure RateFailure rate is the number of failures per unit time of a device; in safety it is split into safe and…

More Safety and SIS Terms

Safety termVoting ArchitectureVoting architecture describes how many of N redundant channels must act to trip, such as 1oo2 or 2oo3.Safety termCommon Cause FailureCommon cause failure is a single cause that makes redundant channels fail together, such as a shared…Safety termProof Test IntervalProof test interval is the time between periodic tests that reveal dangerous undetected failures in a…Safety termProof Test CoverageProof test coverage is the fraction of dangerous undetected failures that a proof test actually reveals.Safety termSpurious TripA spurious trip is an unwanted shutdown caused by a safety system fault rather than a real process hazard.Safety termMean Time Between FailuresMean time between failures is the average operating time between failures of a repairable item.

All safety and sis terms · Full glossary A to Z

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