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Safety and SIS · Glossary

What is Failure Rate?

Symbol: λfailures per hour or FITSafety and SIS

Failure rate is the number of failures per unit time of a device; in safety it is split into safe and dangerous, detected and undetected parts.

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Failure Rate Explained

The four categories are λSD, λSU, λDD and λDU. Dangerous undetected failures (λDU) dominate PFDavg because only proof tests find them.

Rates are often given in FIT, failures per 10⁹ hours, from certified data, FMEDA reports or site experience.

Failure Rate Formula

λ = λSD + λSU + λDD + λDU   and   MTBF ≈ 1 ÷ λ
S, Dsafe, dangerous
D, U (second letter)detected, undetected
Example: 200 FIT = 2 × 10⁻⁷ per hour.
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Calculators for Failure Rate

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

Free Downloads

Free downloadSIL PFDavg Calculation Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Failure Rate?

Failure rate is the number of failures per unit time of a device; in safety it is split into safe and dangerous, detected and undetected parts.

What is a FIT?

One failure in 10⁹ device hours.

Where do failure rates come from?

Device FMEDA reports, certificates, databases such as OREDA, and your own maintenance records.

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Related Terms

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.Safety termMean Time Between FailuresMean time between failures is the average operating time between failures of a repairable item.

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