Safety and SIS · Glossary

What is Mean Time Between Failures?

Symbol: MTBFhours or yearsSafety and SIS

Mean time between failures is the average operating time between failures of a repairable item.

Mean Time Between Failures Explained

For a constant failure rate MTBF is the inverse of λ. It is an average for a population, not a lifetime: an MTBF of 100 years does not mean one unit lasts 100 years.

MTTR, the mean time to repair, is used with MTBF to calculate availability.

Mean Time Between Failures Formula

MTBF = 1 ÷ λ and A = MTBF ÷ (MTBF + MTTR)

λ	failure rate
MTTR	mean time to repair
A	availability

Example: λ = 2 × 10⁻⁶ /h: MTBF = 500,000 h, about 57 years.

Calculators for Mean Time Between Failures

CalculatorMTBF CalculatorFree online calculator with formula and worked example.CalculatorAvailability CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Mean Time Between Failures?

Mean time between failures is the average operating time between failures of a repairable item.

Why do devices fail much sooner than their MTBF?

MTBF is a population average; with 1000 devices, about 1000 ÷ MTBF fail each hour on average.

Is MTTF the same as MTBF?

MTTF is used for non repairable items; for small repair times the values are close.

Learn more on the blog

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on mean time between failures and how it is applied on real plants.

Read Full Article

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