Mean Time Between Failures (MTBF)

Mean time between failures is the average operating time between consecutive failures of a repairable item, calculated as total operating time divided by the number of failures in that time, and used as a basic measure of reliability.

MTBF applies to repairable equipment that is restored to service after each failure. If a machine operates for 2,000 hours in a period and fails four times, its MTBF for that period is 500 hours. Only operating time is counted, not repair or idle time. When the failure rate is constant, as assumed in the exponential distribution, MTBF is the reciprocal of the failure rate, and the probability of surviving a period equal to the MTBF is only about 37 per cent.

MTBF is used in maintenance management to track equipment reliability over time, compare similar assets, evaluate the effect of improvement work and calculate availability together with mean time to repair. Equipment manufacturers quote MTBF figures for components such as power supplies, drives and electronic modules, often predicted from component failure rate models or derived from testing.

MTBF is an average, not a guaranteed failure-free period, and it says nothing about when an individual failure will occur. Comparisons are meaningful only if failures and operating time are defined and recorded consistently, for example whether minor stops count as failures. Manufacturer figures depend on assumed conditions such as temperature and duty, and may differ from field experience. For non-repairable items, mean time to failure is used instead. IEC 60050-192 defines dependability terms, including mean operating time between failures.

Key points

MTBF equals total operating time divided by the number of failures.
Applies to repairable items; MTTF is used for non-repairable ones.
With a constant failure rate, MTBF is the reciprocal of the failure rate.
An average, not a guaranteed failure-free period.

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Mean Time Between Failures (MTBF)

Key points

Where AiVibe comes in

Related terms

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