Mean Time Between Failures

Expectation of the time between failures. NOTE The MTBF of an item can be longer or shorter than the design life of the system.

Source: ISO 20815:2008, Petroleum, petrochemical and natural gas industries – Production assurance and reliability management. Global Standards


Mean Time Between Failures


expected time between successive failures of a repairable item

Note 1 to entry: In the cases illustrated in Figure 4, the MTBF is linked with MTTF and MTTRes by the following

relationship: MTBF = MTTF+MTTRes. More generally It is also linked to the MUT and MDT by MTBF = MUT+MDT.

Note 2 to entry: The acronym MTBF is sometimes defined as the mean operating time between failures (e.g. in

IEV191[14]). This is not at all the same and, in this case, the formula described in Note 1 to entry is no longer valid.

This is very confusing, therefore the traditional definition of the MTBF is retained in this Technical Report.


Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards

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