Meaning – The term ‘mean time between failures’, refers to the mean value of the lengths of time between consecutive failures under stated conditions.
The term is used for repairable systems, while mean time to failure (MTTF) denotes the expected time to failure for a non-repairable system.
The definition of MTBF depends on the definition of what is considered a failure. For complex, repairable systems, failures are considered to be those out of design conditions that place the system out of service and into a state for repair. Failures that occur that can be left or maintained in an unrepaired condition, and do not place the system out of service, are not considered failures under this definition. In addition, units that are taken down for routine scheduled maintenance or inventory control are not considered within the definition of failure
Example of usage – “Mean time between failures (MTBF) describes the expected time between two failures for a repairable system.”