MTBF stands for mean time between failures: the average time a repairable asset runs between one failure and the next. It is calculated by dividing the total operating time in a period by the number of failures in that period. A higher MTBF means the asset fails less often.
The MTBF formula
MTBF = total operating time ÷ number of failures. Only unplanned failures count. Planned shutdowns for servicing are not failures, and the time the asset spends broken down is not operating time.
MTBF applies to repairable assets, which covers most building plant. For items that are replaced rather than repaired, such as lamps or filters, the equivalent measure is MTTF, mean time to failure.
A worked example
Take three chilled water pumps serving a mall over a quarter. Each was scheduled to run 2,000 hours, a total of 6,000 hours. Between them they had four breakdowns, which kept them out of service for 40 hours in total. Operating time is therefore 6,000 − 40 = 5,960 hours, and MTBF is 5,960 ÷ 4 = 1,490 hours.
If one pump caused three of the four failures, the fleet average hides a problem asset. That is why MTBF is most useful per asset, or per asset type, rather than as one figure for the whole building.
MTBF, MTTR and availability
MTBF says how often an asset fails. MTTR says how long each repair takes. Together they give availability: availability = MTBF ÷ (MTBF + MTTR). Using the pumps above, with an average repair of 10 hours (40 hours ÷ 4 failures), availability is 1,490 ÷ 1,500, or about 99.3%.
Raising MTBF is the job of good maintenance strategy: the right planned preventive maintenance, predictive maintenance on critical plant, and root-cause fixes for repeat failures. Lowering MTTR is the job of good response: triage, spares and technician information.
What you need to measure it
- Every failure logged as a work order against the specific asset, not against a floor or a building
- Planned and reactive work kept apart, so servicing does not count as a failure
- Run hours from the BMS or meters where available; scheduled hours as a fallback, with that assumption stated
- At least several months of history before comparing assets, because small numbers of failures make MTBF swing widely
Common questions
What is the full form of MTBF?
MTBF stands for Mean Time Between Failures. It is the average operating time between one failure of a repairable asset and the next.
How do you calculate MTBF?
Divide the total operating time in a period by the number of failures in that period. For example, 5,960 operating hours with 4 failures gives an MTBF of 1,490 hours. Exclude planned shutdowns and downtime from operating time.
What is the difference between MTBF and MTTR?
MTBF measures how long an asset runs between failures; MTTR measures how long it takes to repair it after a failure. You want MTBF high and MTTR low. Together they determine availability.
Is a higher MTBF always better?
For the same asset and duty, yes, because it means fewer failures. But comparing MTBF across different asset types, or between a heavily used and a lightly used asset, can mislead. Compare like with like, over the same period, using the same definition of a failure.