Maintenance · Updated 3 October 2026
MTBF and MTTR: prepare a consistent breakdown log
Define operating time, failure count and repair time consistently before calculating equipment reliability and availability.
Define the equipment and reporting window
MTBF summarizes operating time per failure for repairable equipment. MTTR summarizes repair time per failure. Use the same equipment boundary and reporting period for both. A line-level stop count and a motor-level operating-hour total do not describe the same population.
MTBF = total operating hours ÷ failures. MTTR = total repair hours ÷ failures. This site's simplified inherent availability is operating hours ÷ (operating hours + repair hours). It excludes planned downtime and other delays.
Set up the blank downtime workbook
- Give every breakdown a unique Event ID and the correct Asset ID.
- Enter full start and restoration date-times, including the dates for overnight events.
- Check the calculated downtime hours. An earlier restoration time produces a check message rather than a negative downtime.
- Separate the observed symptom from the confirmed cause.
- Assign a follow-up owner and due date when further work remains.
The workbook dashboard shows ongoing and restored records. A restored record means that a restoration time is entered; it does not prove that the root cause has been eliminated.
Decide what repair time includes
Document whether your MTTR includes only active repair or also waiting, diagnosis and restart checks. The elapsed downtime field may include more than active repair. Do not use it as repair time without checking that your agreed definition matches.
Handle zero failures honestly
With no recorded failures, this failure-count calculation cannot provide a finite observed MTBF. Retain the exposure hours and report zero failures instead of inventing an MTBF. A historical average also does not guarantee when the next breakdown will occur.