Maintenance Glossary Global
MTTR for mining equipment: measuring repair time properly
MTTR is total repair time divided by the number of repairs. Which clock to start and stop on a mine breakdown, with a worked example.
Definition
MTTR
MTTR (mean time to repair, or mean time to restoration) is the average time taken to return a failed machine to working condition, calculated as total repair or restoration time in a period divided by the number of repairs in that period.
Contents 8 sections
The R can mean repair or restoration: in the illustrative example below, one steering pump failure takes 4.25 hours of repair and 9.83 hours of restoration. A site has to say which clock starts and which clock stops before its MTTR can be compared with anything.
MTTR formula
MTTR (hours) = total repair time in the period ÷ number of repairs in the period
The divisor is the count of corrective repairs, one per failure. Planned services are left out, as they are for MTBF.
When the MTTR clock starts and stops
A breakdown on a mine passes through several stages before the machine is loading or hauling again. Each stage has its own start and stop time.
| Stage | Starts | Stops | In active repair time | In restoration time |
|---|---|---|---|---|
| Waiting for a mechanic | Machine stops, fault reported | Mechanic at the machine | No | Yes |
| Diagnosis | Mechanic at the machine | Fault found | Yes | Yes |
| Waiting for parts | Part requested | Part on site | No | Yes |
| Repair | Work starts | Work complete | Yes | Yes |
| Test | Repair complete | Functional check passed | Yes | Yes |
| Waiting for release | Job card signed off | Released to operations | No | Yes |
ISO 14224 defines active maintenance time as the duration of a maintenance action “excluding logistic delay”, and says down time “includes all the delays between the item failure and the restoration of its service” (ISO 14224:2016, 3.1 and 3.16). The US Department of Defense reliability guide lists what MTTR includes: diagnostic time, time to repair and “time required to validate the repair”. Waiting for parts, personnel and administrative delays sit in mean downtime (DoD RAM guide, 2005, Table 3-4).
The MTTR ambiguity
The same four letters stand for different measurements. Selvik and Ford, reviewing downtime terms in ISO 14224 and related standards, found that engineers are “still divided” between mean time to restoration and mean time to repair. They recommend the ISO 14224 abbreviation MTTRes for restoration as “less ambiguous” (Selvik and Ford, 2017). Restoration time, in their breakdown, has four parts: fault detection, preparation and delays, active repair, and delays after repair.
Active repair time measures the workshop’s work on the machine. Restoration time measures what the site lost, including every wait.
Worked example: one haul truck, five breakdowns
MTTR, MTBF and inherent availability
A = MTBF ÷ (MTBF + MTTR)
The DoD guide calls this inherent availability. Selvik and Ford write it with mean time to failure and mean time to restoration: MTTF ÷ (MTTF + MTTRes). In the example:
- With active repair time: 84 ÷ (84 + 3.10) = 96.4%.
- With restoration time: 84 ÷ (84 + 7.77) = 91.5%.
The restoration figure matches what the site lost: 420 operating hours against 38.83 hours of breakdown downtime is also 91.5%. Active repair time shows what the truck would achieve if parts and people were always waiting. Response, parts and release time account for the gap of about five percentage points. For how these figures sit beside mechanical and physical availability, see availability and utilisation.
Mean or median repair time
One overnight wait for an alternator made up 20 of the 38.83 restoration hours. The median restoration time for the five breakdowns is 5.0 hours, against a mean of 7.77. Report the mean for availability, because hours lost add up. Look at the median and the longest stops to see whether the problem is every repair or a few parts delays.
Every figure here depends on the times written on the job card. The Global Mining Guidelines Group says maintenance management and work order systems would capture downtime reasons and failure causes more effectively than its time usage model (GMG, 2020).
Common questions
Does MTTR include waiting for parts?
Mean time to repair, in the narrow sense, does not: it counts diagnosis, repair and testing. Mean time to restoration does, because it runs from the failure until the machine is back in service. State which one a report uses.
When does the MTTR clock start on a mine site?
For restoration time, when the machine stops or the operator reports the fault. For active repair time, when the mechanic starts diagnosis. Record both times on the job card.
How do MTTR and MTBF give availability?
Inherent availability is MTBF divided by the sum of MTBF and MTTR. In the US DoD definition MTTR is active repair time, so the result comes out higher than the availability a site measures from its downtime record.
Sources
- Jon T. Selvik and Eric P. Ford (2017), Down Time Terms and Information Used for Assessment of Equipment Reliability and Maintenance Performance, IntechOpen, read 25 Sep 2026.
- ISO 14224:2016, clause 3 terms and definitions (preview), ISO, read 25 Sep 2026.
- DoD Guide for Achieving Reliability, Availability, and Maintainability (2005), Table 3-4, US Department of Defense, read 25 Sep 2026.
- A Standardized Time Classification Framework for Mobile Equipment in Surface Mining (2020), Global Mining Guidelines Group, read 25 Sep 2026.
