Operations, Maintenance and Reliability
What this domain tests
Operations, Maintenance and Reliability tests reliability metrics, selection of maintenance strategy, and the analysis methods that drive both.
Core rules
- Availability A = MTBF / (MTBF + MTTR).
- For a constant failure rate, reliability R(t) = e^(−λt) with λ = 1/MTBF, so only about 37% of items survive to the MTBF - it is a distribution mean, not a service life.
- Strategies: reactive (run-to-failure, valid for cheap non-critical items), preventive (time-based), predictive/condition-based (vibration analysis, oil analysis, thermography), selected by failure consequence under reliability-centred maintenance.
- FMEA scores RPN = severity × occurrence × detection; act on high-severity items regardless of RPN.
- OEE = availability × performance × quality.
- The bathtub curve runs infant mortality, useful life, then wear-out.
Worked example
A pump has MTBF 450 h and MTTR 50 h, so A = 450/500 = 90%. Adding condition monitoring that cuts MTTR to 25 h raises availability to 450/475 = 94.7%. Alternatively, improving MTBF to 900 h with the original MTTR gives 900/950 = 94.7% - identical. Both routes reach the same availability, so select on cost, spares strategy and criticality rather than on preference.
Traps
Treating MTBF as the expected life of an individual item; applying RPN league tables as absolute rules; time-based replacement of items with no wear-out mode; and counting planned stoppages as failures when calculating OEE availability.