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Availability

A = MTBF / (MTBF + MTTR) - the fraction of calendar time the equipment is capable of operating.

MaintenanceReliability
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Formula, assumptions, and limits

A=MTBFMTBF+MTTRA = \frac{\text{MTBF}}{\text{MTBF} + \text{MTTR}}
SymbolMeaningUnit
MTBFMTBFMean Time Between Failuresh
MTTRMTTRMean Time To Repairh
AAAvailability

Source: IEC 60300 - Dependability management

In short

A = MTBF / (MTBF + MTTR) - the fraction of calendar time the equipment is capable of operating.

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