Operational Availability due to maintenance

The Operational Availability due to maintenance considering operating time and Downtime attributed to maintenance is defined as

  • ratio, during a given period, of the operating time (OT) to the sum of operating time and the downtime (OT+DT).

This ratio, measures, during a given period of time, the percentage of time during which an item performed when required excluding when it didn’t perform due to other reasons than maintenance or faults.

The total downtime (DT) includes the DT during required time and DT during Non-Required time.

DT during RT – Downtime during Required Time: is the time from the instant of failure until restoration

DT during NRT – Downtime during Non-Required Time: is the time spent for preventive maintenance



The EN15341 standard calls the PHA8 indicator maintenance-related availability, and when quantifying it, it considers the operating time and the time components spent on preventive maintenance and fault repairs, i.e., time to restoration (TTR).

The evolution of the PHA8 indicator over time characterizes the downtime attributable to maintenance and repair interventions. The examination point of view focuses exclusively on the time required for the maintenance and restoration interventions and does not consider the standby time, the Idle time and downtime due to external causes. When examining the availability of an item (device, equipment, production line) characterized by the PHA8 indicator, during the analysis it is advisable to compare the values with the previous ones and, in case of a significant increase, to investigate the root causes in order to clarify whether the deviation is caused by preventive or corrective interventions.

The indicator may also be suitable for measuring the efficiency of the maintenance activity and/or the effectiveness of the maintenance organization.