Which term defines the long-term average fraction of time a component or system performs satisfactorily?

The term that defines the long-term average fraction of time a component or system performs satisfactorily is availability. Availability reflects not only the operational state of a system but also takes into account both any downtime due to repairs or maintenance and the system’s ability to operate effectively when needed.

In the context of power substations, availability is crucial as it directly influences the continuity of service. It is assessed over a period, typically involving the operational duration compared to the total time, and it indicates the readiness of the system to perform its intended function without interruptions.

This concept is distinct from reliability, which focuses on the probability of a component performing its required functions without failure over a specified period. While performance rate assesses how well a system meets its performance criteria at any given moment, operational efficiency looks at how effectively resources are used in producing output. Hence, availability encompasses a broader view relevant to long-term operational dependability.