Forced outage

In electrical engineering, forced outage is the shutdown condition of a power station, transmission line or distribution line when the generating unit is unavailable to produce power due to unexpected breakdown.
Forced outage can be caused by equipment failures, disruption in the power plant fuel supply chain, operator error etc.

Forced outage rate

Forced outage rate of a power station unit is the probability that the unit will not be available for service when required.
FOR is defined as the number of hours the unit is on forced outage over the total number of hours in a year.
Two derivative metrics are also used:

equivalent forced outage rate has a different denominator: instead of the total number of hours in the year, only the hours when the unit was needed for load are counted.
weighted EFOR is EFOR weighted by the capacity of the unit.