Fractional Dead Time (FDT)

A probability representing the mean proportion of the total relevant time that a component, equipment, or system, is incapable of providing protection. Also called average unavailability or average probability of being in a failed state. Dimensionless.

The mean fraction of time in which a component or system is unable to operate on demand. (Also know as Unavailability.)

The measure of the fractional time that the system is likely to be in a non-working state.