Average Probability of Failure on Demand PFDavg
average unavailability as per 3.1.12 in the functional safety standard terminology (e.g. IEC 61508)
Note 1 to entry: “Failure on demand” means here “failure likely to be observed when a demand occurs”. PFDavg
encompasses both the failure occurred before the demand and the failure occurring due to the demand itself.
Then this term needs not to be mixed up with the probability of a failure due to a demand (see 3.2.13).
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global Standards