Calculations of the time dependent unavailability of monitored components

The integral equation for the time dependent unavailability of a monitored component, which has a failure intensity of the Weibull form, has been solved numerically and an approximate solution has also been derived. The unavailability of a complex system involves two aspects, the logical consequences of the arrangement of the components, which may be depicted as a fault tree, and also the failure characteristics of the individual components. The unavailability of a component, and hence a system, is a stochastic function of time. The instantaneous unavailability may differ considerably from the average unavailability. In the code FRANTIC II, the instantaneous unavailability of a whole system is calculated. This paper describes the time dependent unavailability of a monitored component and compares the results with the approximation used in FRANTIC II.