The rate of transition from the down state of the two component system is μ1+μ2 and this must be equivalent to μp, i.e.
This value of Rp represents the average period of time during which both components are concurrently out of service, i.e., it represents the period during which the failures overlap. For this reason Rp is generally known as the overlapping repair or outage time of components 1 and 2. The failure event caused by the failure of components 1 and 2 is generally known as overlapping failure event.
Substituting Equation 11.15 into 11.14 givens
If, as is frequently the case in practice, (λ1r1) and (λ2r2) are much less than unity, then
Finally, as in Section 11.2
It should be noted that the three important points made at the end of Section 11.2 are equally applicable in the case of parallel system.
Example 11.2
A system consists of two components, one of which must operate for system success. If the failure rates are 0.05 f/yr and 0.02 f/yr respectively and the average repair times are 0 hr and 25 hr respectively, evaluate the system failure rate, average repair time and unavailability.
From Equations 11.15, 11.17 and 11.18
(where 8760 is the number of hours in a year)