Unlike cold standby components, warm standby components fail even during the standby mode. Generally, though, the failure rate during standby is much less than during operation. Assume that the failure rate of the standby mode is λ. Therefore, the system can reach state 2 due to either the failure of the operating unit or the failure of the standby component. Consequently, these failure rates can be added. Figure 8-16 shows the state transition diagram for a warm standby system.

Using the procedures mentioned earlier:

Therefore:

The reliability of an -unit warm standby can be found from equation (8.88) by substituting λ i.=λ+(n-i)λs, where , i=1,...,n. Moreover, reliability of a k-out-of n warm standby system can be found by substituting λ i.=kλ+(n-k+1–i)λs, where , i=1,...,n-k+1, (In this case, the total number of states will be .n-k+1)