Abstract: Due to the impact of time-dependent unit unavailability, the testing strategies (e.g.,
simultaneous or staggered testing) have significant effect on the system unavailability for
complex standby systems. Several testing strategies have been studied to minimize average
system unavailability for certain systems. For example, uniformly staggered testing has been
shown to be the best for two identical units in parallel with and without common cause failure
between units. However, this result may not be suitable for parallel systems with nonidentical
units. This study provides a discussion to obtain the optimal testing strategy for parallel systems
for the case in which units are not necessary to be identical. Moreover, a maintenance manager
who is aware of the importance of each system in the plant should establish a testing schedule
based on plant-level, not on system- or component-level. That is, the manager should set up
testing schedule for each system based on its contribution to the plant. Therefore, a good testing
schedule should consider the balance between the maintenance cost and risk (or unavailability)
coming with the maintenance task for each system. This study provides a cost-effective model
taking both cost and risk into account to establish a good testing policy (including testing
strategy and test interval) for two-unit parallel systems.