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Availability of a complex system with four subsystems A, B, C, D, in series, has been evaluated. A has 2 i.i.d. units. B, C have several units (not necessarily s-identical). Unit failure rates are constant. Repair (hazard) rate need not be constant. Sensor/switch is imperfect. System fails i.i.f. an A or C unit fails. A failure in B degrades the efficiency of the system but does not cause it to fail. Supplementary variable technique has been used to make the system Markovian. Laplace transform of various time dependent probabilities have been obtained. The time independent numerical computations to evaluate the effect of imperfect/sensor/switch and shelf-life of standby unit on various probability states of the system have been carried out. Derivations are in a separately available Supplement.