Consider a reliability system consisting of n components. The failures and the repair completions of the components can occur only at positive integer-valued times k∈NÅ+Å+∈∈1,2,...∈. At any time k∈NÅ+Å+ each component can be in one of two states: up (i.e., working) or down (i.e., failed and in repair). The system state is also either up or down and it depends on the states of the components through a coherent structure function τ. In this article the authors formulate mathematically the above model and they derive some of its properties. In particular, the authors identify conditions under which the first failure times of two such systems can be stochastically ordered. A variety of special cases is used in order to illustrate the applications of the derived properties of the model. Some instances in which the times of first failure have the NBU (new better than used) property are pointed out.
