Consider a reliability system consisting of n components. The failures
and the repair completions of the components can occur only at positi
ve integer-valued times k is-an-element-of N++ = {1, 2 ...}. At any ti
me k is-an-element-of N++ each component can be in one of two states:
up (i.e., working) or down (i.e., failed and in repair). The system st
ate is also either up or down and it depends on the states of the comp
onents through a coherent structure function tau. In this article we f
ormulate mathematically the above model and we derive some of its prop
erties. In particular, we identify conditions under which the first fa
ilure times of two such systems can be stochastically ordered. A varie
ty of special cases is used in order to illustrate the applications of
the derived properties of the model. Some instances in which the time
s of first failure have the NBU (new better than used) property are po
inted out. (C) 1993 John Wiley & Sons, Inc.