STD-781, Repairable system, Exponen-xi local time since failure i-1, i = 1, 2, ..; x0o 0. tial distribution, Homogeneous Poisson process, Force of mor-hx(x) force of mortality of time to failure. tality, "Failure rate" hxL(xi) force of mortality of ith time between successive failures, i = 1, 2, 3, Reader AIds N(t) number of failures in (0, tl. Purpose: Tutorial E{N(t)# s-expected (mean) number of failures in (0, t]. Special math needed for explanations: Probability v(t) rate of occurrence of failures at time t.