Further Comments on "The Reliability of Periodically Repaired n –l/n Parallel Redundant Systems"

In the original correspondence,1 Cantarella assumed a system consisting ot n parallel identical elements, each with a constant failure rate A. The system is repaired every T hours. An approximation n(n -1) A2T/2 for the average failure rate was derived. In further correspondence, 2 Nilsson derived an exact ciosed form expression for the failure rate. In this comment, we correct an algebraic error in Nilsson's approximation for small values of A. T, and also point out the relevance of both correspondences to the case where n is small, as in a triple modular redundant (TMR) system.