Note on the (N, T) Replacement Rule

A model is presented for analyzing (N, T) replacement policies. This rule consists of a T Age Replacement Policy, combined with replacement after N failures, whichever comes first. Using renewal arguments, an optimal (N, T) policy is established. The model applies to situations where repair and/or replacement costs depend on the number of failures. An example illustrates the procedure with a Poisson process. Under special circumstances, the (N, T) maintenance model reduces to simpler forms. This model can be extended to handle (N, T) replacements of k-out-of-N reliability systems, for example.