Reliability of Intermittently Used Systems

The reliability of 3 models is investigated. The system is not down unless it is failed when there is a demand. Model 1 is a 2-unit parallel system. Model 2 is similar to model 1 except that if there are 2 users, the system is used as two 1-unit systems, one for each user. Model 3 consists of two model 2 systems in series.