A two-dissimilar unit system including a hot standby unit is undertaken for the study. The units fail in two phases partially first and then totally. There is a single repair person who starts repair immediately upon failure. The partially failed unit under repair is subjected to failure while undergoing repair. Repair of the unit is done at its partial and complete failures. On complete failure of the system, a substitute system is connected. The system is repairable and resumes the operation after getting repaired.  The distributions of failure and repair times of the units follow negative exponential with different parameters. A particular case of the system model, with fixed values of the parameters is considered to compare availability of the model having substitute system and without substitute system.