Abstract: The paper deals with cost benefit analysis of a single unit (main unit) system model supported by a helping unit. Main unit has three possible modes – normal (N), partial failure (P) and total failure (F). Whereas the helping unit (protective unit) has two possible modes- normal (N) and total failure (F). The main unit can also work without the protective (helping) unit but with increased failure rate. The system is declared as failed when at least main unit is out of functioning. A single repairman is always available with the system to repair the failed main and protective (helping) unit. The failure and repair times of main unit and protective (helping) unit are taken as independent random variables of discrete nature having geometric distributions with different parameters.