To reduce the system down time, failed spare units can replace units in repairable systems with redundancy Repairmen or manufacturer attends the failed units for corrective maintenance. Failed units are returned for re – use. In this chapter, we consider a series system with spares and we assume that repaired units are as good as new. Since, preventive maintenance will take less time and tends to be cheaper. It can be profitable to perform preventive maintenance in order to return it to its as good as new state in the situation of increasing failure rates. In this mode, we use age – replacement policy for machines If the age or a machine has reached a certain value Mpm, it is taken out for preventive maintenance and replaced by a spare unit. Here we derive an approximation scheme to calculate the expected uptime, the expected downtime and the expected costs per time unit of the system given the total number or units.

Keywords: Preventive Maintenance, Replacement Policy.

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