Evaluation of the cumulative failure rate of a system: The optimal interval for preventive replacement of traveling wave tubes

Changes in the cumulative failure rate of a repairable series system according to its total operating time can be determined by using the renewal process. The result obtained by calculating the change in the cumulative failure rate for the lifetime distribution of 10 W power output travelling wave tubes (TWT's), installed in certain types of NHK's television translators with transmission power of 10 watts, is applied to determining the optimal interval for simultaneous preventive replacement of costly parts like the TWT's. It was found that simultaneous preventive replacement is more economical than the individual replacement after failure when the interval of replacement is between two and three years.