Digital relay protection devices play an important role in ensuring the safe and stable operation of power grids. It is necessary to evaluate the reliability and life of digital relay protection devices. According to the structural characteristics of the digital relay protection device, the digital relay protection device is a conditionally repairable system. In order to analyze the characteristics of the life expectancy of the conditionally repairable system, this paper describes the differences among repairable systems, unrepairable systems and conditional repairable systems. Then the mathematical model of the life expectancy of the conditionally repairable system is derived based on the dynamic of the reliability function. Finally, in the case study, the life expectancy is calculated for the cases where fault distribution of the device is exponentially distributed or Weibull distributed, respectively, and the characteristics of the life expectancy in these two cases are analyzed.

Relay Protection Device,Conditional Repairable System,Exponential Distribution,Weibull Distribution,Life Expectancy Characteristics,exponential distribution