Protection system is the first defense line to ensure the safe operation of power grid, so it is of great significance to study its reliability. Currently, the reliability research of protection system mainly focuses on misoperation and fail-to-operation, yet it does not effectively take advantage of defect information which would occur in great probability. This paper analyzes defect information of protection devices and estimates the parameters of two-parameter Weibull distribution as well as the reliability function, failure rate function and so on. The study case shows that it can supply reference for further reliability assessment of protection systems by calculating the parameters of Weibull distribution and analyzing indices such as reliability, failure rate, and mean time between failures.