This paper discusses a mal-function of a typical 500kV Breaker failure protection detailedly. The reason is that the optical coupler’s inputting circuit is defective. When DC power supply is grounded

the optical coupler can be put through by distributed capacitance

which results in the tripping of the breaker failure protection. We put forward some effectual measures to prevent the mal-function according to the conclusion of this paper. The measures are applied universally in North China Power Systems and their rationality and validity have been proved in the test and practice. We sincerely hope to provide valuable reference for our counterparts of electric power system.