Abstract: This paper introduces the case of single-phase trip of the bus tie circuit breaker on 220 kV side of 500 kV substation due to DC grounding of the three-phase outlet pressure plate during operation. Firstly, the protection action process and the abnormal action is analyzed. And, through the operation time and action voltage of the exit relay are simulated, the causes of single-phase trip are further analyzed, and the improvement measures to prevent accidental touch are put forward, so as to realize the safe and stable operation of the power grid.