Abstract: This article introduces the handling process of an instantaneous fault that occurred in phase A of a 220 kV circuit breaker, and the B and C phases tripped after about 130 ms. Through a thorough investigation and testing of the secondary circuit, the cause of the trip was analyzed, and it was ultimately confirmed that the cause of the trip was due to the three-phase inconsistency of the circuit breaker body. After the relay was powered on, the contact shook and closed, causing the outlet circuit of the three-phase inconsistency of the body to conduct. Corresponding improvement measures have been proposed to provide reference for the analysis and handling of such accidents in the future.