Abstract: This article takes the example of a 10 kV outgoing line interval circuit breaker refusing to operate on the 10 kV I section busbar of a 110 kV substation and the subsequent operation of the 10 kV station grounding transformer protection on the same section busbar as the basis. Based on the on-site monitoring message of the substation and the action record of the protection device, as well as the disassembly analysis of the circuit breaker equipment, the logical sequence of each protection device's action outlet during the fault period is analyzed in detail, and the decomposition situation of the 10 kV circuit breaker is discussed in depth. Optimization measures are proposed to promote the reliable and stable operation of the power grid system by integrating various slow opening situations of circuit breakers.