Abstract: In critical power infrastructure such as nuclear power plants,circuit breakers are important electrical equipment that ensures the safe operation of the system,and their operational reliability is crucial.The study focuses on the occurrence of a refusal to disconnect fault in the auxiliary power supply circuit breaker of a certain nuclear power plant.The control circuit and mechanical linkage mechanism were systematically analyzed,and the root cause of the fault was ultimately confirmed to be the continuous compression of the front panel of the circuit breaker by the cabinet door castelock structure during operation,causing panel deformation and interfering with the operation of the energy storage indicator mechanism,resulting in excessive energy storage and mechanical decoupling.Due to the high concealment and accumulation of faults,three structural optimization suggestions are proposed,including the design of interlocking structure distance,energy absorption design of buffer sleeve,and rigid reinforcement scheme of panel,all of which can be implemented during the equipment manufacturing stage.The research results have important reference value for improving the structural safety margin of medium voltage circuit breakers and the risk prevention and control capabilities of nuclear power plant operation and maintenance.