Abstract: The standby automatic switching device combines the system automatic device with relay protection, which is an economic and effective technical measure to provide uninterrupted power supply for users. The standby automatic switching device mainly takes the voltage and current fixed value as the action criterion, and its action reliability also depends on its own fixed value setting and the integrity of the circuit breaker control circuit involved, This paper mainly analyzes an event that the TV air switch trip of 35kV section II bus in a 110kV substation caused the trip of the medium voltage side of the main transformer, and the failure of the standby automatic switch finally caused the loss of voltage of 35kV section II bus. Through the analysis of the operation mode, preliminary information of the event, the action of the standby automatic switch and the failure causes before the event, the corresponding preventive measures are obtained to reduce the occurrence of similar events.