Abstract: Relay protection plays an extremely important role in the safe and stable operation of power grid. Minor errors in the maintenance of the secondary system may have a very serious impact on the operation of the power system. This paper describes a fault case at the low voltage side of 110 kV main transformer, objectively expounds the causes of the accident through message, waveform and theoretical demonstration, deeply analyzes the abnormal action of 10 kV backup automatic switching in the fault, and puts forward that the logic input of backup automatic switching device should match the output of operation box. While eliminating defects, it is also necessary to check the substations built in the same period, strengthen drawing management, strengthen the training of technicians of the construction unit and other rectification schemes.