Abstract: During the major overhaul of the No. 2 main transformer at a certain 110kV substation, after a trip of a certain 10kV line occurred, the No.3 main transformer low backup protection continued to operate, sequentially tripping the low-voltage side sectionalising circuit breaker 930 and the low-voltage side circuit breaker #903, causing the 10kVII bus and 10kVIII bus to lose power, resulting in a load loss of 11,200 kW. To determine the cause of the fault, other 10kV line protection devices were first checked for any alarms or operations, followed by verification of the correct input of main transformer protection settings, and finally analysis of the 10kV line protection and main transformer backup protection operations as well as fault waveform recordings. It was ultimately concluded that the fault was caused by two 10kV lines installed on the same pole experiencing consecutive faults, with the fault duration exceeding the time setting of the No. 3 main transformer low backup protection, leading to the transformer tripping.