Among all kinds of power substation faults across various voltage levels, the low-voltage side fault of the main transformer is a common typical one. This fault is often dependent on the backup protection action of the relevant equipment for its removal. This inevitably enlarges the fault-impacted areas. In order to remedy this problem, the fault at the low voltage side of the main transformer in a 220 kV substation where the neutral point is grounded through the small resistance is investigated. The symmetrical component method is used to analyze the protection action, which is further combined with fault-recording and phasor diagrams. The insulation breakdown of the bus bridge at the low voltage side of the main transformer outlet is the cause of the dead zone fault. The characteristics of dead zone fault with grounding fault representation are analyzed. This should give full play to shift personnel’s subjective initiative, as well as improving the comprehensive level of equipment operation and maintenance, improving the intelligent level of the integrated dispatching and control system of the primary and distribution network, and changing the power supply mode of the power grid. Some suggestions are put forward to prevent and alleviate the influence of such faults in the future. This work is supported by the Liaoning Provincial Key Research and Development Program (No. 2017106018) and the Liaoning Provincial Key Research and Development Program of Science and Technology (No. 2018106019).