Cutting off the no-load transformer running in the grid will generate operating overvoltage. In a small current grounding system, the magnitude of the so-called overvoltage can reach 3 to 4 times the rated phase voltage; in a large grounding system, the magnitude of the operating overvoltage can also reach 3 times the rated phase voltage. Therefore, in order to test whether the insulation of the transformer can withstand the rated voltage and the operating overvoltage during operation, it is necessary to perform several impulse closing tests before the transformer is put into operation. In addition, when the no-load transformer is put into use, an exciting inrush current will be generated, and its value can reach 6 to 8 times of the rated current. Since the excitation inrush current will generate a large electromotive force, the impact closing test is still an effective measure to consider whether the mechanical strength of the transformer and the relay protection will malfunction.