There is disclosed a method of testing and verifying a performance for insulation to ground of a disconnecting switch when breaking a charging current. This disconnecting switch has a movable electrode (62) and a fixed electrode (64) which are arranged in a tank so as to face each other. One of these electrodes is connected to an electrostatic capacity load (C,) and the other is connected to a power supply (40). This test method comprises the steps of: applying a voltage from the power supply (40) through the disconnecting switch to the load to cause the voltage to be trapped in the load; and applying a voltage of the polarity opposite to that of the trapped charged voltage from the power supply (40) to the disconnecting switch to cause the discharge between those electrodes, thereby verifying the performance for insulation to ground.