It is to ensure safety, one is equipment safety, and the other is personal safety. Because there are distributed capacitances between the primary winding and the secondary winding of the current transformer and between the secondary winding and the ground. Then the voltage division of this distributed capacitance causes the secondary winding to produce a higher voltage to the ground. In addition, the insulation between the primary winding and the secondary winding is damaged due to some reasons, so the high voltage of the primary circuit will be directly added to the secondary circuit, which is very harmful to the secondary equipment and personal safety. Therefore, the secondary side of the current transformer must be grounded, and the secondary circuit has one and only one grounding point, and multiple grounding points are not allowed.