Current transformers, a special type of transformer, are originally isolated on their input and output sides, with no directly connected circuits. However, grounding is crucial for their safe operation. The main purpose of grounding is to prevent high voltage from the primary side from accidentally entering the secondary side, thus preventing safety risks. This function is similar to the neutral wire in a three-phase four-wire system; when the three phases are unbalanced, the current can flow back through the neutral wire, ensuring the safety of the equipment's power supply.
In practical applications, current transformers are usually grounded using a single-point grounding method, a protective grounding measure. Under normal operating conditions, it does not form a circuit to ground, and no current flows into the earth.
If a current transformer is not grounded, it will face several potential risks. First, when an open circuit occurs in the secondary circuit, the current on the primary side will induce high voltage on the secondary side, which will undoubtedly endanger the safety of equipment and personnel. Second, even if the insulation does not break down, any situation that causes high voltage to be generated on the secondary side will cause serious damage to the equipment. After all, the secondary side is designed to ensure a low-voltage, low-current environment to guarantee the safety of personnel interaction. The presence of high voltage not only directly burns out secondary instruments but also poses a risk of electric shock to personnel working nearby.
Furthermore, although the coils of the current transformer are isolated, if the secondary circuit is not grounded, the high voltage on the primary side will be divided by the capacitance between the two coils and the distributed capacitance of the secondary side to ground. In this way, high voltage will directly enter the secondary side, and its magnitude depends on the capacitance of the secondary side to ground. However, once the secondary side is grounded, this distributed capacitance becomes zero, and the high voltage will be directly conducted to the ground. At this point, even if a person touches it, it will not cause an electric shock.