When it comes to current transformers, I believe everyone is familiar with them. Their main function is to convert high values to low values, making them suitable for measuring instruments and relays. The commonly used secondary rated current is 5A. So here's the question: why does one end of the current transformer need to be grounded?

(1) Secondary windings of current transformers and voltage transformers

The secondary windings of current transformers and voltage transformers must be grounded during operation to prevent high voltage from entering the low voltage system through the transformer after its insulation is punctured, which could damage instruments and harm operating personnel. Grounding the secondary winding of the transformer at one point does not affect the normal operation of the equipment and ensures the safety of personnel and equipment.

To ensure safety, the secondary side of the current transformer must be grounded. This serves as a protective measure in case of high voltage hazards on the secondary side of the current transformer, thereby safeguarding both personnel and equipment.

(2) What is the function of a current transformer?

During the production process, the operational status of each primary device in the system is monitored to ensure the safe operation of the power system. A current transformer typically converts the high current of the primary circuit into a secondary low current proportional to it, and outputs the secondary low current to secondary instruments, relay protection, and automatic devices for use. Therefore, the current transformer serves as an AC current source for electrical control, regulation, measurement, and protection components, and also as a hub connecting the electrical secondary circuit with the primary circuit.

(3) Why should a current transformer be grounded?

It is to ensure safety, both equipment safety and personal safety. Because there is distributed capacitance between the primary winding and the secondary winding of the current transformer, as well as between the secondary winding and the ground. The voltage division of this distributed capacitance causes the secondary winding to generate a high voltage to the ground. Additionally, if the insulation between the primary winding and the secondary winding is damaged for some reason, the high voltage of the primary circuit will be directly applied to the secondary circuit, posing great harm to both the secondary equipment and personal safety.

Therefore, the secondary side of the current transformer must be grounded, and the secondary circuit must have only one grounding point. Multiple grounding points are not allowed.

(4) Why can't the secondary side of a current transformer be open-circuited?

Due to an open circuit in the secondary circuit of a current transformer, the secondary current disappears, and consequently, the demagnetization effect also disappears, leading to a high magnetic density in the iron core. Additionally, since the number of turns in the secondary winding is much larger than that in the primary winding, once the secondary side is open-circuited, a very high voltage will also appear, sometimes reaching thousands of volts. Therefore, it not only endangers the secondary side equipment but also poses a threat to personal safety.