Why do we need to wire it? It is to ensure safety, firstly equipment safety and secondly personal safety. Because there is distributed capacitance between the primary and secondary windings of a current transformer, as well as between the secondary winding and ground. So the voltage division of this distributed capacitor causes the secondary winding to generate a relatively high voltage. Also, if the insulation between the primary winding and the secondary winding is damaged due to certain reasons, the high voltage of the primary circuit will be directly added to the secondary circuit, which poses a great threat to the safety of secondary equipment and personnel.

So the secondary side of the current transformer must be grounded, and the secondary circuit has only one grounding point, and multiple grounding points are not allowed.

The grounding of the secondary side of a current transformer is actually set up to prevent insulation breakdown caused by other reasons on the primary side high voltage inside the transformer, and to prevent high voltage from entering the secondary low voltage side, forming a circuit on the silicon steel sheet (iron core) or the secondary side coil, which poses a threat to human safety.

If the current transformer is not installed according to the rules, P1 is the direction of the power supply and P2 is the direction of the line load. Even if the secondary side is grounded, it will pose a threat to human safety. This is because the transformer has a same named end and a different named end in the instantaneous reference direction during operation. Manufacturers of current transformers usually set S1 as the grounding protection terminal according to regulations. It is precisely for this reason that in the installation of several or more current transformers, the arbitrary grounding situation of S1 and S2 cannot be confused. As a qualified power practitioner, this is the most basic common sense.

When the primary side of a current transformer is energized, the secondary side coil must not be open circuited under any circumstances. This is because under normal operating conditions, the primary magnetic potential and secondary magnetic potential of the current transformer are basically balanced, and the excitation magnetic potential is very small. The magnetic flux density in the iron core and the induced potential of the secondary coil are not high. When the secondary circuit is open, all the primary magnetic potential is used for excitation, and the iron core is excessively saturated. The magnetic flux waveform is a flat peak wave, while the secondary potential of the current transformer is a sharp peak wave. Therefore, high voltage will appear in the secondary winding coil, which poses a danger to human body and equipment safety.

When the current transformer is in operation, if it is necessary to disassemble the instrument on the secondary side, the secondary side S1 and S2 of the current transformer must first be short circuited with wires. In order to prevent open circuits, the power safety regulations strictly stipulate that fuses or knife switches are not allowed to be installed on the secondary side of current transformers.