The through-core current transformer does not have a primary winding in its own structure. The current-carrying (load current) wires from L1 to L2 pass through a round (or other shape) iron core made of silicon steel sheet rolling to serve as a primary winding. The secondary winding is directly and evenly wound on the circular iron core, and is connected in series with the secondary load of the current coil such as meters, relays, and transmitters to form a closed loop.

Since the through-core current transformer does not have a primary winding, its transformation ratio is determined by the number of turns of the primary winding through the core of the transformer. The more the through-core turns, the smaller the transformation ratio; on the contrary, the fewer the through-core turns. The larger the transformation ratio, the rated current ratio: where I1——the rated current for one turn through the core; n——the number of turns through the core.

The wiring of the through-core current transformer is similar to that of the ordinary current transformer. The primary side passes through the P1 side of the transformer and comes out from the P2 side. The secondary side wiring is the same as the ordinary transformer.

In the measurement, the coil impedance of the secondary loop of the current transformer is very small, so the secondary loop of the current transformer is close to a short circuit, and in order to unify the specification, the rated value of the secondary winding is generally designed to be 5A. In practical applications, for safety reasons, the secondary side S2 of the current transformer generally needs to be grounded.