1. The primary coil is connected in series in the circuit, and the number of turns is small. Therefore, the current in the primary coil is completely dependent on the load current of the circuit under test, and is independent of the secondary current;
2. The current coil of the instrument and relay connected to the secondary coil of the current transformer is very small, so under normal circumstances, the current transformer operates in a near short circuit state.
The ratio of the primary and secondary rated current of the current transformer is called the rated mutual inductance ratio of the current transformer: kn=I1n/I2n
Since the primary coil rated current I1n has been standardized, the secondary coil rated current I2n is unified to 5 (1 or 0.5) amps, so the current transformer rated mutual inductance ratio has also been standardized. Kn can also be approximated as the turns ratio of the primary and secondary coils of the transformer, that is, kn≈kN=N1/N2 where N1.N2 is the number of turns of the first and second coils.