The current in power generation, transformation, transmission, distribution, and consumption lines varies greatly, ranging from a few amperes to tens of thousands of amperes. For the convenience of measurement, protection, and control, it is necessary to convert to a relatively uniform current. In addition, the voltage on the line is generally high, and direct measurement is very dangerous. The current transformer plays a role in current conversion and electrical isolation.

For pointer type ammeters, the secondary current of current transformers is mostly in the ampere level (such as 5A, etc.). For digital instruments, the sampled signal is generally in the milliampere range (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is in the milliampere range, mainly serving as a bridge between the large transformer and the sampling.

Miniature current transformers are also known as "instrument current transformers". The term "instrument current transformer" refers to a multi current ratio precision current transformer used in laboratories, generally used to expand the instrument range

Current transformers, similar to transformers, work based on the principle of electromagnetic induction. Transformers convert voltage, while current transformers convert current. The winding of the current transformer connected to the measured current (with N1 turns) is called the primary winding (or primary winding); The winding connected to the measuring instrument (with N2 turns) is called the secondary winding (or secondary winding, secondary winding).

The ratio of the current of the primary winding I1 to the current of the secondary winding I2 of a current transformer is called the actual current ratio K. The current ratio of a current transformer when operating at rated current is called the rated current ratio of the current transformer, expressed in Kn.

Kn=I1n/I2n

The function of a current transformer (CT) is to convert a larger primary current into a smaller secondary current through a certain transformation ratio, which is used for protection, measurement, and other purposes. A current transformer with a transformation ratio of 400/5 can convert the actual current of 400A into a current of 5A.