Its core structure comprises a primary winding, a secondary winding, an iron core, and an insulation system. The primary winding has few turns and is connected in series with the main circuit under measurement, while the secondary winding has a larger number of turns and connects to measuring instruments or protective devices.
The operating principle of a current transformer is based on Faraday's law of electromagnetic induction; current transformation is achieved through the turns ratio (transformation ratio) between the primary and secondary windings. During operation, the magnetomotive force generated by the secondary current largely cancels out that generated by the primary current, resulting in a very small excitation current.
When in operation, the secondary circuit of a current transformer must remain closed; the load impedance is very low, meaning the device operates in a state approaching a short circuit. The secondary side must strictly never be left open-circuited, as this would induce dangerously high voltages, endangering both equipment and personnel safety; therefore, the secondary side must be reliably grounded during operation.