Current transformers can be broadly classified into two categories according to their use:

01. Current transformers for measurement (or measuring windings of current transformers): within the normal operating current range, they provide current information of the power network to measuring and metering devices. Measurement of CT current flowing through the equipment under normal operation of the primary equipment. The measured current is used for the metering loop of relay protection device, integrated self-device and metering device. It requires a high measurement accuracy under normal load current conditions.

02. Current transformers for protection (or protection windings of current transformers): in the state of power grid failure, it provides power grid fault current information to relay protection and other devices. Protective CT means that when the primary equipment or line has a fault such as short circuit, when the primary equipment flows through a large fault current, the output current of the secondary side of the CT can basically reflect the magnitude of the primary side current. It requires that the CT is still unsaturated under the condition of large fault current, that is, when the primary side flows N times the rated current, the output current error of the secondary side is still small.