Core Balance Current Transformer or CBCT is a ring-type or rectangle- type current transformer through the center of which a three core cable or three single core cables (forms the primary winding) of three phase system passes. This type of current transformer is normally used for earth fault protection for low and medium voltage systems.
When the system is fault-free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the earth fault relay. Therefore, it is also called the Zero Sequence Current Transformer (ZSCT).