The CBCT , also known as a ZERO sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In CBCT , the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. 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 relay setting.