Current transformer consists at least of two secondary cores. The first core is usually designed for measuring, the second (and third…if exist) is (are) used for protection. The secondary of current transformers are almost connected in star.
The stare point of each secondary core must be earthed. All relays of each core are connected in series. Figure (2) shows the secondary connection of one secondary core of a current transformer and the relays connected with it.
Note:
Due to capacitance between primary and secondary winding of current transformer, high voltage may be induced on the secondary winding. So, the secondary core must be earthed to prevent secondary leads and relays from high voltage.
