Earthing the CT secondary: Safety issue, it prevent any possible floating voltages. (Possible voltages between secondary winding and ground)
Earthing at one point: Prevent differences of potential in the CT secondary circuit because of differences of potential between different points in the station grid. This differences of potential results in circulating currents, and thus incorrect performance of the relays and inaccuracy in the readings of the instruments and meters. It also facilitate the temporary removal and re-establishment of the earth connection, in order to make periodic tests for deterioration of insulation and accidental grounds in the CT secondary circuit.
IEEE standard C57.13.3 serves as the ANSI guide to standardize instrument transformer grounding practices. The grounding of CTs is important to both safety and the correct operation of protective relays. To assure safe and reliable operation, the neutral of the CT secondary should have a single ground location for each circuit. The single ground is irrespective of the number of CTs or the chosen grounding location. Utilizing a single ground eliminates the risk of redundant ground loops and associated problems.
During normal operation more than one ground on a CT circuit is not an obvious problem, other than the difficulties it may cause during testing. However, during a fault condition, multiple grounds allow a different ground potential rise for each CT. The result is a significant current flow through the CT circuit that is not representative of the primary current.
