Basically, a Current Transformer is a step-up transformer. Where on the secondary side, the current is decreased, and the Voltage is increased. A current transformer should never be open-circuited while the main current is passing through the primary winding.
The current transformer is nothing but a step-up voltage transformer, as we are decreasing current, voltage is automatic increases on the secondary side of CT. If we keep the secondary open, CT will face an open circuit voltage on the secondary winding. This open circuit voltage can puncture the insulation of CT and cause burning of CT, as the insulation of CT is not designed to withstand with this voltage due to its cost and size limitation.
That’s why it is recommended to provide a short link on the secondary of CT to avoid this open circuit voltage. In the case of short-circuited secondary, the value of voltage is very less.
Zero current in secondary leads zero back emf. So primary current becomes very high as the primary resistance is very low. Large heat generated may burn the winding and core. If the transformer sustains this heat, even then its operation becomes improper. CT is basically a step-up transformer. So high voltage induced in secondary may saturate the core (core becomes permanently damaged).
So, the operating point advances in a nonlinear region, which makes the transformer operation improper (the secondary current will not replicate the primary according to the turn ratio). High voltage spikes (order of kV) produced on the secondary side may endanger the life of personnel working on the secondary side. High voltage spikes (order of kV) produced on the secondary side are sufficient to break down the transformer insulation.
If the secondary is open-circuited then, Magnet flux produced in the core due to the primary is very high and no flux is produced by the secondary due to open-circuited so the core reaches saturation level, and it may get damaged. As the no of turns in the secondary is very high so voltage produced in the secondary is also higher which may break the insulation causing a short circuit causing damage to people working near it.