Correct option is D
The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.
Current transformer works on the principle of shorted secondary
It means that the burden on the system is equal to 0
Thus, the current transformer produces a current in its secondary which is proportional to the current in its primary
Most important precaution in use of a CT is that in no case should it be open-circuited (even accidentally)
As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened
This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable and the heat dissipation in the core will be very large
Again, due to large flux in the core, the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT
This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there is a good chance that the person who is opening the CT secondary while the primary is energized will get a fatal shock.
