In a current transformer _____ winding is closed.

A. Primary

B. Secondary

C. Both A & B

D. None of the above

Answer

B. Secondary

Explanation 

• In a current transformer, the secondary winding is closed, to protect the insulation of a transformer from generating a high voltage across the secondary winding.
• 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.
• The current transformer works on the principle of shorted secondary
• It means that the burden on the system Zb 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 the 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
• 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 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.

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