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True/false: CT secondary kept open

Short answer

True. In normal CT operation, secondary mmf cancels primary mmf. If secondary is open, net mmf becomes large, core saturates, flux increases rapidly, inducing dangerously high voltage and overheating. Therefore secondary must always be shorted when primary is energized.

State whether the following statement is true or false and justify it: Secondary of CT should not be kept open while the primary winding is energized.

True.

Justification

  • A current transformer is connected in series with the line, so its primary current is fixed by the line and does not depend on the secondary circuit.
  • During normal operation, the secondary mmf (Is Ns) opposes and nearly cancels the primary mmf (Ip Np). Only a very small net mmf is left to set up the flux.
  • If the secondary is opened, Is becomes zero and the demagnetising mmf disappears. The whole primary mmf acts as the magnetising mmf, so the core flux becomes very large and the core saturates.
  • Heavy saturation overheats the core by large iron losses. The rate of change of flux is also very high, so a very high emf (several kV, with sharp peaks) is induced in the many turns of the secondary.
  • This can break the insulation and give a dangerous shock to the operator.

Hence the secondary of a CT must be short-circuited (or kept closed through an ammeter) whenever the primary is energised.

✓ Answer checked by Claude Opus 5.5 (high effort)

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