---
title: "True/false: CT secondary kept open"
subject: "Electrical Machine"
chapter: 2
key: "semester_4/electrical_machine:tf-false-ct-secondary-open"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-false-ct-secondary-open
site: BCT Engineering
---

# True/false: CT secondary kept open

Electrical Machine · Chapter 2: Transformer

- Asked in: 2068 Bhadra, 1+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-false-ct-secondary-open
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-2

## Question

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.

## 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The statement 'Secondary of CT should not be kept open while the primary is energised' is:
   A. False, since the CT is a series device
   B. False, because the secondary is meant to be open
   C. True
   D. True only for a no-load CT
   Answer: C. Opening the secondary causes saturation and a dangerous high emf. (Key point: Truth of statement)

2. Why does the primary current of a CT not fall when the secondary is opened?
   A. Because the CT primary is in series with the line, so the line fixes the current
   B. Because the secondary supplies the current
   C. Because of the core flux
   D. Because the primary has many turns
   Answer: A. Primary current is set by the line, independent of the secondary circuit. (Key point: Primary current in CT)

3. What must be done before disconnecting an instrument from a CT?
   A. Increase the primary voltage
   B. Short-circuit the CT secondary
   C. Open the secondary first
   D. Remove the core
   Answer: B. Shorting the secondary keeps the mmf balanced and avoids a high emf. (Key point: Safe practice)

## Related in Electrical Machine

- [CT secondary should not be kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Act-secondary-open-true-false) — Electrical Machines, chapter 2
- [True/false: auxiliary winding construction](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-auxiliary-winding-differs) — Electrical Machine, chapter 7