---
title: "CT secondary should not be kept open"
subject: "Electrical Machines"
chapter: 2
key: "semester_4/electrical_machines:ct-secondary-open-true-false"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/ct-secondary-open-true-false
site: BCT Engineering
---

# CT secondary should not be kept open

Electrical Machines · Chapter 2: Transformer

- Asked in: 2068 Bhadra, 1+3 marks
- Video: [Why CT Secondary should not keep open](https://www.youtube.com/watch?v=xiz-WhzaaJU) — Barani Tech
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/ct-secondary-open-true-false
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-2

## Question

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

## Short answer

True. If the secondary is open, the opposing mmf is lost, the full primary current magnetizes the core, causing saturation, extremely high induced voltages (insulation breakdown and shock hazard), and core overheating.

## Answer

**True.** The secondary of a current transformer must never be left open while the primary carries current. It should be short-circuited (or connected to a low-impedance burden) first.

**Justification**

In a CT the primary is in series with the line, so the primary current $I_1$ is fixed by the load of the power system and does not depend on the secondary. Under normal working, the secondary current $I_2$ produces an mmf $N_2I_2$ that almost cancels the primary mmf $N_1I_1$. Only a small net mmf (equal to the exciting mmf $N_1I_0$) remains to magnetise the core, so the flux is small.

If the secondary is opened:

1. $I_2 = 0$, so there is no opposing mmf. The full primary mmf $N_1I_1$ acts as magnetising mmf.
2. The core flux rises sharply and the core saturates deeply.
3. The secondary has a very large number of turns. The emf $e_2 = N_2\,d\phi/dt$ becomes a high peaky voltage (several kV), which can break down the insulation and is dangerous to the operator.
4. Core loss rises greatly, so the core overheats and may be damaged.
5. A large residual flux is left in the core after the primary is de-energised, which spoils the ratio accuracy of the CT.

Hence the statement is true.

## Check yourself

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

1. Why does opening the CT secondary not reduce the primary current?
   A. The secondary winding supplies the primary current
   B. The primary current is limited by core saturation
   C. The primary is in series with the line, so its current is set by the power system load
   D. The CT primary has a very high impedance
   Answer: C. The primary carries the line current, which does not depend on the secondary. (Key point: Primary current is fixed)

2. What happens to the magnetising mmf when the CT secondary is opened?
   A. The whole primary mmf N1I1 acts as magnetising mmf
   B. It becomes zero because I2 = 0
   C. It stays equal to the small exciting mmf N1I0
   D. It is cancelled by the secondary voltage
   Answer: A. With I2 = 0 there is no opposing mmf, so the full primary ampere-turns magnetise the core. (Key point: Net mmf when open)

3. Why is an open CT secondary dangerous to the operator?
   A. The primary current drops to zero and the line trips
   B. The deep saturation gives a high peaky emf in the many-turn secondary
   C. The secondary current becomes very large
   D. The core demagnetises completely
   Answer: B. The rapid flux change induces e2 = N2 dphi/dt, which is several kV across the many turns. (Key point: Danger of open secondary)

4. What should be done before working on a CT whose primary is energised?
   A. Leave the secondary open to avoid losses
   B. Connect a very high resistance across the secondary
   C. De-energise nothing and open the secondary slowly
   D. Short-circuit the secondary or connect a low-impedance burden
   Answer: D. A closed low-impedance secondary keeps the mmf nearly balanced and the flux small. (Key point: Safe practice)

## Related in Electrical Machines

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- [DC series motor started at no load: true/false](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-no-load-true-false) — Electrical Machines, chapter 4
- [DC series motor should not start on no-load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-motor-no-load-start) — Electrical Machines, chapter 4
- [Auxiliary winding construction differs from main winding](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aaux-winding-construction-true-false) — Electrical Machines, chapter 7
- [Primary current and efficiency of 11 kV/380 V Δ/Y transformer](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adelta-star-11kv-380v-40kw-efficiency) — Electrical Machines I, chapter 2, asked 3×

## Related in other subjects

- [True/false: CT secondary kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-false-ct-secondary-open) — Electrical Machine, 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
- [CT secondary open and PT secondary short](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-open-pt-short) — Switchgear and Protection, chapter 2