---
title: "Current transformer vs conventional; CT open"
subject: "Electrical Machine"
chapter: 2
key: "semester_4/electrical_machine:current-transformer-secondary-open"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/current-transformer-secondary-open
site: BCT Engineering
---

# Current transformer vs conventional; CT open

Electrical Machine · Chapter 2: Transformer

- Asked in: 2073 Bhadra, 8 marks
- Video: [What happen if Secondary Terminals of C.T. Left Open.](https://www.youtube.com/watch?v=o156tP8CQkE) — SunlitUri
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/current-transformer-secondary-open
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-2

## Question

How current transformer is different from conventional transformer. Explain how CT is used to measure high currents. Also explain, what happens if the secondary of CT is open when there is high current flowing in primary side.

## Short answer

A CT has primary in series with line (few turns) and secondary connected to ammeter (many turns), providing electrical isolation. If secondary opens under primary current, demagnetizing mmf vanishes, core saturates, flux spikes induce extreme secondary voltage (several kV) causing insulation failure and operator hazard.

## Answer

#### Current transformer (CT) versus a conventional (power) transformer
| Basis | Current transformer | Power transformer |
|---|---|---|
| Purpose | Measure/step down large alternating currents for instruments and relays | Transfer power at a changed voltage |
| Primary | A few turns (or a single bar) in series with the line | Many turns, connected across the supply |
| Secondary | Many turns, connected to an ammeter / relay coil, nearly short-circuited | Connected to the load |
| Primary current | Fixed by the line (load), independent of secondary | Depends on the secondary load |
| Burden | Very low (a few VA) | Rated power (kVA) |
| Voltage across secondary | Very low in service | Rated voltage |
| Flux in core | Low in service; very high if secondary is open | Nearly constant |

#### Use of a CT to measure high currents
```text
 line ──[ primary: Np (1-2 turns) ]── load
              core
        [ secondary: Ns turns ]──(A)──
```
- The primary is placed in series with the high-current line. The secondary has many turns and is connected to a standard 5 A (or 1 A) ammeter, which acts nearly as a short circuit.
- Ip Np ≈ Is Ns, so Is = Ip (Np/Ns). Line current = Is × (Ns/Np). E.g. a 1000/5 A CT with 5 A on the meter indicates 1000 A in the line.
- The meter and the line are separated electrically, so the instrument can be safely used for high-voltage lines.

#### Secondary open with high primary current
- In normal service, the secondary mmf almost cancels the primary mmf, so the net mmf (and flux) is small.
- If the secondary is opened, Is = 0 and nothing cancels the primary mmf. The whole primary ampere-turns Ip Np acts as magnetising mmf, and the flux in the core rises to very high values.
- Core saturates heavily and the core loss gives severe heating of the core.
- The emf in the secondary (many turns, high rate of change of flux) becomes extremely large. Peaks of several kV can appear, endangering the insulation and the operator.
- Therefore the CT secondary should never be opened under load. It must be short-circuited before disconnecting an instrument.

## Check yourself

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

1. How is the primary of a CT connected and what is its construction?
   A. Few turns, in series with the line
   B. Many turns, across the supply
   C. Many turns, in series with the line
   D. Few turns, across the supply
   Answer: A. A CT primary has one or a few turns carrying the line current in series. (Key point: CT primary)

2. A 1000/5 A CT shows 5 A on the ammeter. What is the line current?
   A. 5 A
   B. 200 A
   C. 1000 A
   D. 5000 A
   Answer: C. Line current = Is × (Ns/Np) = 5 × 200 = 1000 A. (Key point: CT current relation)

3. Why does the core flux rise sharply if a CT secondary is opened with the primary energised?
   A. The primary current drops
   B. The secondary mmf that cancels the primary mmf disappears, so all primary ampere-turns magnetise the core
   C. The core resistance increases
   D. The primary voltage rises
   Answer: B. With Is = 0, nothing opposes Ip Np, which becomes the magnetising mmf. (Key point: CT secondary open flux)

4. What is the dangerous result of opening the CT secondary under load?
   A. Zero output voltage
   B. A very high secondary emf (several kV peaks) and core overheating
   C. The primary current becomes zero
   D. The CT turns into a step-up power transformer with a safe output
   Answer: B. High dΦ/dt in the many secondary turns gives huge emf and saturation heating. (Key point: Dangers of open CT)

## Related in Electrical Machine

- [CT secondary open; auto transformer economy](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Act-open-autotransformer-economical) — Electrical Machines, chapter 2
- [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
- [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
- [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×
- [25 kVA transformer: load voltage and primary current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atf-25kva-11kv-load-voltage) — Electrical Machines, chapter 2
- [3-phase 500 kVA transformer primary current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atf-500kva-11kv-400v-primary-current) — Electrical Machines, chapter 2

## Related in other subjects

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- [Terzaghi limitations; primary vs secondary](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aterzaghi-theory-limitations-primary-secondary) — Soil Mechanics, chapter 9
- [Primary and secondary energy at 45% exceedance](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aprimary-secondary-energy-45pct) — Hydro Power, chapter 3
- [Primary/secondary energy and pondage volume](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aenergy-and-pondage-volume) — Hydro Power, chapter 3