---
title: "CT working principle and applications"
subject: "Switchgear and Protection"
chapter: 2
key: "semester_6/switchgear_and_protection:ct-definition-principle-applications"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-definition-principle-applications
site: BCT Engineering
---

# CT working principle and applications

Switchgear and Protection · Chapter 2: Current and Potential Transformers

- Asked in: 2074 Bhadra, 4 marks
- Video: [What is Current Transformer (CT) and how it works?](https://www.youtube.com/watch?v=kcHir5x87kc) — The Engineering Power
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ct-definition-principle-applications
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2

## Question

Define current transformer, its working principle and their application areas.

## Short answer

A current transformer (CT) is an instrument transformer with primary in series with the line, delivering a reduced secondary current (1A or 5A) to meters and relays. It operates on electromagnetic induction with secondary nearly short-circuited; primary ampere-turns are balanced by secondary ampere-turns, leaving only small exciting current. Applications include metering (ammeters, energy meters), protection (overcurrent, differential, distance relays), core-balance earth-fault detection, SCADA, and isolation from high voltage.

## Answer

A **current transformer (CT)** is an instrument transformer whose primary winding carries the line current in series with the circuit and whose secondary delivers a proportionally reduced current (1 A or 5 A) to meters and relays, with a known phase relation.

#### Working principle
It works on the principle of electromagnetic induction, like a transformer operating with its secondary nearly short-circuited.
```text
     I_p (line current)
  ═══════════╦═══════════  1 or few turns
          ┌──╨──┐
          │core │  flux Φ
          └──╥──┘
        N_s turns
        ┌────╨────┐
        │  A / R  │ ammeter/relay
        └─────────┘ low impedance burden
```
- The primary current is fixed by the load of the line, not by the CT.
- It sets up primary ampere-turns N_p·I_p. The secondary current I_s produces opposing ampere-turns; only a small difference remains to magnetise the core:
```text
 N_p·I_p ≈ N_s·I_s   →   I_s ≈ I_p × (N_p/N_s)
```
- *Example:* a 500/5 A CT gives 5 A at 500 A primary; turns ratio 1:100.
- The flux density is low in normal operation; if the secondary is opened, the full primary ampere-turns magnetise the core, causing saturation and dangerously high secondary voltage. Hence the secondary must never be open-circuited.

#### Types
Wound type, bar type, window/ring type, bushing type, core-balance CT.

#### Application areas
1. **Metering:** ammeters, wattmeters, energy meters, power-factor meters on HV/LV feeders and tariff metering.
2. **Protection:** overcurrent and earth-fault relays, differential protection of transformers, generators and busbars, distance and directional relays.
3. **Core-balance CTs:** earth-leakage and sensitive earth-fault protection of cables and motors.
4. **Control and monitoring:** SCADA transducers, maximum demand indicators, power quality meters.
5. **Isolation:** keeps meters and personnel away from high voltage.

## Check yourself

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

1. What is the CT working principle?
   A. Ampere-turns balance: N_p I_p is approximately N_s I_s, with only a small exciting difference magnetising the core
   B. Voltage balance across primary and secondary
   C. Capacitive division
   D. Primary current set by the CT secondary burden
   Answer: A. The secondary ampere-turns oppose the primary's; I_s = I_p x N_p/N_s approx. (Key point: Principle)

2. What determines the primary current of a CT?
   A. The CT burden
   B. The line load, not the CT
   C. The turns ratio
   D. The secondary resistance
   Answer: B. The CT is in series with the line, so the primary current is fixed by the load. (Key point: Primary current)

3. A 500/5 A CT has what turns ratio?
   A. 1:10
   B. 1:500
   C. 1:100
   D. 100:1
   Answer: C. 500/5 = 100, so N_p:N_s = 1:100. (Key point: Ratio example)

4. Why must a CT secondary never be opened?
   A. The current doubles
   B. The primary opens
   C. Ratio becomes 1:1
   D. No opposing m.m.f.: full primary ampere-turns magnetise the core, causing saturation and dangerous high voltage
   Answer: D. Without secondary m.m.f. the core saturates and high voltage is induced across the many secondary turns. (Key point: Open secondary)

## Related in Switchgear and Protection

- [Applications of CTs/PTs; CT burden](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-pt-applications-burden) — Switchgear and Protection, chapter 2
- [Isolator construction, principle and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-construction-principle) — Switchgear and Protection, chapter 4
- [CT with ammeter numerical](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-ammeter-numerical) — Switchgear and Protection, chapter 2
- [Accuracy classes of CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-accuracy-class) — Switchgear and Protection, chapter 2
- [IDMT settings for 20 MVA transformer and feeder](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-feeder-idmt-20mva) — Switchgear and Protection, chapter 7, asked 2×
- [Percentage differential relay: 125 MVA transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apercentage-differential-125mva) — Switchgear and Protection, chapter 7

## Related in other subjects

- [Current transformer vs conventional; CT open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Acurrent-transformer-secondary-open) — Electrical Machine, chapter 2
- [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
- [LVDT construction, working and applications](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Alvdt-construction-working-principle) — Instrumentation and Measurement, chapter 5
- [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3