---
title: "Need of instrument transformers; CT errors"
subject: "Switchgear and Protection"
chapter: 2
key: "semester_6/switchgear_and_protection:instrument-transformers-need-ct-errors"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/instrument-transformers-need-ct-errors
site: BCT Engineering
---

# Need of instrument transformers; CT errors

Switchgear and Protection · Chapter 2: Current and Potential Transformers

- Asked in: 2075 Baisakh, 4 marks
- Video: [Current Transformers (CT) | Eaton PSEC](https://www.youtube.com/watch?v=32Vw40nUSwA) — Eaton
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/instrument-transformers-need-ct-errors
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-2

## Question

Explain the need of instrument transformers in system. Mention the errors on CT.

## Short answer

Instrument transformers provide isolation, standardize meter and relay ratings, and allow remote placement. CT errors include ratio error from the loss component of exciting current, phase angle error from the magnetizing component, and composite error. These errors increase with burden, saturation, and remanent flux.

## Answer

**Instrument transformers** (current transformers and potential/voltage transformers) reduce the large currents and high voltages of the power system to standard low values (1 A or 5 A; 110 V or 110/√3 V) for meters and relays.

#### Need of instrument transformers
1. **Isolation:** meters, relays and operators are isolated from the high-voltage system, improving safety.
2. **Standardisation:** relays and meters of standard rating (5 A / 1 A, 110 V) can be used on any system voltage or current.
3. **Economy:** low-rated instruments with thin wiring are much cheaper than high-voltage, high-current instruments.
4. **Remote location:** relays and meters can be placed in a control room away from the switchyard.
5. **Protection functions:** CTs and VTs supply the current and voltage signals needed by overcurrent, differential, distance and directional relays.
6. **Combination:** several CT cores can serve separate metering and protection circuits.

#### Errors of CT
Because part of the primary ampere-turns supplies the exciting current I₀, the secondary current is not exactly I_p/K_n.
1. **Ratio (current) error:** `(K_n·I_s − I_p)/I_p × 100 %`. Caused mainly by the loss component of I₀; increases with burden.
2. **Phase angle error (phase displacement):** angle between I_p and reversed I_s, caused mainly by the magnetising component of I₀.
3. **Composite error:** r.m.s. difference between ideal and actual secondary current over one cycle including harmonics; important for protective CTs under saturation.

Errors increase with high burden, saturation (high fault current, DC offset), remanent flux, low primary current and poor core material.

## Check yourself

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

1. Why are instrument transformers used?
   A. To raise voltage
   B. Isolation and standardised low ratings for meters and relays
   C. To increase line current
   D. To remove relays
   Answer: B. They provide safety, standardisation, economy. (Key point: Need)

2. Standard secondary values are:
   A. 1 A or 5 A; 110 V
   B. 100 A; 11 kV
   C. 50 A; 440 V
   D. 10 A; 33 kV
   Answer: A. Per the answer. (Key point: Standard values)

3. Which are CT errors?
   A. Hysteresis only
   B. Burden and fault
   C. Ratio, phase angle and composite
   D. Voltage and frequency only
   Answer: C. Three errors as listed. (Key point: Errors list)

4. Errors increase with:
   A. Low burden
   B. Good core material
   C. Small DC offset
   D. High burden, saturation, remanent flux, low primary current
   Answer: D. These factors increase exciting current. (Key point: Error increase)

## Related in Switchgear and Protection

- [Knee point voltage and CT errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-errors) — Switchgear and Protection, chapter 2
- [Different errors of CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aerrors-of-ct) — Switchgear and Protection, chapter 2
- [CT purpose, phasor diagram and errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-phasor-ratio-phase-error) — Switchgear and Protection, chapter 2
- [Knee point voltage; measuring vs protective CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-measuring-protective-ct) — Switchgear and Protection, chapter 2
- [Knee point voltage and CT selection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-selection) — Switchgear and Protection, chapter 2
- [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

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- [Need for difference equation; LCCDE and system function](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-difference-equation-lccde) — Digital Signal Analysis and Processing, chapter 1, asked 2×
- [Frequency response of a CT LTI system](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Act-frequency-response) — Signals and Systems, chapter 5, asked 3×