---
title: "Impedance type distance relay"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:impedance-distance-relay-application"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/impedance-distance-relay-application
site: BCT Engineering
---

# Impedance type distance relay

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2080 Chaitra, 4 marks
- Video: [distance relay | impedance relay | distance protection relay | distance relay protection | in hindi](https://www.youtube.com/watch?v=oQK0YHjdf44) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/impedance-distance-relay-application
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

Explain the working principle of impedance type distance relay and also explain its application area.

## Short answer

An impedance relay uses a balanced beam with current coil (operating torque ∝ I²) and voltage coil (restraining torque ∝ V²). It trips when V/I < Z_set; its R‑X characteristic is a circle centered at origin, so a directional unit is added. Applied to medium‑length transmission lines, with stepped zones, where overcurrent grading is difficult.

## Answer

An **impedance relay** is a distance relay that compares the voltage and current at the relay location and operates when the ratio V/I (the impedance seen up to the fault) is less than its setting Z_s.

#### Working principle (balanced beam type)

```text
   PT ── voltage coil ┐       ┌ current coil ── CT
                      ▼       ▼
        restraining ◄─[ beam ]─► operating
                       pivot
                     trip contact
```

- Current coil gives operating torque ∝ I²; voltage coil gives restraining torque ∝ V².
- Torque equation (neglecting spring): T = K₁I² − K₂V². Relay trips when K₁I² > K₂V², i.e. Z = V/I < √(K₁/K₂) = Z_s.
- On the R–X diagram the characteristic is a circle of radius Z_s centred at the origin. Points inside the circle → trip. It is non-directional, so it is used with a directional unit.
- Under normal load, Z is large (outside circle); during a fault within reach, Z drops inside the circle and the relay trips.

#### Application area

- Protection of **medium-length transmission lines**, with three zones (zone 1 instantaneous for 80–90% of line).
- Back-up protection of adjacent line sections.
- Phase-fault protection where overcurrent grading is difficult (interconnected networks).
- Not ideal for short lines (arc resistance error) or very long lines (power-swing tripping).

## Check yourself

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

1. In a balanced-beam impedance relay, which torque is the operating torque?
   A. Voltage coil torque proportional to V^2
   B. Current coil torque proportional to I^2
   C. Spring torque
   D. Brake magnet torque
   Answer: B. Current gives operating torque K1*I^2 and voltage gives restraining torque K2*V^2. (Key point: Torque equation)

2. On the R-X diagram the impedance relay characteristic is:
   A. A straight line parallel to the R axis
   B. A circle centred at the origin
   C. A circle passing through the origin
   D. A polygon
   Answer: B. The trip condition Z < Z_s gives a circle of radius Z_s centred at the origin, hence non-directional. (Key point: R-X characteristic)

3. Why is the impedance relay not ideal for short lines?
   A. Arc resistance causes a large error in the measured impedance
   B. It cannot measure current
   C. It has no voltage coil
   D. It trips too slowly for zone 1
   Answer: A. On short lines the arc resistance is comparable to line impedance, which causes reach error; it is best for medium-length lines. (Key point: Application limits)

## Related in Switchgear and Protection

- [Static distance relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-distance-relay) — Switchgear and Protection, chapter 8, asked 3×
- [Impedance relay from universal torque equation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aimpedance-relay-torque-equation) — Switchgear and Protection, chapter 7, asked 2×
- [Distance protection relay and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adistance-protection-relay) — Switchgear and Protection, chapter 7
- [CT working principle and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-definition-principle-applications) — 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
- [Static differential relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-relay) — Switchgear and Protection, chapter 8, asked 3×

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