---
title: "Types of overcurrent relays"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:overcurrent-relay-types"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overcurrent-relay-types
site: BCT Engineering
---

# Types of overcurrent relays

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2075 Baisakh, 4 marks
- Video: [Overcurrent Relay](https://www.youtube.com/watch?v=AT8LFPkcHM0) — 5MINUTES ELECTRICAL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overcurrent-relay-types
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

What are the various types of over current relays? Discuss their area of application.

## Short answer

Types include instantaneous (no intentional delay, near source), definite time (fixed delay, radial feeders), IDMT (inverse with definite minimum, common distribution), very inverse (steeper, feeders with varying fault current), extremely inverse (I² relation, coordinating with fuses or motors), and directional (trips only for one direction, parallel feeders and ring mains).

## Answer

An **overcurrent relay** operates when the current through it exceeds a preset value (pick-up). By their time-current characteristic they are classified as below.

| Type | Characteristic | Application |
|---|---|---|
| Instantaneous | Trips with no intentional delay (< 0.1 s) when current exceeds setting | Near the source, where fault current is much larger than at the far end; high-set unit with IDMT |
| Definite time | Fixed delay, independent of current magnitude | Radial feeders where source impedance is large, back-up of other relays |
| IDMT (inverse definite minimum time) | Inverse at low PSM, definite minimum at high PSM | Most common for distribution feeders, transformers; grading of radial systems |
| Very inverse | Steeper: t ∝ 1/I approx. | Feeders where fault current falls a lot with distance |
| Extremely inverse | t ∝ 1/I² approx. | Feeders with high inrush/cold-load pick-up; grading with fuses; motors, earthing transformers |
| Directional overcurrent | Trips only for current in one direction | Ring mains, parallel feeders, reverse power on generators |

- Instantaneous + IDMT units are often combined in one relay to clear close-in faults fast.
- Earth-fault relays are overcurrent relays fed from residual CT current, with a lower setting (20–40%).

## Check yourself

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

1. Which relay type is most commonly used for distribution feeders and transformers?
   A. Instantaneous
   B. Definite time
   C. IDMT
   D. Extremely inverse only
   Answer: C. IDMT (inverse at low PSM, definite minimum at high PSM) is the most common choice and is used for grading radial systems. (Key point: IDMT application)

2. An extremely inverse relay has an approximate time-current relation of:
   A. t independent of I
   B. t proportional to 1/I
   C. t proportional to 1/I^2
   D. t proportional to I
   Answer: C. Extremely inverse is about t proportional to 1/I^2, suited to inrush/cold-load pick-up and grading with fuses. (Key point: Extremely inverse relay)

3. Why is a directional overcurrent relay used on ring mains and parallel feeders?
   A. It trips only for current in one direction
   B. It trips faster than any other type
   C. It needs no CT
   D. It has a fixed delay independent of current
   Answer: A. Fault current can flow either way in rings and parallel feeders, so direction sensing is needed for selectivity. (Key point: Directional overcurrent use)

## Related in Switchgear and Protection

- [Voltage-monitored overcurrent relay timing](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avoltage-monitored-overcurrent) — Switchgear and Protection, chapter 7
- [Time-current grading for relays X, Y, Z](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-grading-xyz) — Switchgear and Protection, chapter 7
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- [Static vs electromagnetic relays; components](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-vs-electromagnetic-relays) — Switchgear and Protection, chapter 8, asked 4×
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- [Design IDMT grading for relays A and B](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aradial-feeder-idmt-design-ab) — Switchgear and Protection, chapter 7

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