- 2075 Baisakh · 4 marks
Types of overcurrent relays
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).
What are the various types of over current relays? Discuss their area of application.
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%).
✓ Answer checked by Claude Opus 5.5 (high effort)
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