---
title: "Operation of auto reclosure"
subject: "Switchgear and Protection"
chapter: 6
key: "semester_6/switchgear_and_protection:auto-reclosure"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/auto-reclosure
site: BCT Engineering
---

# Operation of auto reclosure

Switchgear and Protection · Chapter 6: Circuit Breakers

- Asked in: 2075 Bhadra, 4 marks
- Video: [What is Auto reclosing and duty cycle of a circuit breaker? TheElectricalGuy](https://www.youtube.com/watch?v=mc30ud7v1cE) — Gaurav J - TheElectricalGuy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/auto-reclosure
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6

## Question

Explain the operation of auto reclosure.

## Short answer

Auto reclosure automatically recloses breaker after a dead time (0.2-0.5s) to restore supply for transient faults (80-90% of overhead line faults). If fault clears, breaker stays closed after reclaim time; if permanent, it trips again and locks out after 1-3 shots. Improves supply continuity, transient stability, and reduces outage time.

## Answer

**Auto reclosure** is the automatic re-closing of a circuit breaker a short time after it has tripped on a fault, so that the line is restored without an operator if the fault was temporary.

#### Why it is used
About 80–90% of faults on overhead lines are **transient** (lightning flashover, birds, tree branches touching in wind). Once the arc is cleared and the air deionises, the line can be safely re-energised.

#### Operation
1. A fault occurs; the protective relay trips the breaker.
2. The breaker stays open for a **dead time** (typically 0.2–0.5 s for high-speed reclosing on EHV lines; a few seconds on distribution lines) so the fault arc path deionises.
3. The auto-reclose relay sends a close signal to the breaker.
4. If the fault has cleared (transient fault), the breaker stays closed and the relay resets after a **reclaim time**.
5. If the fault is still present (permanent fault), the protection trips the breaker again.
6. After a preset number of attempts (usually 1–3 shots), the breaker **locks out** and stays open until it is reset manually.

```text
 Fault  Trip   dead time   Reclose
   │     │◄────────────►│
 ──┴─────┘               └─────── (stays closed: transient)
                         └─ trip again → lockout (permanent)
```

#### Types
- **Single-shot / multi-shot** (number of reclosures).
- **Three-pole** (all phases) or **single-pole** (only the faulted phase, for single line-to-ground faults; keeps power transfer and stability).
- **High-speed** (below 0.5 s) and **delayed** auto reclosing.

#### Advantages
- Improves continuity of supply and reduces outage time.
- Improves transient stability of the system.
- Reduces operating staff and patrolling.

## Check yourself

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

1. Why is auto reclosure effective on overhead lines?
   A. Most faults (80-90%) are transient and clear after the arc deionises
   B. Most faults are permanent
   C. It prevents faults from occurring
   D. It replaces protective relays
   Answer: A. Lightning flashovers, birds and branches are temporary, so re-energising after a short dead time usually succeeds. (Key point: Why used)

2. What is the purpose of the dead time?
   A. To let the breaker cool
   B. To count the number of shots
   C. To let the fault arc path deionise before re-energising
   D. To reset the lockout manually
   Answer: C. The breaker stays open for a short time so the arc path deionises and the fault can clear. (Key point: Dead time)

3. What happens if the fault is permanent?
   A. The breaker stays closed
   B. The protection trips again and after the preset shots the breaker locks out
   C. The dead time is doubled indefinitely
   D. The line is earthed automatically
   Answer: B. After 1-3 unsuccessful attempts the breaker locks out until manually reset. (Key point: Permanent fault)

4. What is the benefit of single-pole auto reclosing?
   A. It works for three-phase faults only
   B. It needs no relays
   C. It opens all three phases
   D. It trips only the faulted phase for single line-to-ground faults, keeping power transfer and stability
   Answer: D. The healthy phases keep carrying power, improving transient stability. (Key point: Single-pole reclosing)