---
title: "Vacuum circuit breaker"
subject: "Switchgear and Protection"
chapter: 6
key: "semester_6/switchgear_and_protection:vacuum-cb-construction"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/vacuum-cb-construction
site: BCT Engineering
---

# Vacuum circuit breaker

Switchgear and Protection · Chapter 6: Circuit Breakers

- Asked in: 2075 Baisakh, 8 marks; 2077 Chaitra, 6 marks (asked 2 times)
- Video: [Working Principle of Vacuum Circuit Breaker | Step by Step Explained | TheElectricalGuy](https://www.youtube.com/watch?v=xxuuo1yjaZg) — 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/vacuum-cb-construction
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6

## Question

Describe the construction, principle of operation and application of vacuum circuit breaker.

## Short answer

A vacuum circuit breaker has contacts in a sealed vacuum bottle (10^-5 to 10^-7 torr) with metal bellows and vapor shield. Arc burns in metal vapor and extinguishes at first current zero as vapor condenses rapidly, restoring dielectric strength. Used for 3.3–33 kV, capacitor switching, motor switching, frequent operations.

## Answer

A **vacuum circuit breaker (VCB)** is a breaker whose contacts open inside a sealed chamber evacuated to about 10⁻⁵ to 10⁻⁷ torr. Since vacuum has very high dielectric strength and no gas to ionise, the arc (formed only of metal vapour) is extinguished at the first current zero.

#### Construction
```text
        Fixed terminal
            |
     +------+------+   ceramic/glass
     |   [fixed]   |   envelope
     |  ( shield ) |   metal vapour
     |   [moving]  |   condensing shield
     +------+------+
          bellows      metal bellows
            |
      Moving stem --> operating mechanism
```
1. **Vacuum interrupter (bottle):** sealed envelope of glass or ceramic, with metal end plates.
2. **Fixed and moving contacts:** made of copper–chromium (Cu-Cr) alloy, with spiral or axial-magnetic-field (AMF) shaped faces to keep the arc moving and diffuse.
3. **Metal bellows:** stainless-steel bellows let the moving contact travel (about 8–20 mm) without breaking the vacuum.
4. **Vapour condensing shield:** metal shield around the contacts on which the metal vapour condenses, protecting the envelope insulation.
5. **Operating mechanism:** spring-charged (motor-wound) mechanism with closing and trip coils.
6. **Frame, insulators and terminals.**

#### Principle of operation
1. When the trip coil is energised, the mechanism separates the contacts in vacuum.
2. At separation, the last point of contact is heated and **metal vapour** is released; the arc burns in this vapour (there is no gas).
3. Because of contact shape, the arc is driven around or spread over the contact face, keeping it diffuse and limiting erosion.
4. As the current approaches zero, vapour production falls; the vapour **condenses rapidly** on the contacts and shield within microseconds.
5. At current zero the gap recovers its very high dielectric strength within a few microseconds (much faster than in oil or air), so the arc does not restrike. Interruption occurs at the **first current zero**.

#### Applications
- Medium voltage systems: **3.3 kV to 33 kV** (mainly 11 kV and 33 kV indoor/outdoor switchgear in Nepal's distribution substations).
- Capacitor bank and reactor switching (restrike-free).
- Motor switching, arc furnace switching (frequent operation).
- Railway traction, mining and ring main units.

#### Advantages (brief)
- Compact, silent, no fire or explosion risk, no gas/oil maintenance.
- Very long contact life; suitable for frequent operation.
- Fast recovery; handles high RRRV.
- Limitation: current chopping can produce overvoltage on inductive loads (reduced by Cu-Cr contacts or surge suppressors); economic mainly up to about 36 kV.

## Check yourself

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

1. Why is a vacuum interrupter good at extinguishing the arc?
   A. Arc burns in oil
   B. The arc consists only of metal vapour and vacuum has very high dielectric strength
   C. It uses SF6
   D. It relies on an air blast
   Answer: B. With no gas to ionise, the vapour condenses quickly and the gap recovers at the first current zero. (Key point: Arc medium)

2. What is the function of the metal bellows?
   A. Cool the contacts
   B. Let the moving contact travel without breaking the vacuum
   C. Condense vapour
   D. Hold the spring
   Answer: B. Stainless-steel bellows allow 8-20 mm movement while keeping the seal. (Key point: Bellows)

3. What does the vapour condensing shield do?
   A. Limits current
   B. Collects metal vapour so the envelope insulation stays clean
   C. Drives the contacts
   D. Produces the vacuum
   Answer: B. Vapour condenses on the shield, protecting the glass or ceramic wall. (Key point: Vapour shield)

4. Where are VCBs mainly used?
   A. EHV 400 kV lines
   B. Medium voltage 3.3-33 kV, capacitor, motor and furnace switching
   C. Only LV 230 V
   D. Only generators above 100 kV
   Answer: B. They suit frequent operation and restrike-free duty up to about 36 kV. (Key point: Application)

## Related in Switchgear and Protection

- [HVDC circuit breaker operation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahvdc-circuit-breaker) — Switchgear and Protection, chapter 6
- [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
- [SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asf6-cb-construction) — Switchgear and Protection, chapter 6, asked 2×
- [Air blast circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aair-blast-cb) — Switchgear and Protection, chapter 6
- [Arc extinction in vacuum circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-arc-extinction) — Switchgear and Protection, chapter 6
- [Minimum oil circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aminimum-oil-cb) — Switchgear and Protection, chapter 6

## Related in other subjects

- [Vacuum versus SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-vs-sf6) — Power Plant Design, chapter 4
- [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2×
- [Choice of generator circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-circuit-breaker-choice) — Power Plant Design, chapter 4, asked 2×
- [Construction and operation of synchronous generator](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-generator-construction-principle) — Electric Machines II, chapter 1