--- title: "Arc extinction in vacuum circuit breakers" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:vacuum-cb-arc-extinction" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/vacuum-cb-arc-extinction site: BCT Engineering --- # Arc extinction in vacuum circuit breakers Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2072 Asoj, 8 marks - 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-arc-extinction - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe arc extinction process in vacuum circuit breakers. Also explain the preferred voltage levels of its application and its advantages over OCB and air blast circuit breakers. ## Short answer Metal-vapour arc is extinguished at current zero as vapour condenses on contacts and shield, vacuum dielectric strength recovers in microseconds. Preferred 3.3–36 kV. Advantages: no fire/explosion risk, silent, minimal maintenance, compact, long contact life. ## Answer A **vacuum circuit breaker** interrupts current between contacts in a sealed vacuum of about 10⁻⁵ to 10⁻⁷ torr. With no gas present, the arc is a **metal-vapour arc** that is extinguished at the first current zero because the vapour condenses almost instantly. #### Arc extinction process ```text [fixed contact] | | | metal vapour + ions |~|~| (arc in vapour) [moving contact] -> separates condensing shield around gap ``` 1. **Contact separation:** the moving Cu-Cr contact is pulled away by the mechanism. Contact pressure falls; the last bridging point melts and evaporates. 2. **Metal-vapour arc:** the current continues through the ionised metal vapour emitted from **cathode spots**. At low current the arc is **diffuse** (many cathode spots spread over the contact face). 3. **Arc control:** at high currents the arc tends to constrict. Specially shaped contacts (spiral/radial-magnetic-field or axial-magnetic-field) make the arc rotate or stay diffuse, preventing local overheating and erosion. 4. **Current approaching zero:** vapour production falls with current; the cathode spots die out. 5. **Condensation:** the vapour and ions spread out and condense on the contacts and on the **metal shield** within microseconds, so the gap again becomes a near-perfect vacuum. 6. **Dielectric recovery:** a vacuum gap has very high breakdown strength; it recovers within a few microseconds, faster than the restriking voltage rises. The arc is cleared at the **first current zero** with a short contact gap (about 10–20 mm). 7. **Current chopping:** if the arc becomes unstable before natural zero at very low currents, the current may be chopped, causing overvoltage in inductive circuits; Cu-Cr contact material keeps the chopping current very low (a few amperes). #### Preferred voltage levels - Most economical and widely used for **medium voltage, 3.3 kV to 36 kV** (11 kV and 33 kV in Nepal's distribution system). - For higher voltages, several interrupters must be connected in series, so SF6 breakers are preferred above about 36 kV (though VCBs up to 72.5 kV and some 145 kV designs exist). #### Advantages over OCB and ABCB | Point | VCB | OCB | Air blast CB | |---|---|---|---| | Arc medium | Vacuum, sealed | Oil, needs replacement | Compressed air system | | Fire/explosion risk | None | Oil fire risk | None but high-pressure air | | Noise | Silent | Quiet | Very noisy blast | | Maintenance | Almost none | Frequent oil filtration | Compressor upkeep | | Size/weight | Compact, light | Bulky tank | Large with air plant | | Contact life | Very long (10,000+ ops) | Short | Medium | | Arc time | Short, first current zero | Longer | Short | | Frequent switching | Excellent | Poor | Fair | Other advantages: no exhaust of gas or flame, suitable for any position, unaffected by atmosphere and altitude, handles high RRRV and capacitor switching without restrike. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What sustains the arc in a vacuum interrupter? A. Ionised air B. Ionised SF6 C. Ionised metal vapour from cathode spots on the contacts D. Hot oil vapour Answer: C. With no gas present, the arc is a metal-vapour arc from cathode spots. (Key point: Nature of vacuum arc) 2. Why is the arc cleared at the first current zero? A. Compressed gas is blown across it B. Vapour production falls and the vapour condenses on contacts and shield within microseconds, restoring vacuum C. Oil cools the gap D. The contacts are pressed together Answer: B. The vapour condenses on the metal shield and contacts, so the gap recovers its high dielectric strength rapidly. (Key point: Why arc is extinguished quickly) 3. VCBs are most economical and widely used at A. 400 kV and above B. medium voltage, 3.3 kV to 36 kV C. below 400 V only D. 765 kV Answer: B. At higher voltages several interrupters are needed in series, so SF6 is preferred above about 36 kV. (Key point: Preferred voltage level) 4. Which is an advantage of VCB over OCB and air blast CB? A. Requires oil filtration B. Needs a compressor plant C. No fire risk, silent, almost no maintenance and long contact life D. Short life under frequent switching Answer: C. VCBs are sealed, compact, silent and excellent for frequent switching. (Key point: Advantages over OCB and ABCB) ## Related in Switchgear and Protection - [Air blast circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aair-blast-cb) — Switchgear and Protection, chapter 6 - [Compare bulk oil, minimum oil, air blast CBs](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acompare-bocb-mocb-abcb) — Switchgear and Protection, chapter 6 - [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2× - [Classification of circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-classification) — Switchgear and Protection, chapter 6 - [Arcing contacts, arc runners and splitters](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aarcing-contacts-arc-runners) — Switchgear and Protection, chapter 6 - [Fault clearing process with trip circuit](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afault-clearing-trip-circuit) — Switchgear and Protection, chapter 1 ## Related in other subjects - [Typical voltage levels and why three-phase](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-voltage-levels-three-phase) — Power System Analysis I, chapter 1 - [SP, DP and TP miniature circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asp-dp-tp-mcb) — Industrial Power Distribution and Illumination, chapter 3 - [AC voltage controller RL load extinction angle](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aacvc-rl-extinction-angle) — Power Electronics, chapter 6 - [Why NOS preferred over DOS](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Anos-vs-dos-realization) — Distributed System, chapter 3, asked 2×