--- title: "Air blast circuit breaker" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:air-blast-cb" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/air-blast-cb site: BCT Engineering --- # Air blast circuit breaker Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2079 Chaitra, 6 marks - Video: [SGP215 Air Blast Circuit Breaker](https://www.youtube.com/watch?v=tDpE5YDNrio) — Lectures in Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/air-blast-cb - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe with neat sketch the construction and principle of operation of an air blast circuit breaker. ## Short answer An air blast circuit breaker uses compressed air at 20–30 bar blown axially across the arc to cool, sweep away ionized gas, and extinguish it at current zero. The moving contact is pushed open by air pressure; the fast blast ensures very short arcing time, suits frequent operation and auto-reclosing. ## Answer An **air blast circuit breaker (ABCB)** is a breaker in which a blast of high-pressure compressed air (about 20–30 kg/cm²) is directed on the arc to cool it, sweep away ionised gas and extinguish it at current zero. It was widely used from 66 kV to 400 kV before SF6 breakers became common. #### Construction - **Compressed air reservoir** charged by an air compressor plant. - **Blast valve** that admits air from the reservoir to the arc chamber on a trip signal. - **Arc extinction chamber** containing a fixed contact (often a nozzle) and a moving contact held closed by a spring. - **Exhaust** to atmosphere through a silencer. - **Isolating switch / resistor** connected in parallel or series in some designs to control restriking voltage. ```text Air reservoir ──► Blast valve ──► Arc chamber ──► Exhaust (20-30 bar) │ ┌─────────┐ ┌─┴────────┐ Fixed ─────│ nozzle │ ~~ │ moving │◄─ spring contact └─────────┘arc └──────────┘ air flows ───────────────► ``` #### Types (by direction of air flow) 1. **Axial blast:** air flows along the arc through a nozzle; moving contact is pushed back by the air pressure itself. Most common. 2. **Cross blast:** air flows across the arc and drives it into arc splitters, lengthening it; suited to medium voltages and high current. 3. **Radial blast:** air flows radially into the gap. #### Principle of operation 1. When the breaker trips, the blast valve opens and high-pressure air enters the arc chamber. 2. The air pressure pushes the moving contact away against its spring; an arc is drawn. 3. The fast air stream cools the arc, removes hot ionised particles and lengthens the arc. 4. The arc resistance rises and at the next current zero the arc is extinguished. 5. Fresh dry air fills the gap, so the dielectric strength builds up faster than the restriking voltage. 6. After extinction a series isolator opens (in some designs) and the blast valve closes; the contacts are held open. #### Advantages - No fire risk (no oil); suitable for indoor use. - Very fast arc extinction and short arcing time; little contact burning. - Suitable for frequent operation and repetitive auto-reclosing. - Arcing products are blown away; low maintenance of the medium. #### Disadvantages - Needs a compressor plant and air storage; costly upkeep. - Very noisy operation (needs silencers). - Prone to current chopping at low currents and high RRRV, so resistors across contacts are used. - Air leakage at fittings must be watched. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an air blast circuit breaker extinguish the arc? A. A high-pressure air stream cools the arc and sweeps away ionised gas so it goes out at current zero B. Air is evacuated so the arc cannot form at all C. Oil vapour is blown across the arc to absorb heat D. The air blast forces the arc to carry a steady DC so it dies out Answer: A. Compressed air (20-30 bar) cools, lengthens and removes ionised particles; the arc is extinguished at the next current zero. (Key point: Principle of ABCB) 2. Which part admits compressed air to the arc chamber when the breaker trips? A. Silencer B. Blast valve C. Arc splitter D. Turbulator Answer: B. The blast valve connects the air reservoir to the arc chamber on a trip signal; the silencer only quietens the exhaust. (Key point: Construction) 3. In an axial blast ABCB, what does the air flow along the arc do to the moving contact? A. Holds it closed against the spring B. Has no effect on it C. Pushes it back, opening the contacts D. Locks it in the fixed contact permanently Answer: C. In the axial blast type the air pressure itself pushes the moving contact away against its spring, drawing the arc. (Key point: Axial blast type) 4. Why are resistors connected across the contacts in some air blast breakers? A. To increase the compressor pressure B. To heat the air before the blast C. To reduce noise from the exhaust D. To control current chopping and the rate of rise of restriking voltage Answer: D. ABCBs are prone to current chopping and high RRRV, so resistors are used to limit them. 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