--- title: "Puffer type SF6 circuit breaker" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:puffer-sf6-cb" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/puffer-sf6-cb site: BCT Engineering --- # Puffer type SF6 circuit breaker Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2074 Bhadra, 8 marks - Video: [Puffer Type SF6 Circuit Breaker](https://www.youtube.com/watch?v=wH7ixCoAsYM) — AISSMS INSTITUTE OF INFORMATION TECHNOLOGYY - IOIT - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/puffer-sf6-cb - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe the construction and working of a puffer type SF6 circuit breaker. Also mention the properties of SF6 gas which make it a good arc quenching medium. ## Short answer A cylinder on the moving contact compresses SF6 against a fixed piston, blowing gas through a nozzle to cool and deionize the arc at current zero. SF6 is electronegative, has 2.5–3 times air's dielectric strength, rapid recovery, high thermal conductivity, enabling breakers for 33–765 kV. ## Answer A **puffer type SF6 circuit breaker** is a single-pressure SF6 breaker in which the gas flow needed to quench the arc is produced by the **motion of the moving contact itself**: a cylinder attached to the moving contact compresses gas against a fixed piston and blows it through a nozzle onto the arc. #### Construction ```text Fixed contact (hollow, arcing tip) | | =====|nozzle|===== PTFE nozzle | arc | +-------[moving]------+ | puffer cylinder | moves down with contact | (gas compressed) | +====[fixed piston]===+ | operating rod -> spring/hydraulic mechanism Whole unit in SF6 at about 5-6 bar ``` 1. Interrupter enclosure (porcelain or earthed metal tank) filled with SF6 at one pressure (about 5–6 bar). 2. **Fixed contact** with arcing tip, and **main contacts** that carry normal current. 3. **Moving contact** fixed to a **puffer cylinder**; a stationary **piston** sits inside the cylinder. 4. Insulating **PTFE nozzle** that guides the gas jet along the arc. 5. Operating mechanism, gas density monitor, and an absorbent (alumina) for decomposition products. #### Working 1. On tripping, the moving contact and cylinder move away from the fixed contact. 2. The main contacts part first; the current transfers to the arcing contacts, which then part and draw an arc. 3. As the cylinder moves over the fixed piston, the gas inside is **compressed** (pressure rises 2–3 times). 4. When the arcing contact leaves the nozzle throat, the compressed SF6 is **blown axially** through the nozzle across the arc. 5. The gas cools the arc and its electronegative molecules capture electrons; at **current zero** the gap de-ionises rapidly and dielectric strength builds up faster than the restriking voltage. 6. The used gas recombines and remains in the enclosure; no compressor is needed. 7. Modern **self-blast (auto-puffer)** designs also use arc heat to raise pressure, reducing the operating energy. #### Properties of SF6 that make it a good arc quencher 1. **Electronegative:** captures free electrons to form heavy, slow negative ions, so arc conductivity falls very quickly after current zero. 2. **High dielectric strength:** about 2.5–3 times that of air at the same pressure; at a few bar it is comparable to oil. 3. **Fast dielectric recovery:** strength builds up about 100 times faster than in air after current zero. 4. **Good heat transfer:** high thermal conductivity in the 2000–3000 K range cools the arc boundary. 5. **Chemically stable and inert:** non-flammable, non-toxic and non-corrosive in pure form; dissociated products recombine after the arc. 6. **Low arc voltage and energy,** so low contact erosion. 7. **Heavy gas** (about 5 times as dense as air) that liquefies easily under pressure, so it can be stored and transported as a liquid in cylinders. These properties allow short arcing time, compact interrupters and use from 33 kV up to 765 kV and in GIS. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a puffer breaker, what produces the gas blast? A. A separate compressor B. A high-pressure reservoir C. A puffer cylinder moving with the contact compresses gas against a fixed piston D. Heating by the main contacts Answer: C. The motion of the moving contact itself compresses SF6 and blows it through the nozzle, so no compressor is needed. (Key point: Source of gas flow) 2. A puffer breaker uses SF6 at A. a single pressure of about 5-6 bar B. two pressures of 14 bar and 2.8 bar C. atmospheric pressure D. vacuum Answer: A. It is a single-pressure design; gas is compressed 2-3 times only during the stroke. (Key point: Pressure arrangement) 3. When is the compressed SF6 blown on the arc? A. Before the contacts part B. After the arc has died C. When the arcing contact leaves the nozzle throat, blown axially across the arc D. Only when the breaker is closed Answer: C. Once the arcing contact clears the nozzle throat, the gas jet flows axially through it onto the arc. (Key point: Timing of gas blast) 4. Which property makes SF6 a good arc quencher? A. It is electronegative and captures free electrons, giving fast dielectric recovery B. It is highly flammable C. It has low dielectric strength D. It is lighter than air Answer: A. It is electronegative, has dielectric strength 2.5-3 times air, good heat transfer and is chemically stable. (Key point: Properties of SF6) ## Related in Switchgear and Protection - [Compare MOCB/BOCB and ABCB/puffer SF6](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acompare-mocb-bocb-abcb-sf6) — Switchgear and Protection, chapter 6 - [SF6 CB: properties and limitations](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asf6-cb-properties-limitations) — Switchgear and Protection, chapter 6 - [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× - [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× - [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 ## 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 - [Gas flames, arc welding and heat input](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aoxyacetylene-flames-smaw-heat-input-numerical) — Manufacturing and Production Processes, chapter 12 - [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×