--- title: "SF6 circuit breaker" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:sf6-cb-construction" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/sf6-cb-construction site: BCT Engineering --- # SF6 circuit breaker Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2071 Bhadra, 8 marks; 2073 Bhadra, 6 marks (asked 2 times) - Video: [SF6 Circuit Breaker Working Principle | Explained | TheElectricalGuy](https://www.youtube.com/watch?v=4c_eJrh0F68) — 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/sf6-cb-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe with a neat sketch the construction and principle of operation of a SF6 circuit breaker. ## Short answer SF6 circuit breaker uses electronegative SF6 gas under pressure (2.8–5 bar) in an interrupter with fixed and moving contacts, nozzle, and gas system. On opening, SF6 is blown axially along the arc, capturing electrons to form heavy ions, de-ionizing the arc at current zero. Applied from 33 kV to 765 kV, GIS. ## Answer An **SF6 circuit breaker** is a breaker in which the contacts separate in sulphur hexafluoride (SF6) gas under pressure. SF6 is **electronegative**: it captures free electrons to form heavy negative ions, which rapidly de-ionises the arc path and restores dielectric strength after current zero. #### Construction ```text Fixed contact (hollow) +---------[ ]---------+ | SF6 gas at about | | 2.8 to 5 bar | | arc -> nozzle | | [moving] | | insulating nozzle | +----------+------------+ | HP SF6 tank --valve-- LP tank --compressor | operating mechanism ``` Main parts: 1. **Interrupter unit:** fixed (hollow) contact and moving contact with **arcing probes/contacts** and **current-carrying main contacts**; an insulating (PTFE) **nozzle** directs gas flow. 2. **Gas system:** in the two-pressure design, a high-pressure reservoir (about 14 bar) and low-pressure chamber (about 2.8 bar), with compressor, filters, valves and gas monitoring (density switches). Modern **puffer** breakers use a single pressure (about 5–6 bar). 3. **Arc-quenching chamber** of porcelain or metal enclosure (live tank, dead tank or GIS). 4. **Operating mechanism:** spring, hydraulic or pneumatic. 5. **Alarms and lockouts** for low gas pressure, and **absorbers** (activated alumina) to take up decomposition products. #### Principle of operation 1. In the closed position the contacts are surrounded by SF6 at low pressure. 2. On a trip signal the moving contact is pulled away; an arc forms between the arcing contacts. 3. At the same time a valve opens (two-pressure type) or the puffer piston compresses gas, and **SF6 is blown axially along the arc** through the nozzle. 4. SF6 molecules capture the free electrons and form heavy, slow negative ions (SF6⁻, SF5⁻), so the conductivity of the arc column falls sharply. 5. The gas also cools the arc (high thermal conductivity near arc temperatures). 6. At **current zero** the arc path de-ionises quickly and the dielectric strength rises faster than the restriking voltage, so the arc is extinguished. 7. The gas returns to the low-pressure side and is recompressed and reused; it recombines after the arc, so little is lost. #### Applications - 33 kV to 765 kV outdoor switchyards and gas-insulated substations (GIS). - 132 kV and 220 kV substations in Nepal's grid widely use SF6 breakers. #### Advantages - Excellent arc quenching and dielectric strength (about 2.5–3 times air); silent operation. - Compact, closed system, no moisture or fire risk; low maintenance. - No current chopping problem; suitable for high RRRV. - Disadvantage: SF6 is a strong greenhouse gas and needs careful handling; costly, needs leak monitoring. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is SF6 effective at extinguishing the arc? A. It captures free electrons to form heavy negative ions, lowering arc conductivity B. It is a very light gas that carries heat away by convection only C. It reacts chemically with the contacts to absorb the arc energy D. It is a good conductor so it shorts out the arc Answer: A. SF6 is electronegative; it captures free electrons and forms heavy, slow negative ions, so the arc path de-ionises rapidly after current zero. (Key point: Electronegativity of SF6) 2. In the principle of operation, how is the arc quenched after contact separation? A. Oil is forced across the contacts B. SF6 is blown axially along the arc through the nozzle C. The contacts are moved into a vacuum chamber D. The arc is lengthened in air by magnetic blow-out coils Answer: B. A valve (two-pressure) or puffer piston forces SF6 axially along the arc through the insulating nozzle. (Key point: Gas flow along the arc) 3. Which statement about gas pressures is correct? A. Both types use about 14 bar at all times B. Two-pressure types use a single 5-6 bar pressure, puffer types use two C. Two-pressure types have a high-pressure reservoir (about 14 bar) and low-pressure chamber (about 2.8 bar); puffer types use a single pressure of about 5-6 bar D. Puffer types need a compressor and high-pressure reservoir Answer: C. The older two-pressure design needs HP and LP systems with a compressor, while modern puffer breakers work at one pressure. (Key point: Two-pressure vs puffer design) 4. When is the arc finally extinguished in the SF6 breaker? A. When dielectric strength rises faster than the restriking voltage after current zero B. When the restriking voltage rises faster than the dielectric strength C. When the gas pressure is released to atmosphere D. When the main contacts re-close Answer: A. Fast de-ionisation lets dielectric strength build faster than the restriking voltage at current zero, so the arc does not restrike. (Key point: Condition for extinction at current zero) ## Related in Switchgear and Protection - [Puffer type SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apuffer-sf6-cb) — Switchgear and Protection, chapter 6 - [HVDC circuit breaker operation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahvdc-circuit-breaker) — 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× - [Air blast circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aair-blast-cb) — 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 - [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 ## 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