--- title: "Classification of circuit breakers" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:cb-classification" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-classification site: BCT Engineering --- # Classification of circuit breakers Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2070 Bhadra, 8 marks - Video: [Types of Circuit Breaker with Detailed Classifications | TheElectricalGuy](https://www.youtube.com/watch?v=QePD4AmNuU0) — 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/cb-classification - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question What is the most important basis for the classification of circuit breaker? How they are classified? Describe briefly. Mention their field of application. ## Short answer The most important classification basis is the arc quenching medium. Circuit breakers are classified as air break (low voltage, arc splitters), oil (bulk/minimum, distribution), air blast (compressed air, EHV), SF6 (puffer type, transmission, GIS), and vacuum (medium voltage, indoor). Applications range from LV panels to 765 kV substations. ## Answer The **most important basis** for classifying circuit breakers is the **medium used for arc extinction**, because the medium decides the breaking capacity, voltage range, size, speed and maintenance of the breaker. #### Classification by arc quenching medium 1. **Air break circuit breaker** - Arc is drawn in air at atmospheric pressure and lengthened by arc runners, blow-out coils and arc splitters (high resistance method). - *Application:* low and medium voltage up to about 11 kV; LV switchboards, motor control, DC circuits. 2. **Oil circuit breaker** - Arc in oil decomposes it into hydrogen gas which cools and deionises the arc. - **Bulk oil:** oil for quenching and insulation; up to 33 kV (old installations). - **Minimum oil:** oil for quenching only, porcelain for insulation; 3.3–132 kV. - *Application:* outdoor distribution substations (now being replaced). 3. **Air blast circuit breaker** - Compressed air (20–30 bar) blown axially or across the arc. - *Application:* 66–400 kV transmission; where frequent operation and fast auto-reclosing are needed; arc furnaces. 4. **SF6 circuit breaker** - Electronegative SF6 gas blown over the arc (puffer or double-pressure type). - *Application:* 33–765 kV substations, GIS (gas-insulated switchgear). 5. **Vacuum circuit breaker** - Contacts open in a vacuum of 10⁻⁶ to 10⁻⁷ torr; the arc is a metal-vapour arc that vanishes at current zero. - *Application:* 3.3–33 kV indoor switchgear, capacitor and motor switching, railways. #### Other bases of classification - **By voltage:** low voltage (below 1 kV), medium voltage (1–72.5 kV), high voltage (72.5–245 kV) and extra high voltage (above 245 kV). - **By location:** indoor and outdoor breakers. - **By operating mechanism:** spring, pneumatic, hydraulic, solenoid. - **By external design:** dead tank (enclosure at earth potential) and live tank (interrupter at line potential). - **By interrupting principle:** high resistance interruption (DC and LV) and low resistance or current-zero interruption (AC HV). #### Summary table | Type | Voltage range | Typical use | |---|---|---| | Air break | up to 11 kV | LV/MV panels, DC | | Bulk oil | up to 33 kV | Old distribution | | Minimum oil | 3.3–132 kV | Distribution substations | | Air blast | 66–400 kV | Transmission, arc furnaces | | SF6 | 33–765 kV | Transmission, GIS | | Vacuum | 3.3–33 kV | Indoor MV switchgear | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the most important basis for classifying circuit breakers? A. The medium used for arc extinction B. The location (indoor or outdoor) C. The operating mechanism D. The colour of the housing Answer: A. The quenching medium decides breaking capacity, voltage range, size, speed and maintenance. (Key point: Basis of classification) 2. How does an air break CB extinguish the arc, and where is it used? A. By SF6 gas, at 33-765 kV B. By lengthening, cooling and splitting the arc in air, in LV/MV up to about 11 kV and DC C. By vacuum, at 3.3-33 kV D. By compressed air at 20-30 bar, at 66-400 kV Answer: B. Air break breakers use the high resistance method in atmospheric air, for low/medium voltage and DC circuits. (Key point: Air break CB) 3. Which breaker is used for 3.3-33 kV indoor switchgear, capacitor and motor switching? A. Air blast B. Bulk oil C. Vacuum D. Minimum oil Answer: C. Vacuum breakers extinguish a metal-vapour arc at current zero and suit indoor MV applications. (Key point: Vacuum CB application) 4. Which pairing of breaker and voltage range is correct? A. Bulk oil: 66-400 kV B. Vacuum: 66-400 kV C. Air break: 33-765 kV D. SF6: 33-765 kV and GIS Answer: D. SF6 breakers serve 33-765 kV substations and gas-insulated switchgear; air blast serves 66-400 kV. (Key point: Application of air blast and SF6) ## Related in Switchgear and Protection - [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 - [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× - [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 - [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 - [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 - [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 - [Reactor to protect circuit breaker of small station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-protect-circuit-breaker) — Power Plant Equipment, 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×