--- title: "Minimum oil circuit breaker" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:minimum-oil-cb" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/minimum-oil-cb site: BCT Engineering --- # Minimum oil circuit breaker Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2072 Magh, 8 marks - Video: [SGP213 Minimum Oil Circuit Breaker](https://www.youtube.com/watch?v=W8KO04U1xvM) — 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/minimum-oil-cb - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Describe with a neat sketch the principle of operation of a minimum oil circuit breaker. Why is it called so? ## Short answer A minimum oil circuit breaker uses a small quantity of oil solely for arc extinction, while porcelain provides insulation to earth. The arc decomposes oil into hydrogen gas; a turbulator directs the gas flow to cool and deionize the gap. It is called “minimum oil” because the oil volume is about 10% of a bulk oil breaker, used only for quenching, not insulation. ## Answer A **minimum oil circuit breaker (MOCB)** is an oil circuit breaker in which oil is used **only for arc extinction**, while insulation between live parts and earth is provided by solid insulation (porcelain or organic material). It is used from 3.3 kV to about 132 kV. #### Why it is called "minimum oil" In a bulk oil CB a large earthed steel tank is filled with oil, which serves both for arc quenching and for insulating live parts from the earthed tank, so a lot of oil is needed. In the MOCB the interrupter is housed in a small insulated chamber, which is itself kept live, so only the small quantity of oil needed for arc quenching is used (about 10% of a bulk oil CB of the same rating). Hence the name "minimum" or "small" oil circuit breaker. #### Construction - **Supporting chamber:** porcelain, filled with oil, physically separated from the interrupting chamber; it carries the breaker and gives insulation to earth. - **Circuit breaking chamber:** a porcelain/insulating tube holding the fixed contact, a moving contact (rod), and a **turbulator** (side-vent or axial-vent arc control device). - **Top chamber:** a metal chamber with an oil separator to cool and separate gases from oil; a vent releases gas. - **Operating mechanism:** spring or solenoid, connected to the moving contact through an insulating rod. ```text ┌──────────────┐ Top chamber │ gas separator│──► vent ├──────────────┤ │ fixed contact│ │ ┌──┴──┐ │ Breaking chamber │ │turbu│ │ (oil, porcelain) │ │lator│ │ │ └──┬──┘ │ │ moving contact│ ├──────┼───────┤ │ support │ Supporting chamber │ chamber (oil)│ (porcelain) └──────┼───────┘ operating rod ``` #### Principle of operation 1. In the closed position the moving contact rod is held engaged in the fixed contact. 2. On a trip signal the mechanism pulls the moving contact down, and an arc is struck between the contacts in oil. 3. The arc heat decomposes oil into gas (mainly **hydrogen, about 70%**) forming a gas bubble at high pressure. 4. The turbulator directs this gas and oil flow through side vents (cross blast) or axially across the arc, which cools it, lengthens it and deionises the gap. 5. Hydrogen has high thermal conductivity, so the arc loses heat fast; at current zero the arc is extinguished and fresh oil fills the gap. 6. Gases rise to the top chamber where oil is separated and the gas is vented. #### Advantages - Very small oil requirement, less fire risk, lighter and smaller. - Lower cost and less space than bulk oil CB. - Suitable for indoor and outdoor substations. #### Disadvantages - Small oil volume gets carbonised quickly, so frequent oil replacement is needed. - Not suitable for frequent operation or high-speed reclosing. - Danger of explosion if oil level falls. - Difficult to remove gases from the small space. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the minimum oil circuit breaker so called? A. It uses oil only for arc quenching, while insulation to earth is by solid porcelain, so little oil is needed B. It uses oil only for insulation, not for arc quenching C. It contains no oil at all D. It has a small tank that is earthed and filled with oil for both insulation and quenching Answer: A. The interrupter sits in a small live insulated chamber so oil serves only for quenching (about 10% of a bulk oil CB). (Key point: Why called minimum oil) 2. Which gas, produced by oil decomposition, mainly cools the arc in an MOCB? A. Oxygen B. Nitrogen C. Hydrogen D. Carbon dioxide Answer: C. Arc heat decomposes oil into about 70% hydrogen, which has high thermal conductivity and cools the arc quickly. (Key point: Arc quenching gas) 3. What is the function of the turbulator in the breaking chamber? A. It stores spring energy for closing B. It directs gas and oil flow across or along the arc to cool, lengthen and deionise it C. It separates gas from oil in the top chamber D. It insulates the moving contact from earth Answer: B. The turbulator (side-vent or axial-vent) channels the high-pressure gas/oil flow over the arc. (Key point: Turbulator) 4. Why does an MOCB need frequent oil replacement? A. The oil evaporates through the vent B. Porcelain absorbs the oil C. The small oil volume gets carbonised quickly D. Hydrogen dissolves the oil Answer: C. With little oil, arcing carbonises it quickly, so it deteriorates and must be changed often. 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