--- title: "Compare bulk oil, minimum oil, air blast CBs" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:compare-bocb-mocb-abcb" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/compare-bocb-mocb-abcb site: BCT Engineering --- # Compare bulk oil, minimum oil, air blast CBs Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2070 Magh, 8 marks - Video: [High Voltage AC Circuit Breaker Types and Working](https://www.youtube.com/watch?v=Ku1fnkc_t-g) — Electrical lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/compare-bocb-mocb-abcb - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Compare the merits and demerits of bulk oil circuit breakers, minimum oil circuit breakers and air blast circuit breakers. ## Short answer Bulk oil CB uses large oil for quenching and insulation, heavy, fire risk, slow; minimum oil CB uses 10% oil in insulated chambers, lighter, lower fire risk but oil carbonizes quickly. Air blast CB uses compressed air, no fire risk, very fast, suitable for auto-reclosing but requires a compressor and is noisy. ## Answer Bulk oil (BOCB), minimum oil (MOCB) and air blast (ABCB) circuit breakers differ in the quenching medium and in what gives insulation to earth. BOCB uses a large earthed oil tank, MOCB uses a little oil in insulated live chambers, and ABCB uses compressed air. #### Merits and demerits | Breaker | Merits | Demerits | |---|---|---| | Bulk oil | Simple and reliable; oil absorbs arc energy and gives good insulation; arc produces hydrogen which cools well; CTs can be mounted inside bushings | Large oil volume, fire and explosion risk; heavy and bulky; slow; oil carbonises and needs maintenance; not good for frequent operation | | Minimum oil | Needs only about 10% of oil; small, light, cheaper; less fire risk; good for indoor and outdoor use up to 132 kV | Oil deteriorates quickly (carbonisation); frequent oil change; not suited for frequent switching or high-speed reclosing; explosion risk if oil level is low | | Air blast | No fire risk; very fast arc extinction and short arc time; low contact erosion; suitable for repeated operation and auto-reclosing; clean medium | Needs compressor and air storage plant; very noisy; current chopping and high overvoltages need resistors; sensitive to RRRV; air leakage | #### Comparison of characteristics | Point | Bulk oil | Minimum oil | Air blast | |---|---|---|---| | Quenching medium | Oil | Oil | Compressed air (20–30 bar) | | Insulation to earth | Oil in earthed tank | Porcelain/solid | Porcelain/air | | Medium quantity | Very large | Small | Air from reservoir | | Speed of operation | Slow | Medium | Fast | | Fire hazard | High | Low | None | | Maintenance | Oil filtering, high | Oil change often | Compressor upkeep | | Noise | Low | Low | Very high | | Voltage range | Up to 33 kV (old up to 132 kV) | 3.3–132 kV | 66–400 kV | | Auto-reclosing | Poor | Poor | Very good | #### Summary - BOCB is cheap but bulky and hazardous; now mostly obsolete. - MOCB saves oil and space; popular for medium voltage before vacuum CBs. - ABCB is best for EHV and frequent switching where speed matters, at the cost of the air plant and noise. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What provides insulation to earth in a bulk oil CB compared with a minimum oil CB? A. Oil in an earthed tank in BOCB; porcelain/solid insulation in MOCB B. Porcelain in BOCB; oil in MOCB C. Compressed air in both D. SF6 in BOCB; oil in MOCB Answer: A. BOCB uses the oil in a large earthed tank for insulation, whereas MOCB uses solid insulation and keeps its interrupter live. (Key point: Insulation to earth) 2. Which statement about fire hazard is correct? A. Air blast has the highest fire risk B. Minimum oil has no fire risk C. Bulk oil and air blast have the same fire risk D. Bulk oil has high fire risk, minimum oil low, and air blast none Answer: D. Oil volume determines fire risk: large in BOCB, small in MOCB, and no oil at all in ABCB. (Key point: Fire risk) 3. Which breaker is best suited to frequent operation and auto-reclosing? A. Bulk oil B. Air blast C. Minimum oil D. All three equally Answer: B. ABCB has fast extinction, fresh air each time and little contact erosion; oil breakers carbonise and are poor for reclosing. (Key point: Auto-reclosing suitability) 4. Which is a demerit of the air blast breaker? A. Large oil volume B. Slow operation C. It needs a compressor plant and is very noisy D. It cannot be used above 33 kV Answer: C. ABCB needs compressor and air storage and has very loud exhaust, besides current chopping and leak issues. (Key point: ABCB demerits) ## Related in Switchgear and Protection - [Compare MOCB/ABCB and ABCB/bulk oil CB](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acompare-mocb-abcb-bocb) — Switchgear and Protection, chapter 6 - [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 - [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 - [Cross jet explosion pot in bulk oil CB](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Across-jet-explosion-pot) — 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 - [Forward vs reverse engineering merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aforward-reverse-engineering-merits) — Object Oriented Analysis and Design, chapter 4, asked 3× - [Cofferdam types, uses, merits and demerits](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acofferdam-types-merits) — Foundation Engineering, chapter 5, asked 7× - [Incineration: merits and demerits](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asolid-waste-incineration) — Sanitary Engineering, chapter 10, asked 7× - [Merits and demerits of river and ridge routes](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ariver-ridge) — Transportation Engineering I, chapter 5, asked 7×