--- title: "Compare MOCB/ABCB and ABCB/bulk oil CB" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:compare-mocb-abcb-bocb" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/compare-mocb-abcb-bocb site: BCT Engineering --- # Compare MOCB/ABCB and ABCB/bulk oil CB Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2071 Magh, 8 marks - Video: [Basic Differences between 4 types of Circuit Breakers: Vacuum, Oil, SF6 & AirBlast CB in HINDI](https://www.youtube.com/watch?v=SdDOw8N5a4o) — Study 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/compare-mocb-abcb-bocb - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Compare the performance and characteristics of i) minimum oil circuit breaker and air blast CB ii) Air-blast circuit breaker and bulk oil circuit breaker. ## Short answer i) Minimum oil CB uses oil, has moderate speed and some fire hazard; air blast CB uses compressed air, is very fast, fire-free, but noisy and needs a compressor. ii) Air blast CB is fast, compact, no fire hazard, but requires an air plant; bulk oil CB is heavy, slow, high fire risk, but simple and needs no auxiliary equipment. ## Answer The comparison is based on how each breaker quenches the arc and the practical consequences in speed, safety, maintenance and application. #### i) Minimum oil CB vs Air blast CB | Point | Minimum oil CB | Air blast CB | |---|---|---| | Arc quenching medium | Small quantity of oil; arc produces hydrogen gas | Compressed air at 20–30 bar | | Insulation to earth | Porcelain/solid material | Porcelain and air | | Arc extinction | By gas bubble and turbulator (cross/axial vent) | By air blast cooling and sweeping the arc | | Arcing time | Longer | Very short | | Speed | Medium | High | | Fire risk | Small (some oil present) | None | | Repeated operation / auto-reclose | Not suitable | Very suitable | | Auxiliary plant | None | Compressor and air receiver | | Noise | Quiet | Very noisy | | Current chopping | Low | Significant; resistors needed | | Maintenance | Oil change after a few fault clearances | Compressor and valves | | Range | 3.3–132 kV | 66–400 kV | #### ii) Air blast CB vs Bulk oil CB | Point | Air blast CB | Bulk oil CB | |---|---|---| | Medium | Compressed air | Large volume of oil in earthed steel tank | | Insulation | Air/porcelain | Oil insulates contacts from tank | | Size and weight | Smaller, lighter for the same rating | Very heavy and bulky | | Fire and explosion hazard | Nil | High | | Speed of operation | Fast (2–3 cycles) | Slow (5–8 cycles) | | Arc energy and contact erosion | Low | High | | Medium after operation | Fresh air every time | Oil carbonises, needs filtering | | Frequent operation | Suitable | Unsuitable | | Noise | Very high | Low | | Auxiliary plant | Compressor needed | None | | Cost | High (with air plant) | Low | | Application | EHV transmission, frequent switching | Old distribution systems, up to 33 kV | #### Remarks - Oil breakers are simple and need no auxiliary plant, but carry fire risk and are slow. - Air blast breakers are fast and clean, but costly and noisy; they have largely been replaced by SF6 breakers at EHV. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which ordering of speed of operation is correct (fastest first)? A. Bulk oil, minimum oil, air blast B. Minimum oil, air blast, bulk oil C. Air blast, bulk oil, minimum oil D. Air blast, minimum oil, bulk oil Answer: D. Air blast is fast (2-3 cycles), MOCB medium and BOCB slow (5-8 cycles). (Key point: Speed of operation) 2. Which breaker needs a compressor and air receiver as auxiliary plant? A. Air blast CB B. Minimum oil CB C. Bulk oil CB D. Both oil CBs Answer: A. Only the air blast breaker needs a compressed-air plant; oil breakers need none. (Key point: Auxiliary plant) 3. What is the typical voltage range of the air blast CB? A. Up to 11 kV B. 3.3-132 kV C. 66-400 kV D. Only 765 kV Answer: C. ABCB is used for EHV transmission, 66-400 kV, whereas MOCB covers 3.3-132 kV. (Key point: Voltage range) 4. Why is the air blast CB better than bulk oil for frequent operation? A. It is quieter B. It has fresh air every time, while oil carbonises and needs filtering C. It uses less energy to close D. It has no moving contacts Answer: B. Fresh air replaces the medium each time and contact erosion is low, while oil carbonises with each operation. 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