--- title: "Compare MOCB/BOCB and ABCB/puffer SF6" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:compare-mocb-bocb-abcb-sf6" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/compare-mocb-bocb-abcb-sf6 site: BCT Engineering --- # Compare MOCB/BOCB and ABCB/puffer SF6 Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2075 Bhadra, 8 marks - Video: [Differences between ACB, VCB, OCB, ABCB, and SF6 Breakers](https://www.youtube.com/watch?v=3y3NLSIzyNA) — BrainVOLT - 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-bocb-abcb-sf6 - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Compare the performance and characteristics and working principle of: i) Minimum oil circuit breaker vs Bulk oil circuit breaker ii) Air blast circuit breaker vs puffer type SF6 circuit breaker. ## Short answer i) Minimum oil CB uses only quenching oil with solid insulation, is smaller and lower fire risk than bulk oil CB which uses large oil for quenching and insulation; MOCB oil deteriorates faster. ii) Air blast CB uses external compressed air, noisy, high chopping; puffer SF6 CB generates its own SF6 blast, silent, very fast, low maintenance, no exhaust. ## Answer The comparison covers working principle, performance and characteristics of each pair. #### i) Minimum oil CB vs Bulk oil CB **Working principle (common):** the arc between separating contacts decomposes oil into a gas bubble (mainly hydrogen); high pressure and cooling by hydrogen deionise the gap so that the arc goes out at current zero. | Point | Minimum oil CB | Bulk oil CB | |---|---|---| | Role of oil | Arc quenching only | Arc quenching and insulation to earth | | Tank | Small porcelain chamber, kept live | Large earthed steel tank | | Oil quantity | About 10% of BOCB | Very large | | Arc control | Turbulator (axial/side vent) always used | Plain break or explosion pot | | Size, weight | Small, light | Heavy, bulky, large foundations | | Fire risk | Low | High | | Oil deterioration | Fast (small volume) | Slow | | Maintenance | Frequent oil change | Oil filtering, tank cleaning | | Range | 3.3–132 kV | Up to 33 kV | #### ii) Air blast CB vs Puffer type SF6 CB **Working principle:** - *ABCB*: a blast of compressed air from a separate reservoir is blown on the arc; it cools the arc and sweeps away ions, and air is exhausted to atmosphere. - *Puffer SF6*: the moving contact drives a piston that compresses SF6 in a cylinder and blows it through a nozzle over the arc. SF6 is electronegative and absorbs free electrons, so the gap recovers very fast. Gas stays in a sealed enclosure. | Point | Air blast CB | Puffer SF6 CB | |---|---|---| | Medium | Air at 20–30 bar | SF6 at about 5–7 bar | | Gas supply | External compressor and receiver | Self-generated by piston; no compressor | | Dielectric strength | Low (needs high pressure) | 2.5–3 times air | | Arc time constant | Larger | Very small | | Current chopping | High; resistors needed | Negligible | | Noise | Very loud exhaust | Silent (closed system) | | Breaks per pole at EHV | More | Fewer | | Maintenance | Compressor, valves, leaks | Very low; gas leak monitoring | | Environment | Clean (air) | SF6 is a strong greenhouse gas | | Cost | High (with air plant) | Moderate | | Present use | Mostly replaced | Standard for 33–765 kV | #### Remarks SF6 puffer breakers combine the speed of air blast breakers with silent, compact and low-maintenance operation, which is why they have replaced both oil and air blast types at transmission voltages. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the difference in the role of oil in MOCB and BOCB? A. MOCB oil quenches and insulates; BOCB oil quenches only B. Oil is used only for insulation in both C. Oil quenches in both, but BOCB oil also insulates live parts from the earthed tank D. Neither uses oil for quenching Answer: C. Both quench the arc with oil (hydrogen bubble); only BOCB also relies on oil for insulation to its earthed tank. (Key point: Role of oil) 2. How is gas supplied in a puffer-type SF6 breaker? A. A piston driven by the moving contact compresses SF6 and blows it over the arc B. From an external compressor and receiver C. From a separate high-pressure SF6 reservoir at 20-30 bar D. By oil decomposition Answer: A. The puffer piston is moved by the contact, so no compressor is needed; the gas stays in a sealed enclosure. (Key point: Puffer principle) 3. Why does the gap in an SF6 breaker recover its dielectric strength so quickly? A. SF6 is a good conductor of heat only B. SF6 is electronegative and absorbs free electrons C. SF6 is exhausted to atmosphere D. SF6 has lower dielectric strength than air Answer: B. Electronegative SF6 captures free electrons, rapidly deionising the gap; its dielectric strength is 2.5-3 times that of air. (Key point: Why SF6 recovers fast) 4. Which statement about oil deterioration is correct? A. It is slow in MOCB because of the small volume B. Neither deteriorates C. It is the same in both D. It is fast in MOCB because of the small oil volume, slow in BOCB Answer: D. A small oil volume gets carbonised quickly, hence frequent oil change in MOCB. (Key point: Oil deterioration) ## 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 - [Compare bulk oil, minimum oil, air blast CBs](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acompare-bocb-mocb-abcb) — Switchgear and Protection, chapter 6 - [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 - [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 - [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× ## 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