- 2070 Magh · 8 marks
Compare bulk oil, minimum oil, air blast CBs
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.
Compare the merits and demerits of bulk oil circuit breakers, minimum oil circuit breakers and air blast circuit breakers.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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