- 2071 Magh · 8 marks
Compare MOCB/ABCB and ABCB/bulk oil CB
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.
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.
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- Air blast breakers are fast and clean, but costly and noisy; they have largely been replaced by SF6 breakers at EHV.
✓ Answer checked by Claude Opus 5.5 (high effort)
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