Oil and vacuum were the two great interrupting media of the twentieth century — oil came first, vacuum replaced it at medium voltage. They extinguish the arc in completely different ways: oil by cooling the arc in a bubble of hydrogen gas, vacuum by letting the arc burn in metal vapour that condenses instantly at current zero. This post compares them side by side.
Figure 3.2: Oil vs vacuum circuit breaker — interrupting chamber. (Schematic, not to scale.)
| Feature | Oil Circuit Breaker (OCB) | Vacuum Circuit Breaker (VCB) |
|---|---|---|
| Interrupting medium | Mineral oil | High vacuum, 10−6–10−8 Torr, in a sealed bottle |
| Interruption physics | Arc heat decomposes oil into a pressure gas bubble (~70% H2). The high thermal conductivity of hydrogen cools the arc | Arc burns in evaporated metal vapour from the Cu-Cr contacts; the vapour condenses on the walls at current zero |
| Contact travel / gap | Long contact travel, 100–300 mm | Extremely short contact gap, 8–15 mm |
| Operating speed | Slow — 3–5 cycles to clear | Fast — 1.5–2 cycles to clear |
| Service life | Oil carbonises with every operation; needs periodic filtration or replacement | Very long — over 20,000 operations; negligible contact wear |
| Fire and safety | Fire risk — oil is flammable, and the tank can explode under severe faults | Zero fire hazard; no oil, no toxic gas |
| Voltage application | Older bulk / minimum-oil breakers up to 132 kV (now largely superseded) | Dominant standard for medium voltage — 11 kV to 33 kV, up to 66 kV |
The comparison explains the history: oil breakers once covered everything up to 132 kV, but their slow clearing, fire risk and oil maintenance lost out wherever vacuum could reach. Vacuum now dominates medium voltage, while higher voltages went to SF6 — because, as the dielectric curve shows, vacuum strength saturates above about 36–66 kV.
Key points to remember
- OCB: oil decomposes into a hydrogen bubble (~70% H2), 100–300 mm travel, 3–5 cycles, fire risk, up to 132 kV.
- VCB: vacuum 10−6–10−8 Torr, metal-vapour arc, 8–15 mm gap, 1.5–2 cycles, over 20,000 operations.
- VCB is the dominant medium-voltage breaker (11–33 kV, up to 66 kV); oil is now largely superseded.
- Oil carbonises and needs filtration; vacuum needs almost no maintenance.
Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q3(b).