Not every circuit breaker suits every voltage. A breaker that works perfectly at 400 V would fail catastrophically at 132 kV, and a breaker built for 132 kV would be absurdly expensive at 400 V. The choice is decided mainly by one thing: how the insulating medium between the opening contacts recovers its dielectric strength. This post explains the dielectric curve of the four common media and shows which breaker belongs to each voltage level.
The dielectric curve: breakdown voltage vs contact gap
When a circuit breaker's contacts separate, an arc forms between them. The arc is extinguished at a current zero, and immediately afterwards the full system voltage — the transient recovery voltage — appears across the small contact gap. Whether the breaker succeeds depends on whether the dielectric strength of the medium in the gap builds up faster than this voltage. The figure below plots the dielectric breakdown voltage of four media against the contact gap distance.
Figure 3.1: Dielectric breakdown voltage vs contact gap for the four interrupting media. High vacuum is strongest at very small gaps but saturates above about 15 mm; SF6 rises almost linearly with gap. (Schematic, not to scale — curve shapes only; values are illustrative.)
The four curves are:
- High vacuum (10−6 Torr) — the highest breakdown strength of all at small gaps, but the curve flattens and saturates above about 15 mm. Making the gap bigger adds almost no extra strength.
- SF6 gas (5 bar) — breakdown voltage rises almost linearly with gap distance, so it keeps scaling up to the highest voltages.
- Transformer oil — in between: stronger than air, weaker than SF6 at large gaps.
- Air (1 bar) — the weakest of the four, and the reason air-break breakers are limited to low voltages.
Selection by voltage level
| Voltage level | Breaker used | Why this medium wins here |
|---|---|---|
| LV — below 1 kV (≤ 1000 V) | MCB, MCCB, ACB | Air at 1 atmosphere is strong enough for such small gaps, so simple air-break construction is cheapest and adequate. |
| MV — 3.3 kV to 36 kV | VCB (Vacuum Circuit Breaker) dominates | Vacuum gives the highest breakdown strength at the short gaps used here (8–15 mm) and the fastest dielectric recovery after current zero. |
| HV / EHV / UHV — 66 kV to 765 kV | SF6 Circuit Breaker dominates | Vacuum saturates above 36–66 kV, so it cannot scale to these voltages. SF6 strength rises linearly with gap, so it scales all the way up. |
Why vacuum cannot go to EHV — and SF6 can
Look again at the vacuum curve: it rises steeply at first, then goes flat. Above roughly 15 mm of gap — which corresponds to about 36–66 kV of system voltage — opening the contacts wider buys almost no extra dielectric strength, because the breakdown mechanism in vacuum changes from gap-dependent to field-emission limited. A vacuum breaker for 400 kV would therefore need many breaks in series, which is impractical.
SF6, by contrast, is a gas whose breakdown voltage keeps rising with gap (and with pressure). One SF6 interrupter can therefore be built for 132 kV, 220 kV, 400 kV and beyond simply by making the gap longer. That linear scaling is the whole reason SF6 dominates high-voltage switching.
Key points to remember
- Breaker selection follows the dielectric curve of the interrupting medium, not the brand or the size of the switchgear.
- LV (< 1 kV): MCB / MCCB / ACB in air. MV (3.3–36 kV): VCB — vacuum is strongest at short gaps of 8–15 mm.
- HV and above (66 kV and up): SF6 breaker — its strength scales linearly with gap.
- Vacuum saturates above about 15 mm gap (36–66 kV), so it cannot be scaled to EHV on its own.
- Air at 1 bar is the weakest medium — which is why air-break breakers stay at low voltage.
Based on Power System Protection & Switchgear, Paper II, Q3(a), Exam 11 June 2026.