Arc extinction in vacuum circuit breakers

A vacuum circuit breaker interrupts current between contacts in a sealed vacuum of about 10⁻⁵ to 10⁻⁷ torr. With no gas present, the arc is a metal-vapour arc that is extinguished at the first current zero because the vapour condenses almost instantly.

Arc extinction process

   [fixed contact]
        | | |  metal vapour + ions
        |~|~|  (arc in vapour)
   [moving contact] -> separates
   condensing shield around gap

1. Contact separation: the moving Cu-Cr contact is pulled away by the mechanism. Contact pressure falls; the last bridging point melts and evaporates.
2. Metal-vapour arc: the current continues through the ionised metal vapour emitted from cathode spots. At low current the arc is diffuse (many cathode spots spread over the contact face).
3. Arc control: at high currents the arc tends to constrict. Specially shaped contacts (spiral/radial-magnetic-field or axial-magnetic-field) make the arc rotate or stay diffuse, preventing local overheating and erosion.
4. Current approaching zero: vapour production falls with current; the cathode spots die out.
5. Condensation: the vapour and ions spread out and condense on the contacts and on the metal shield within microseconds, so the gap again becomes a near-perfect vacuum.
6. Dielectric recovery: a vacuum gap has very high breakdown strength; it recovers within a few microseconds, faster than the restriking voltage rises. The arc is cleared at the first current zero with a short contact gap (about 10–20 mm).
7. Current chopping: if the arc becomes unstable before natural zero at very low currents, the current may be chopped, causing overvoltage in inductive circuits; Cu-Cr contact material keeps the chopping current very low (a few amperes).

Preferred voltage levels

• Most economical and widely used for medium voltage, 3.3 kV to 36 kV (11 kV and 33 kV in Nepal's distribution system).
• For higher voltages, several interrupters must be connected in series, so SF6 breakers are preferred above about 36 kV (though VCBs up to 72.5 kV and some 145 kV designs exist).

Advantages over OCB and ABCB

Other advantages: no exhaust of gas or flame, suitable for any position, unaffected by atmosphere and altitude, handles high RRRV and capacitor switching without restrike.