Vacuum circuit breaker

A vacuum circuit breaker (VCB) is a breaker whose contacts open inside a sealed chamber evacuated to about 10⁻⁵ to 10⁻⁷ torr. Since vacuum has very high dielectric strength and no gas to ionise, the arc (formed only of metal vapour) is extinguished at the first current zero.

Construction

        Fixed terminal
            |
     +------+------+   ceramic/glass
     |   [fixed]   |   envelope
     |  ( shield ) |   metal vapour
     |   [moving]  |   condensing shield
     +------+------+
          bellows      metal bellows
            |
      Moving stem --> operating mechanism

1. Vacuum interrupter (bottle): sealed envelope of glass or ceramic, with metal end plates.
2. Fixed and moving contacts: made of copper–chromium (Cu-Cr) alloy, with spiral or axial-magnetic-field (AMF) shaped faces to keep the arc moving and diffuse.
3. Metal bellows: stainless-steel bellows let the moving contact travel (about 8–20 mm) without breaking the vacuum.
4. Vapour condensing shield: metal shield around the contacts on which the metal vapour condenses, protecting the envelope insulation.
5. Operating mechanism: spring-charged (motor-wound) mechanism with closing and trip coils.
6. Frame, insulators and terminals.

Principle of operation

1. When the trip coil is energised, the mechanism separates the contacts in vacuum.
2. At separation, the last point of contact is heated and metal vapour is released; the arc burns in this vapour (there is no gas).
3. Because of contact shape, the arc is driven around or spread over the contact face, keeping it diffuse and limiting erosion.
4. As the current approaches zero, vapour production falls; the vapour condenses rapidly on the contacts and shield within microseconds.
5. At current zero the gap recovers its very high dielectric strength within a few microseconds (much faster than in oil or air), so the arc does not restrike. Interruption occurs at the first current zero.

Applications

• Medium voltage systems: 3.3 kV to 33 kV (mainly 11 kV and 33 kV indoor/outdoor switchgear in Nepal's distribution substations).
• Capacitor bank and reactor switching (restrike-free).
• Motor switching, arc furnace switching (frequent operation).
• Railway traction, mining and ring main units.

Advantages (brief)

• Compact, silent, no fire or explosion risk, no gas/oil maintenance.
• Very long contact life; suitable for frequent operation.
• Fast recovery; handles high RRRV.
• Limitation: current chopping can produce overvoltage on inductive loads (reduced by Cu-Cr contacts or surge suppressors); economic mainly up to about 36 kV.