Air blast circuit breaker

An air blast circuit breaker (ABCB) is a breaker in which a blast of high-pressure compressed air (about 20–30 kg/cm²) is directed on the arc to cool it, sweep away ionised gas and extinguish it at current zero. It was widely used from 66 kV to 400 kV before SF6 breakers became common.

Construction

• Compressed air reservoir charged by an air compressor plant.
• Blast valve that admits air from the reservoir to the arc chamber on a trip signal.
• Arc extinction chamber containing a fixed contact (often a nozzle) and a moving contact held closed by a spring.
• Exhaust to atmosphere through a silencer.
• Isolating switch / resistor connected in parallel or series in some designs to control restriking voltage.

 Air reservoir ──► Blast valve ──► Arc chamber ──► Exhaust
  (20-30 bar)                         │
                     ┌─────────┐    ┌─┴────────┐
          Fixed ─────│ nozzle  │ ~~ │ moving   │◄─ spring
          contact    └─────────┘arc └──────────┘
                     air flows ───────────────►

Types (by direction of air flow)

1. Axial blast: air flows along the arc through a nozzle; moving contact is pushed back by the air pressure itself. Most common.
2. Cross blast: air flows across the arc and drives it into arc splitters, lengthening it; suited to medium voltages and high current.
3. Radial blast: air flows radially into the gap.

Principle of operation

1. When the breaker trips, the blast valve opens and high-pressure air enters the arc chamber.
2. The air pressure pushes the moving contact away against its spring; an arc is drawn.
3. The fast air stream cools the arc, removes hot ionised particles and lengthens the arc.
4. The arc resistance rises and at the next current zero the arc is extinguished.
5. Fresh dry air fills the gap, so the dielectric strength builds up faster than the restriking voltage.
6. After extinction a series isolator opens (in some designs) and the blast valve closes; the contacts are held open.

Advantages

• No fire risk (no oil); suitable for indoor use.
• Very fast arc extinction and short arcing time; little contact burning.
• Suitable for frequent operation and repetitive auto-reclosing.
• Arcing products are blown away; low maintenance of the medium.

Disadvantages

• Needs a compressor plant and air storage; costly upkeep.
• Very noisy operation (needs silencers).
• Prone to current chopping at low currents and high RRRV, so resistors across contacts are used.
• Air leakage at fittings must be watched.