Air Circuit Breaker Features & Operation

An air circuit breaker employs atmospheric air as its interrupting medium. The arc is drawn between its contacts and extended via arc runners on to an arc chute where it is presented with a large cooling surface of arc splitter plates. These break the arc into a number of series arcs, the principle being to increase the resistance of the arc and extract energy from it via the metallic splitter.

As the circuit breaker contacts open the arc is formed and enhanced by strong thermal convection effects and electromagnetic forces to stretch across splitter plates. The elongation helps cooling and deionization of the air/contact metallic vapor mixture. The long arc resistance also boosts the arc power factor and thus aids arc extinction at zero current as current and circuit breaker voltage are more in phase. Transient recovery voltage oscillations are also damped thus reducing overvoltages. The arc products are carefully vented away from the main contact area and out of the switchgear enclosure.

Air Circuit Breakers Applications & Limitations

Air circuit breakers are typically applied at low voltage levels but are available with high current ratings up to 6000 A and short circuit ratings up to 100 kA at 500 V. The physical size of such units, which contain large arc chutes, makes them uneconomic as voltages increase above 3.6 kV.

The application at low voltage (LV) is where a difficult performance is needed in terms of load, number of operations and fault level.

Molded-case circuit breakers have replaced many LV applications where previously air circuit breakers were used mainly due to economic factors but where high performance, maintainability and long-term reliability are essential requirements air circuit breakers are used for instance a typical application is in generating station LV auxiliary supplies.

The main application of high voltage (HV) air circuit breakers has been in applications where the exclusion of flammable materials is a fundamental requirement; a typical application being in the generating-station LV auxiliary supplies. However such high-rated circuit breakers are extremely costly and they have been replaced by high performance vacuum or SF₆ circuit breakers.

Lastly another application of air circuit breakers is for use with dc supplies. DC circuit breakers are extensively employed in traction applications where ratings of up to 3 kV may be used.

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