How Do Vacuum Circuit Breakers Work?

Breaker Basics

• To understand vacuum breakers, you need to know a few things about how circuit breakers work. Circuit breakers contain metal electrical contacts held together by a spring-loaded trip mechanism. This trip linkage contains a mechanical heat sensor that trips off the breaker when it detects the rising temperatures generated by overloading the electrical conductor. The linkage also contains a magnetic sensor that trips off the breaker when it detects the sudden rise in the magnetic field around the conductor caused by a short circuit. In either an overload or short circuit situation, the breaker’s trip mechanism pulls apart the contacts to stop the flow of electricity in the circuit.

Arcing Problem

• When high-voltage circuit breakers trip off and pull apart their electrical contacts, high-voltage electricity will arc between the contacts for a few microseconds. This jolt could ionize air and make it electrically conductive, giving the electricity a path past the breaker. For this reason, high-voltage circuit breakers are filled with materials that cannot be ionized and don’t conduct electricity. The arcing still happens as the circuit is broken but the arc can’t go anywhere before it falls apart as the breaker’s contacts pull away from each other.

Vacuum Power

• A vacuum breaker surrounds the contacts with a vacuum -- empty space that has nothing solid, liquid or gas in it. A vacuum is a near-perfect insulator. There’s nothing that can ionize and nothing that can carry the arcing electricity to the wrong places. For this reason, a vacuum is an ideal arc extinguishing medium. Other high-voltage breaker designs employ a blast of high-pressure air to blow the arc apart, or are filled with special oil that quenches the arcing. But the air-blast breakers are more complicated, and the quenching oil used in oil-filled breakers can burn and is very toxic to the environment if it escapes.

Applications

• Vacuum circuit breakers, oil-quenched breakers and air-blast breakers all perform well in transmission line applications where they trip off only occasionally. In transmission line protection applications, all three types require only minimal maintenance. Vacuum breakers perform slightly less well than other types in handling line spikes and surges without tripping off, but perform better than other types in applications such as powering arc furnaces where high-voltage current must frequently be connected and disconnected.