Figure 11.7 - Air-break Circuit Breaker (ACB) for Alternators

Interruption of current flow results in the production of an arc between contact faces. Arcing is severe with overload current but is not a serious problem during normal operation. As mentioned earlier, in order to prevent damage to main contacts, separate arcing supplementary contacts are used. These are made of arc-resisting alloy such as silver tungsten and easily replaced if damaged. They should be inspected after a faulty operation of the breaker. The arcing contact shown, has a spring which pushes it forward so as to hold it in the closed position until after the main contacts have opened.

Air-break circuit breakers with arcing contacts have always been used for d.c. switchboards but further development was necessary to improve arc control before such breakers could replace oil circuit breakers and be used for a.c. Later development has meant that they can be used on systems with voltages up to 3.3kV.

Arc control requires that the arc be elongated and removed from the gap between the arcing contacts. Electromagnetic forces associated with the arc and thermal action will cause it to move up the arc runners to the arc chute, which is provided for the purpose. Thus the arc is elongated and finally chopped into sections and also cooled with the help of the splitter plates in the chute.

Arc chutes are made of insulated arc-resisting material. They confine the arc and produce a funnel effect, which assists thermal action. Splitter plates are made of metal (steel or copper) in some breakers, and in others, made of insulator material. Some breakers have horizontal rods fitted to cool and split the arc. Arc runners are fixed and not of the moving type as in some designs. Interruption of the arc is assisted by the current dropping to zero during the cycle. However, with three phases the zero points in each phase are staggered Contact opening is therefore followed by the current falling to zero and this means that for the next part of the cycle an arc has to be struck across a gap.

Successful removal of ionized gas (from the arc, a result of the contact opening) will increase the resistance in the air gap between the contacts. When gas remains, it provides a path across which the arc can re-strike. The rate at which the gas is removed is such that the arc will not re-strike more than two or three times.

Breaking speed is made as high as possible by powerful throw-off springs and light construction of the moving arm assembly. Anti-bounce devices prevent any rebound at the end of the opening movement.

Rapid closing of the breaker also helps to prevent damage and most are power, neither than manually, closed. Power is provided by a solenoid or by a spring, which is automatically rewound (re-charged) by a motor and left ready after each closing operation. Where springs are used, an emergency hand-tensioning method is arranged for use with a dead switchboard, so that the spring can be wound up and made ready for closing the breaker.

After operation of a spring-activated breaker, the motor can usually be heard charging the spring for the next time. Feeder and distribution circuits are usually protected by the moulded-case type or the miniature type of circuit breakers.

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