Circuit Breakers
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A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
Principles:
Closing: In dc circuits the current reaches about 95% of its final steady state value after time 3¦Ó, where ¦Ó is the time constant. Similarly, when an alternating current is closed, the current reaches a steady value after a transient process. The time depends upon the resistive, inductive and capacitive elements of the circuit. The highest switching current is achieved if switching is effected at zero voltage (very high peak currents can develop if the switch is closed on short circuit conditions).
Opening: Below a threshold voltage, any circuit can be opened without any arc formation. In practice, however, the commonly used switches do produce an arc while interrupting the current. The arc must be either kept limited or extinguished at the earliest in order not to damage the contacts.
Contacts: Copper is by far the most widely used contact material. But since non-conducting layers are formed on copper contacts as a result of switching, a wiping action is provided while designing copper contacts. These are also plated wih a layer of silver in many applictions. In low voltage circuits, silver is also in use as contact material.
Arc extinction: Since switching almost invariably gives rise to arcing, extinguishing such arcs assumes vital importance to prolong contact life. The following methods are employed:
a) Lengthening of the arc till it extinguishes b) Intensive cooling (in jet chambers) c) Division into partial arcs d) Zero point quenching e) Connecting capacitors in parallel with contacts in dc circuits f) Use of vacuum g) Use of air h) Use of oil
Insulation: The contacts need to be kept properly insulated from other metal parts including the body. Different insulating materials are in use. The most commonly used material is cast epoxy. Besides, PVC, polystyrene, polycabonate and ceramics are also in use.
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