5. Description of different Circuit Breaker
5. 1. Air-break Circuit-breaker
The air at atmospheric pressure is used as an arc extinguishing medium in Air-Break Circuit-Breakers. These circuit breaker employ the high resistance interruption principle. the arc is rapidly lengthened by means of the arc runners and arc chutes and the resistance of the arc is increased by cooling, lengthening and splitting the arc. The arc resistance increases to such an extent that the voltage drop across the arc becomes more than the supply voltage and the arc gets extinguished. Magnetic field is utilized for lengthening the arc in high voltage air-break circuit-breakers.
Air-break circuit breakers are used in d..c. circuit and a.c. circuits up to 12KV.
The air-break circuit breakers are generally indoor type and installed on vertical panels or indoor draw-out type switchgear.
A.C. air circuit breaker are widely used in indoor medium voltage and low voltage switchgear. Typical reference values of ratings of air-break circuit breaker are :
460 V , 400-3500 A, 40-75 KA
3.3 KV , 400-3500 A, 13.1-31.5 KA
6.6 KV , 400-2400 A, 13.1-20 KA
5.1.1 Construction of Air-break Circuit-breaker
In the air-break circuit-breaker the contact separation and arc extinction takes place in air at atmospheric pressure. As the contacts are opened, arc is drawn between them. The arc core is a conducting 'path' of plasma. The surrounding medium contains ionized air. By cooling the arc, the diameter of arc core is reduced. The arc is extinguished by lengthening the arc, cooling the arc and splitting the arc. The arc resistance is increased to such an extent that the system voltage cannot maintain the arc and the arc get extinguished.
Figure 4 illustrates the normal arrangement of an air break circuit breaker.
This type of breaker is used for medium and low voltages.
There are tow sets of contacts : Main contacts (1) and Arcing contacts (2). Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. The arcing contacts (2) are hard, heat resistance and are usually of copper alloy. While opening the contacts, the main contacts dislodge first. The current is shifted to the arcing contacts. The arcing contacts dislodge later and arc is drawn between them (3). This arc is forced upwards by the electromagnetic forces and thermal action. The arc ends travel along the arc Runners (Arcing horns). The arc moves upwards and is split by arc splitter plates (arc chutes) (5) as shown by the arrow (4). The arc extinguished by lengthening, cooling, splitting etc. In some breakers the arc is drawn in the direction of the splitter by magnetic field.
Furthermore, air-break circuit-breaker have been developed with current limiting feature, magnetic blow-up of arc, etc. Air break a.c. circuit breakers are widely used for industrial switchgear, auxiliary switchgear in generation station. Air break principle employing lengthening of arc, arc runners, magnetic blow-ups are used for d.c. circuit breaker up to 15 KV.
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