Types of Circuit Breaker

AIR BREAK CIRCUIT BREAKER

Construction

In this type of Circuit Breaker arc extinction and contact separation takes place in air at atmospheric pressure. Figure (a) shows closed current carrying contacts. As the contacts are opened, arc is drawn between them. The arc core is conducting part of plasma. Ionized air is contained in surrounding medium. 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 can’t maintain the arc and arc gets extinguished.

The last figure illustrates normal arrangement of an air break circuit breaker. This type of breaker is used for medium and low voltages.

There are two sets of contact: Main contact (1) and Arcing contact (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 contact, the main contacts dislodge first. The current is shifted to arcing contacts. The arcing contacts dislodge later and arc is drawn between them (3). This arc is forced upwards b the electromagnetic forces and the thermal action. The arc ends travel along the Arcing horns. The arc moves upwards and is split by splitting plated (5). The arc is extinguished by lengthening, cooling, splitting, etc.  But this Air break Principle is used for DC Circuit Breaker upto 15KV.

Arc Extinction in A.C. Air Break Circuit Breaker

In A.C. Air Break Circuit Breaker, the arc is lengthened, cooled and splitted so as to increase the resistance of the arc. The rapid increase in the arc resistance causes the reduction in the fault current and the fault current does not reach the prospective high value. The arc extinction process is assisted by current zeros and A.C. wave. The voltage drop across  the arc goes on increasing with the increase in arc resistance and at a current zero, when  the recovery voltage across the contacts is less than the arc voltage, the arc gets extinguished.

Arcing Horns

As soon as the arc leaves the vicinity to the contacts it communicates to the pair of run out horns. In doing so outer blow out system is switched on. There blow out coils provide a magnetic field such that the arc footing is subjected to strong magnetic field. We know from the electromagnetic theory that force is experienced by current carrying element of length dl meters, carrying current I amperes and placed in magnetic field B Wb/m² is given by,

                                         

dF=I(dl*B)

By virtue of this force the arc travels upwards and its length increases. The tips of the arc run along the arc runners and come to its extreme point. As the length of arc increases at a particular length the system voltage is unable to maintain the arc and the arc is extinguished. For systems having low inductance the energy      (½ LI² ) is small and arc gets extinguished before reaching the extremity of runners. For high inductance circuits special techniques such as air blast, additional larger acing horns etc are used.

*other types of CB will be posted soon