1. Introduction
         In this chapter, The constructional aspects of circuit breakers have been briefly discussed. The theoretical aspects regarding transient variation of current and voltage, are extinction process and the various types of circuit breakers have been described in detail in other chapter.
          The circuit breaker are automatics switches which can interrupt fault currents. In some application like single phase traction system, single pole circuit breakers are used. The part of the circuit breakers connected in one phase is called the 'pole'. A circuit breaker suitable for three phase system is called a 'triple-pole circuit breaker'.
           Each pole of the circuit breaker comprise one or more 'interrupter' or 'arc-extinguishing chambers'. The interrupters are mounted on support insulators. The interrupter encloses a set of 'fixed and moving contacts'. The moving contacts can be drawn apart by means of the operating links of the operating medium. The operating mechanism of the circuit breaker gives the necessary energy for opening and closing of contacts of the circuit breaker.
           The arc produced by the separation of current carrying contacts is interrupted by a suitable medium and by adopting suitable techniques for arc extinction . The circuit breaker can be classified on the basis of the arc extinction medium.

2. The Trip-Circuit
           The basic connection of the circuit breaker control, for the opening operation, is illustrated in figure 1.

              The protected circuit is shown by dashed line. When a fault occurs in the protected circuit. the relay connected to the CT and PT actuates and closes its contacts. Current flows from the battery (source) in the trip circuit. As the trip coil of the circuit breaker (C.B.) is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation. Auxiliary switch is an important item in the circuit. such auxiliary switch has two position "open" and "close" corresponding to the position of the circuit breaker. In each position, some auxiliary circuits are opened and some are closed. The auxiliary circuits serve several purposes such as:

( i ).  Indication
     breaker open or close by lamps, near C.B. and at a remote place.
( ii ). Electric Interlocks
      The is electrically interlocked with isolators through the auxiliary switch.
( iii ). Connection 
       For auxiliary circuit of the operating mechanism.
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