Page 338 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

14 Energy storage handle

15 Draw out /in hole
16 Rocker repository
17 Trip reset button
Internal construction of air circuit breaker

The internal parts of an ACB in Fig.2
1 Sheet steel supporting structure
2 Current transformer for protection trip unit
3 Pole group insulating box
4 Horizontal rare terminals

5 Plate for fixed main contacts
6 Plates for fixed arcing contacts
7 Plate for main moving contacts
8 Plates for moving arcing contacts
9 Arcing chamber

10 Terminal box for fixed version - sliding contacts for withdrawable version
11 Protection trip unit
12 Circuit breaker closing and opening control
13 Closing springs
14 Spring loading arrangement

15 Manual releasing lever
Principle of operation of air circuit breaker
• When the circuit breaker opens the circuit either under the normal condition or in the fault condition, some Arc
is produced between the main contacts and some current flows to the load, called transition current through
the arc.
• This Arc and the current should be suppressed /eliminated especially during the fault condition otherwise the
severity of the fault level will be more and damages the circuit which leads to the electric fire.
• During the period of Arc some voltage appears across the main contacts called transition voltage, which will
be more than the rated system / supply voltage.
• To quench the Arc, this transition voltage should be reduced or the Arc voltage to be increased. The minimum
voltage required to maintain the arc is called as Arc voltage. In ACB, the Arc voltage is increased in the
following three ways.
• Arc voltage can be increased by cooling arc plasma by air. The temperature of arc plasma is reduced, more
voltage will be required to maintain the arc.

• By splitting the arc into a number of series in Arc chute will increases the arc voltage.
• Arc voltage can be increased by lengthening the arc path. As length of arc path is increased its resistance of
the arc path will increase hence the arc voltage is increased.
Some ACB contains two pairs of contact . The main pair carries the current and the made of copper. An additional
pair of contact (Arc contact) is made of carbon. When the breaker is opened, the main contact opens first. and the
arc contact remains in touch. The arcing gets initiated when arc contacts are separated.
Hence transition voltage will be reduced.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116