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Figure 5   Cutaway View of Molded Case Circuit Breaker
Figure 6   Large Air Circuit Breaker

Electrical Science Volume 4 of 4
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Electrical Distribution Systems CIRCUIT BREAKERS For moderate overload currents, it will operate more slowly.   Molded case breakers with much larger current ratings also have a magnetic trip element to supplement the thermal trip element. The magnetic unit utilizes the magnetic force that surrounds the conductor to operate the circuit breaker tripping linkage. When the separable contacts of an air circuit breaker are opened, an arc develops between the two contacts.  Different manufacturers use many designs and arrangements of contacts and their surrounding chambers.   The most common design places the moving contacts inside of an arc chute.  The construction of this arc chute allows the arc formed as the contacts open to draw out into the arc chute.   When the arc is drawn into the arc chute, it is divided into small segments and quenched.  This action extinguishes the arc rapidly, which minimizes the chance of a fire and also minimizes damage to the breaker contacts. Molded case circuit breakers come in a wide range of sizes and current ratings.   There are six frame sizes available:   100, 225, 400, 600, 800, and 2,000 amps.   The size, contact rating, and current  interrupting  ratings  are  the  same  for  all  circuit  breakers  of  a  given  frame  size.    The continuous  current  rating  of  a  breaker  is  governed  by  the  trip  element  rating.    The  range  of voltage available is from 120 to 600 volts, and interrupting capacity ranges as high as 100,000 amps. Much larger air circuit breakers are used in large commercial and industrial distribution systems. These circuit breakers are available in much higher continuous current and interrupting ratings than the molded case circuit breaker.  Breakers of this type have current ratings as high as 4,000 amps, and interrupting ratings as high as 150,000 amps. Most large air circuit breakers use a closing device, known as a "stored energy mechanism," for fast, positive closing action.   Energy is stored by compressing large powerful coil springs that are attached to the contact assembly of a circuit breaker.  Once these springs are compressed, the latch may be operated to release the springs, and spring pressure will shut the circuit breaker. Circuit  breaker  closing  springs  may  be  compressed  manually  or  by  means  of  a  small  electric motor.  This type of circuit breaker can be classified as either a manually- or electrically-operated circuit breaker. When a large air circuit breaker is closed, the operating mechanism is latched.   As the circuit breaker is closed, a set of tripping springs, or coils, are compressed, and the circuit breaker may then  be  tripped  by  means  of  a  trip  latch.    The  trip  latch  mechanism  may  be  operated  either manually or remotely by means of a solenoid trip coil. As   previously   stated,   circuit   breakers   may   be   operated   either   manually   or   electrically. Electrically-operated circuit breakers are used when circuit breakers are to be operated at frequent intervals or when remote operation is required. Rev. 0 Page 9 ES-15







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