Electrical Distribution Systems
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
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
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.