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 3.5.5.1 Oilless Circuit Breakers. The oilless class includes: air, magnetic-blast,
compressed-air, gas-filled, and vacuum circuit breakers.
(a) In the air type, the arc is extinguished, or interrupted, in substantially static air in
which the arc moves into arcing chutes containing metal or insulating fins. The metal fins break
the arc into many short segments while the insulating fins stretch the arc. Both methods are
effective in extinguishing the arc. A circuit breaker arc chute interruption is shown in Figure
3-12.
(b) The magnetic-blast type differs from the air type in that it uses a magnetic field to
blow the arc into the arc chute which elongates and cools the arc.
(c) In the compressed-air and gas-filled types, high-pressure air or gas is forced through
the arcing path to elongate the arc and increase the resistance to the fault current.
(d) In a vacuum circuit breaker the arc is drawn in a vacuum. The high dielectric
strength and the rapid recovery rate of a vacuum gap makes arc extinguishing extremely fast and
clean.
3.5.5.2 Oil Circuit Breakers. As an interrupting medium, oil is much better than air at
room pressure. Its dielectric strength is greater and, in addition, the arc generates hydrogen gas
from the oil which helps to cool the arc. The use of a small arcing chamber to build up pressure
greatly increases the interrupting capacity of an oil circuit breaker. The major disadvantages of
oil circuit breakers are their inherent fire hazard and their relatively high cost.
3.5.6 Major Component Parts. All power circuit breakers, regardless of the type and
construction, have certain structural components that are designated similarly for all. Such
components include contacts, bushings, and the operating mechanism. Following are brief
descriptions of major components of circuit breakers and their functions.
3.5.6.1 Contacts. The contacts are the most vital part of a circuit breaker. They consist of
a pair of separable members that are opened and closed by mechanical means. Normally the
contact members are held together under pressure and the electric current flows from one to the
other through the point or points of contact. Circuit breakers are provided with main contacts
made of silver for carrying the continuous current. In addition, there is a pair of arcing contacts
made out of tungsten which protect the main contacts from damage due to arcing. The arcing
contacts, which close before and open after the main contacts, carry current only during the
interrupting process and are easily renewable. Twice during each cycle of an alternating current
the current amplitude drops to zero value and the arc is extinguished for an instant. In order to
interrupt the circuit, therefore, it is only necessary to prevent reignition of the arc after the
current reaches zero value. Whether or not the arc is extinguished immediately after the current
is zero depends upon whether the dielectric strength of the arc gap builds up at a faster rate than
3-29
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