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the current waveform and permanently exceeds the system recovery voltage. This tends to
reestablish the flow of current across the arc gap. In the case of an oil circuit breaker, the
dielectric strength build up is accomplished by forcing clean oil into the arc path. In the case of a
magnetic-blast circuit breaker, the arc is magnetically forced into the arc chute where it is
elongated, cooled, and restricted to a point where the dielectric strength of the arc gap
permanently exceeds the system recovery voltage. Bushings. The bushing is the structure used to insulate the current-carrying parts
from ground and from other live parts. It has a through conductor for carrying current from the
line connection to the contacts. In many breaker designs it is also used to support the main
contacts and interrupters. Bushings for oil circuit breakers are normally made of a porcelain or
glass outer shell with an internal insulation material of: paper, oil, compound, ceramic materials,
or a combination of these. Housing. Circuit breakers are enclosed in some form of a housing to protect the
essential parts from the environment, and also to protect personnel from live parts. Padmount
housings normally enclose vacuum breakers in freestanding, tamperproof metal enclosures that
have a neat appearance. A padmounted vacuum circuit breaker is shown in Figure 3-13. The
two basic, most common forms of housings used are the tank and cubicle. In oil circuit breakers
the tank is used to contain the oil in which the contacts and interrupters are immersed. The
cubicle-type circuit breaker is one designed for use in a switchgear assembly. The switchgear
cubicle is an indoor or outdoor assembly enclosed on all sides and top containing circuit
breakers, buses, connections, and auxiliary devices. Design of the cubicle is such that when a
circuit breaker is installed in its operating position, all operating parts are adequately insulated
with a protective grounded barrier between all live parts and the operator. Cubicle-type
switchgear is normally divided into the following types:
(a) A line-up of low voltage metal-enclosed air circuit breaker switchgear is shown in
Figure 3-14. It consists of a front section; and a cable-entrance section. Each circuit breaker
must be isolated from all other equipment. Circuit breakers may be either the fixed or draw-out
type. The fixed type is rigidly mounted and has no provisions for quick removal. The draw-out
type can be easily disconnected and removed from the switchgear cubicle while the bus remains
energized. Vents are provided in the circuit breaker compartments for cooling and to release
ionized gases that form when the circuit breaker opens to interrupt fault currents. Other sections
of the switchgear must also be ventilated to allow circulation of air for cooling. Barriers between
the bus compartment and the cable compartment are not required, but they may be furnished to
permit connecting or disconnecting of the cables without danger of contacting the energized bus.
If barriers are provided, all cable terminations should be insulated after installation, so that there
will also be no danger of workmen contacting these hot terminals when making changes in the
cable connections. Aside from having a weatherproof structure, outdoor metal-enclosed
switchgear is similar to indoor equipment. Heaters, however, must be provided to keep the
inside cubicle temperature greater than the outside temperature to prevent condensation. Indoor


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