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direct-acting automatic tripping devices. They are
circuit breaker is designed for either manual local
used to protect single-load circuits and all feeder
closing or electrical remote closing. It has an open
metallic frame construction mounted on a drawout
circuits coming from a load center distribution panel.
mechanism and is normally used to supply heavy
Where the requirements are low enough, the type
loads and to protect the equipment from high
AQB may be used on generator switchboards. When
short-circuit currents.
it becomes necessary to replace one of the older type
AQB circuit breakers, you should replace it with the
Type ACB circuit breakers are used to connect
newer AQB-A101, AQB-A250, AQB-A400,
ship's service and emergency generators to the power
AQB-A600, or AQB-A800, as required.
distribution system. They are also used on bus ties and
shore connection circuits, and on some feeder circuits
from the ship's service switchboard. In these
A Q B type of circuit breakers, such as the
applications, they operate usually with a pilot device,
AQB-A250 (fig. 2-35, views A, B, and C), have
such as a relay or a switch. An electrically operated
several advantages over the older types. They are
circuit breaker has an electromagnet that acts as a
designed for front or rear connections as required.
solenoid to trip a release mechanism that causes the
They may be mounted so as to be removable from
breaker contacts to open. The energy to open the
the front without removing the circuit breaker
breaker is derived from a coiled spring. The
cover. The outside dimensions of these new
electromagnet is controlled by the contacts in the pilot
breakers are the same for both the two-pole and
three-pole circuit breakers.
Circuit breakers designed for high currents have a
The 250 part of the circuit breaker designation
double-contact arrangement. The complete contact
indicates the frame size of the circuit breaker. The
assembly consists of the main bridging contacts and
current-carrying parts of a 250-ampere frame size
the arcing contacts. All current-carrying contacts are
circuit breaker have a continuous rating of 250
high-conductivity, arc-resisting silver or silver-alloy
amperes. The voltage ratings of the AQB-A250 are
500 volts ac, 670 Hz or 250 volts dc.
Each contact assembly has a means of holding the
Trip units for this breaker are available with
arcing to a minimum and extinguishing the arc as
current ratings of 125, 150, 175, and 250 amperes.
soon as possible. The arc control section is called an
The trip unit houses the electrical tripping
arc chute or arc runner. The contacts are so arranged
mechanisms, the thermal element for tripping the
that when the circuit is closed, the arcing contacts
circuit breaker on overload conditions, and the
close first. Proper pressure is maintained by springs to
instantaneous trip for tripping on short-circuit
ensure that the arc contacts close first. The main
contacts then close.
In addition, 100-, 160-, and 250-ampere rating
When the circuit opens, the main contacts open
trip units with a special calibration are available for
first. The current is then flowing through the arc
use with generator circuit breakers. Regardless of the
contacts, which prevents burning of the main
trip unit used, the breaker is still a 250-ampere frame
contacts. When the arc contacts open, they pass under
sire. The automatic trip devices of the AQB-A250
the front of the arc runner, creating a magnetic field
circuit breaker are "trip free" of the operating handle;
that blows the arc up into the arc quencher and quickly
in other words, the circuit breaker cannot be held
opens the circuit.
closed by the operating handle if an overload exists.
When the circuit breaker has tripped due to overload
Type ACB circuit breakers are either manually
or short circuit, the handle rests in a center position.
(hand operated) or electrically operated. Electrically
To reclose the circuit breaker after automatic tripping,
operated ACB breakers may be operated from a
move the handle to the extreme OFF position. This
remote location, making it unnecessary for personnel
resets the latch in the trip unit. Then move the handle
to approach them in order to open or close the circuit.
to the ON position.
circuit breakers are mounted in supporting and
AQB-LF type of circuit breaker, such as the
enclosing housings of insulating material and have

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