AQB-A250
The newer AQB type of circuit breakers, such as the
AQB-A250, have several advantages over the older
types. The outside dimensions of these new breakers
are the same for both the two-pole and three-pole circuit
breakers. They are designed for front and rear
connections. They may be mounted so as to be
removable from the front without removing the circuit
breaker cover. The voltage rating of the AQB-A250 are
500 volts ac, 60/400 hertz or 250 volts dc.
The 250 part of the circuit breaker designation
indicates the frame size of the circuit breaker. In a
250-ampere frame size circuit breaker, the
current-carrying parts of the breaker have a continuous
rating of 250 amperes. Trip units (fig. 2-52) for this
breaker are available with current ratings of 125, 150,
175, 225, and 250 amperes.
The trip units houses the electrical tripping
mechanisms, the thermal elements for tripping the
circuit breaker on overload conditions, and the
instantaneous trip for tripping on short-circuit
conditions.
In addition, 100-, 160-, and 250-ampere rating trip
units with a special calibration are available for use with
generator circuit breakers. Regardless of the trip unit
used, the breaker is still a 250-ampere frame size. The
automatic trip devices of the AQB-A250 circuit breaker
are "trip free" of the operating handle; in other words,
the circuit breaker cannot be held closed by the
operating handle if an overload exists. When the circuit
breaker has tripped due to overload or short circuit, the
handle rests in a center position. To reclose the circuit
Figure 2-52.--AQB-A250 circuit breaker complete front view,
with cover and arc suppressor removed.
breaker after automatic tripping, move the handle to the
extreme OFF position. This resets the latch in the trip
setting of the AQB-A250 trip units maybe adjusted by
unit. Then move the handle to the ON position.
the instantaneous trip adjusting wheels (12) shown in
The AQB-A250 circuit breaker may have auxiliary
figure 2-53, view A. These trip adjusting wheels are
switches, shunt trip (for remote tripping), or
marked for five positions, LO-2-3-4-HI. The trip unit
label (not shown) will list the instantaneous trip value
undervoltage release attachments. A shunt trip cannot
obtainable for each marked position. Identical settings
be provided in the same breaker with an undervoltage
must be made on each pole of the circuit breaker.
release. Figure 2-53 shows a trip unit with a shunt trip
NEVER remove a circuit breaker cover to perform
(view A) and a trip unit with an undervoltage trip (view
adjustments while the circuit breaker is in the closed
B). The coil for a shunt trip has a dual rating for ac and
(ON) position.
dc voltages. The undervoltage trip coils are wound for
a specific voltage, such as 450 ac or 250 dc and have
Terminal mounting block assemblies
used in conjunction with the circuit breaker
rated pickup and dropout values. The instantaneous trip
2-44
