has blown, the circuit breaker cannot be reclosed until
the blown fuse is replaced. Any attempt to remove the
fuse unit when the circuit breaker is in the closed
position will automatically trip the breaker.
The AQB-LF250 circuit breaker is interchangeable
with the AQB-A250 circuit breaker except a larger
cutout is required in the switchboard front panel to
accommodate the fuse unit of the AQB-LF250.
The AQB-LF250 circuit breaker is a 250-ampere
frame size. However, the circuit breaker has an
interrupting rate of 100,000 amperes at 500 volts ac, 60
hertz. The AQB-A250 circuit breakers has an
interrupting rating of 20,000 amperes at 500 volts ac, 60
hertz.
While the AQB-A250 circuit breaker could be
either front or back connected, the AQB-LF250 is
designed only for back (drawout type) connection. It
uses the same type of slip connector terminal studs as
shown in figure 2-54.
NQB-A250
The NQB-A250 circuit breaker (fig. 2-58) is similar
to the AQB-A250 circuit breaker except the NQB-A250
has no automatic tripping devices. This type of circuit
Figure 2-58.--NQB-A250 circuit breaker, front view with
breaker is used for circuit isolation and manual transfer ,
cover removed.
applications. The NQB-A250 is still a 250-ampere
frame size. The current-carring parts of the breaker are
NLB
capable of carrying 250 amperes. Technically, this
circuit breaker is simply a large on-and-off switch.
The NLB circuit breakers are identical to the ALB
Some types of AQB and NQB breakers are provided
circuit breakers except that they have no automatic
with electrical operators mounted on the front of the
tripping device. They are used only as on-off switches.
breaker. These are geared motor devices for remote
operation of the breaker handle.
Maintenance
ALB
When you work on circuit breakers, there are
several precautions you should take. The most
The ALB circuit breakers are designated
important precaution you should remember to take is to
low-voltage, automatic circuit breakers. The
de-energize all control circuits to which the circuit
continuous duty rating ranges from 5 through 200
breaker is connected. The procedures differ somewhat
amperes at 120 volts ac or dc. The breaker is provided
with the type of mounting being used.
with a molded enclosure, drawout type of connectors,
When working on drawout circuit breakers, make
and nonremovable and nonadjustable thermal trip
sure that they are switched to the open position. Then
elements.
the circuit breaker may be removed.
This circuit breaker is a quick-make, quick-break
When working on fixed-mounted circuit breakers,
type. If the operating handle is in the tripped (midway
open the disconnecting switches ahead of the breakers.
between ON and OFF) position, indicating a short
If disconnecting switches are not provided for
circuit or overload, the operating handle must be moved
isolation, you need to de-energize the supply bus to the
to the extreme off position. This automatically resets
circuit breaker.
the overload unit and the breaker can again be closed.
2-48
