Figure 2-39.-Type NQB-A250 circuit breaker, front view with cover removed.

The AQB-LF250 circuit breaker is a 250-ampere

This circuit breaker is a quick-make, quick-break

frame size. However, the circuit breaker has an

type. If the operating handle is in the tripped (midway

interrupting rate of 100,000 amperes at 500 volts ac.

between ON and OFF) position, indicating a short

The AQB-A250 circuit breaker has an interrupting

circuit or overload, the operating handle must be

moved to the extreme off position. This automatically

rating of 20,000 amperes at 500 volts ac.

While the AQB-A250 circuit breaker could be

either front or back connected, the AQB-LF250 is

designed for back (drawout type) connection only.

NQB CIRCUIT BREAKER.-- The NQB type of

circuit breaker, such as the NQB-A250 circuit breaker

(fig. 2-39), is similar to the AQB-A250 circuit breaker,

except the NQB-A250 has no automatic tripping

devices. This type of circuit breaker is used for circuit

isolation and manual transfer applications. The

NQB-A250 is still a 250-ampere frame size as the

current-carrying parts of the breaker are capable of

carrying 250 amperes. Technically this circuit breaker

is simply a large on-and-off switch. Some types of

AQB and NQB breakers are provided with electrical

operators mounted on the front of the breaker. These

are geared motor devices for remote operation of the

breaker handle.

ALB CIRCUIT BREAKER.-- The ALB circuit

breaker is a designated low-voltage, automatic circuit

breaker. The continuous duty rating ranges from 5 to

200 amperes at 120 volts ac or dc. The breaker is

provided with a molded enclosure, drawout-type

connectors, and nonremovable and nonadjustable

Figure 2-39.-Type NQB-A250 circuit breaker, front view

thermal trip elements.

with cover removed.

Figure 2-38.-Current-limiting fuse unit assembly for type AQB-LF250 circuit breaker.

2-27