Types of Air Circuit Breakers

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

AQB-A CIRCUIT BREAKERS.-- The newer

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

device.

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

inserts.

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

conditions.

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.

AQB CIRCUIT BREAKERS.-- Type AQB

AQB-LF CIRCUIT BREAKER.-- T h e

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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