Figure 2-34.--Type ACB circuit breaker.

procedures are the same for any branch circuit.

CIRCUIT BREAKERS

Therefore, a description will be given on the steps

necessary to (1) locate the defective circuit and (2)

Circuit breakers have three fundamental

follow through on that circuit and find the trouble.

purposes: to provide circuit protection, to perform

normal switching operations, and to isolate a

Assume that, for some reason, several of the lights

defective circuit while repairs are being made.

are not working in a certain section. Because several

lights are out, it will be reasonable to assume that the

Circuit breakers are available in manually or

voltage supply has been interrupted on one of the

electrically operated types that may or may not

branch circuits.

provide protective functions. Some types may be

operated both ways, while others are restricted to one

To verify this assumption, first locate the

mode.

distribution box feeding the circuit that is

inoperative. Then make sure that the inoperative

Types of Air Circuit Breakers

circuit is not being supplied with voltage. Unless the

circuits are identified in the distribution box, the

Air circuit breakers are used in switchboards,

voltage at the various circuit terminations will have

switch gear groups, and in distribution panels. The

to be measured. For the following procedures, use the

types installed on naval ships are ACB, AQB, AQB-A,

circuits shown in figure 2-33 as an example.

AQB-LF, NQB-A, ALB, and NLB. They are called air

circuit breakers because the main current-carrying

To pin down the trouble, connect the voltage tester

contacts interrupt in air.

to the load side of each pair of fuses in the branch

distribution box. No voltage between these terminals

ACB CIRCUIT BREAKERS.-- Figure 2-34

indicates a blown fuse or a failure in the supply to the

shows the exterior of a type ACB circuit breaker. This

distribution box. To find the defective fuse, make

certain S1 is closed, connect the voltage tester across

A-A1, and next across B-B1 (fig. 2-33). The full-phase

voltage will appear across an open fuse, provided

circuit continuity exists across the branch circuit.

However, if there is an open circuit at some other

point in the branch circuit, this test is not conclusive.

If the load side of a pair of fuses does not have the

full-phase voltage across its terminals, place the

tester leads on the supply side of the fuses. The full-

phase voltage should be present. If the full-phase

voltage is not present on the supply side of the fuses,

the trouble is in the supply circuit from the feeder

distribution box.

Assume that you are testing at terminals A-B (fig.

2-33) and that normal voltage is present. Move the

test lead from A to A1. Normal voltage between A1 and

B indicates that fuse A-A1 is in good condition. To test

fuse B-B1, place the tester leads on A and B, and then

move the lead from B to B1. No voltage between these

terminals indicates that fuse B-B1 is open. Full-phase

voltage between A and B1 indicates that the fuse is

good.

This method of locating blown fuses is preferred to

the method in which the voltage tester leads are

connected across the suspected fuse terminals,

because the latter may give a false indication if there

Figure 2-34.--Type ACB circuit breaker.

is an open circuit at any point between either fuse

and the load in the branch circuit.

2-23