procedures are the same for any branch circuit.
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
provide protective functions. Some types may be
operated both ways, while others are restricted to one
To verify this assumption, first locate the
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
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
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
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.