Figure 2-2.-Continuity test.
When testing for grounds, you use an
testing such components is to make a resistance
measurement and compare the indicated resist-
ohmmeter or some other continuity tester. By
ance with the resistance given on schematics or
measuring the resistance to ground at any point
in maintenance manuals.
in a circuit, you can determine if the point goes
to ground. Look at figure 2-2. After removing the
VOLTAGE TESTS
jumper from pin A of plug 1, you can test each
cable conductor for grounds. To do this, connect
Voltage checks are made while the power is
one meter lead to ground and the other to each
applied; therefore, you must follow the prescribed
of the pins of one of the plugs. A low resistance
safety precautions to prevent injury to person-
shows that a pin is shorting to ground. For this
nel and equipment damage. Voltage tests are
test you remove both plugs from their units; if
important. You will use them to isolate mal-
you remove only one plug, a false indication is
functions to major components and to maintain
possible.
subassemblies, units, and circuits.
The voltmeter is used to test voltages. When
SHORT TEST
you use the voltmeter, make sure the meter is the
A short circuit occurs when two conductors
correct one for the type current (ac or dc) under
test. Also, make sure it has a scale with a suitable
accidentally touch each other directly or through
range. Since defective parts in a circuit may cause
another conducting element. In a short circuit,
higher than normal voltages to be present at the
enough current may flow to blow a fuse or open
point of test, use the highest voltmeter range
a circuit breaker. It is possible to have a short
between two cables carrying signals and the short
available first. Once you get a reading, you can
not blow the fuse.
determine if a lower scale is possible. If so, you
You use an ohmmeter to check for a short.
use the lower scale as it provides a more accurate
reading.
By measuring the resistance between two con-
Another consideration in the circuit voltage
ductors, you may detect a short between them by
test is the resistance and current in the circuit. A
a low resistance reading. Refer to figure 2-2. If
low resistance in a high-current circuit may cause
you remove the jumper and disconnect both plugs,
a considerable voltage drop. The same resistance
you can make a short test. You can do this
in a low-current circuit may cause a minimal
by measuring the resistance between the two
voltage drop. You can check abnormal resistance
suspected conductors.
Shorts not only happen in cables, they occur
in part of a circuit with either an ohmmeter or
a voltmeter. Where practical, use an ohmmeter
in many components, such as transformers, motor
because the test is then on a dead circuit.
windings, capacitors, etc. The major method for
