2. If you do not find the trouble, or open, by a
visual check, use a voltmeter to determine whether the
circuit is live (operating) up to the point of the
component.
A SHORT CIRCUIT results when two bare
conductors of different potential come into contact with
each other. If a conductor inadvertently contacts a
metallic part of a wiring system, such as a motor frame
or conduit, the system is sometimes said to be
GROUNDED instead of having a short circuit.
Grounds or short circuits can be (1) solid, (2) partial, or
(3) floating. This situation presents a serious safety
hazard because the machinery may be in operation,
even though it has a short circuit. This condition is
especially true in motors and some appliances.
A solid ground or short circuit is one in which a full-
voltage reading is obtained across the terminals of a
blown fuse when the load is disconnected from the
circuit. The circuit resistance, in this case, is quite low,
and the current is quite high so that the fuse will blow.
A partial short or ground is one in which the
resistance between the phase wires, or between the
phase wire and the ground, is partially lowered.
However, enough current still remains to blow the fuse.
Grounds of this type are generally more difficult to
locate than are solid grounds.
A floating ground is a condition in which the
resistance of the defect in a system varies from time to
time. Grounds of this type may be present in an
electrical system for some time before their existence
becomes known. A floating ground is indicated when
fuses are blown on the phase side of a circuit a number
of times, and a circuit test shows no defects in the
system. In grounds of this type, fuse trouble may not
occur for several days. Then the ground recurs, and the
fuses are blown again.
The procedures used to repair the troubles
mentioned thus far are usually fairly simple. In the case
of an open, short, or ground in NM cable, the bad section
from box to box can simply be replaced using the same
procedures outlined for installation. Another method
would be to cut the cable at the trouble spot, install
junction boxes, and add a short piece of cable to replace
the bad section. Although the latter is the cheapest, it
may not be possible if the trouble is concealed.
Remember, you must have at least 6 inches of free
conductor in a junction box to make the splices.
Once you find out where the trouble is in a conduit
system, the repair procedure is even easier. All that is
required is to pull the open, shorted, or grounded
conductor out and replace it with a new one. This
replacement can be done by attaching the new
conductor to the one that is to be removed. In any case, a
little common sense and knowledge of the NEC©
requirements will dictate the action you should take.
METERS
Many times a visual inspection does not uncover an
apparent problem; therefore, you must advance to
troubleshooting with meters. In electrical
troubleshooting, you will use voltmeters, ohmmeters,
ammeters, and the meter that incorporates many
meters—the multimeter.
When using a voltmeter, you have to connect the
power to the circuit before testing. On the other hand,
you cannot use the ohmmeter on an energized circuit.
You need to start voltmeter tests at the power input end
of the circuit, whereas you start ohmmeter tests at the
ground end
Electrical circuit troubles develop either in the
wiring or in the operating unit. If you analyze the
problem carefully and take systematic steps to locate it,
not only will you save much time and energy, but you
will also prevent damage to expensive equipment.
Either dead circuits or live circuits can be tested
with instruments. Circuit defects can sometimes be
located more easily by one method than the other,
depending upon the type of circuit and the trouble.
To test a dead circuit, disconnect the device from
the outlet or disconnect switch. Equipment for this
method of testing includes such units as ohmmeters and
battery-powered test lamps. Asuitable continuity tester
can be made easily from a flashlight in an emergency.
An ohmmeter that contains its own batteries is excellent
for continuity testing. A basic factor to consider in
choosing continuity test equipment is to use relatively
low-voltage instruments, reducing the danger of
sparking.
WARNING
When connections are made in the
presence of combustible vapors, sparking is
a serious fire hazard.
When you test live circuits, energize the circuit
under test from the power source. Generally, you will
test with a voltmeter. Make certain that the voltmeter is
designed for the type of current to be tested and has a
5-38
