For your safety, before you begin testing
with an ohmmeter, BE SURE that the circuit
that you are about to test is de-energized.
Isolate the circuit being tested to prevent
reading resistance from other circuits.
Many times you can determine the approximate
location of an open by simply studying the circuit
diagram before doing any actual circuit testing. For
example, suppose both lamps in figure 5-74 light when
the circuit switch is placed in the BRIGHT position,
but neither lamp lights when the switch is placed in the
DIM position. Because the lamps light when the switch
is in one position, you can gather that all wires and
lamps are good. The only units that could be faulty are
the resistor, half of the switch, or the wires that connect
the switch and the resistor. By using the ohmmeter as
you did before, you can check the continuity of these
A short circuit exists when there is a direct
connection between two wires or conductors of
different potentials. If the trouble is not found by a
visual inspection, you must isolate it step by step. First,
disconnect all the equipment in the circuit and install a
new fuse or place the circuit breaker to the ON position.
If the short is clear, then the trouble will be found in the
equipment. However, if the short circuit does not clear
and the fuse burns out again or the circuit breaker trips,
then the trouble is in the wiring.
To find the short in the electrical wiring, you first
disconnect the wires at both ends of the circuit and test
each wire with an ohmmeter.
If there is a short between the wires, a low-
resistance reading will appear on the ohmmeter. If no
short exists between the wires, a high-resistance reading
will appear on the ohmmeter. You should continue this
procedure until the short is found.
Let us assume that a light circuit is faulty. Using
figure 5-75 as an example, you see a circuit with three
lights controlled by a switch with a short at the junction
box of the middle lamp. Disconnect the wires at the fuse
panel to isolate the circuit and to prevent feedback from
the other circuits.
Connect one lead of the ohmmeter to neutral and the
other to the wire you have just disconnected. With the
switch open, the ohmmeter will read infinity. Closing
the switch will cause the ohmmeter to read continuity,
showing that the short is beyond the switch. You can
now proceed to the nearest junction box and test at the
first light. Remove all light bulbs from the circuit.
Disconnect Point A and connect the ohmmeter
between the neutral and the wire leading to the first
Figure 5-75.Faulty lighting circuit.