scale of adequate range. Ensure that the circuit is
disconnected from the power source before making the
necessary circuit changes; then reapply the power.
Be extremely careful not to touch the hot
conductors when you use this method of
testing because these live points of the circuit
are exposed when the junction box covers
Let us troubleshoot a circuit with a voltmeter. The
power to the circuit must be turned on. The first and
most logical place to check is the fuse or circuit breaker
panel. Set the voltmeter to the proper scale. If you do
not know the value of the incoming voltage, set the
meter to the highest scale; then work down to the proper
scale. Check each incoming phase by connecting one
lead of the voltmeter to the neutral and the other to each
On a three-phase 120/208, 240-volt service, you
must get 120 volts on each phase to ground. Less than
120 volts on one phase means that phase is open and you
are getting a feedback from equipment connected to the
lead side of the panel. Sometimes there will be a slight
variation of normal voltage from the different phases;
therefore, to determine if one phase is dead, check
between the phases.
To perform this test, place one lead of the voltmeter
on Phase A and the second lead on Phase B and read the
voltage. It should read approximately 208 or 240,
depending upon the system. After you have taken this
reading, move the second lead to Phase C and take the
reading. After this reading, move the first lead to Phase
B and take the reading. You have now read between all
phases and a lower than normal reading indicates an
Which phase is dead? Assume that Phase B has a
blown fuse. When you take your reading between
Phases A and B (fig. 5-69). you get a low-voltage
reading. Your next reading, between Phases A and C,
reads normal. But the next reading, between Phases B
and C, again is a low reading. Each time you read to
Phase B, you get low voltage. This reading is a good
indication that Phase B is open.
Another way to determine which phase is open is to
place one voltmeter lead on the top of the fuse and the
other lead on the bottom of the same fuse. If you get a
voltage reading across the fuse, that fuse is open.
Figure 5-69.Checking a three-phase circuit for a blown fuse.
Assuming that everything is all right at the main
panel, let us examine a single 120-volt circuit.
Illustrations help explain the procedure for locating an
open in a circuit. Figure 5-70 shows a circuit with a
lamp in series with a single-throw switch and fuse and
the normal voltage readings at the various points of the
circuit. If the lamp fails to light, check the circuit in
progressive steps through the circuit and lamp from the
last point where voltage is known to be present. In
figure 5-71, we have voltage at one connection of the
Figure 5-70.Circuit with fuse, switch, and lamp.
Figure 5-71.Circuit with blown fuse.