scale of adequate range. Ensure that the circuit is disconnected from the power source before making the necessary circuit changes; then reapply the power.
WARNING 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 are removed.
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 phase separately.
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 open phase.
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.Continue Reading