Figure 5-72. - Circuit with burned-out lamp.
fuse and no voltage at the other. Since the fuse is a conducting unit. normally the same voltage reading should occur between both sides of the fuse and the ground. The only conclusion in this case, then, is that the fuse is open. Figure 5-72 shows that there is a voltage reading when the voltmeter is connected across the lamp. The logical assumption is that the lamp is inoperative. To be sure the lamp is inoperative, you must check it with an ohmmeter. Fuses, switches, and lamps are vulnerable; and you should check them first in a circuit. In figure 5-73. the lamp does not light and the voltmeter shows voltage from the ground screw of the lamp to the neutral wire. These conditions indicate an open in the ground wire. When you connect the voltmeter at another point at the right of the lamp and no voltage is indicated, you can assume that there is an open in the wiring between this point and the lamp connection. In figure 5-74, you find two lamps wired in parallel so that they can be controlled by a double-throw switch. With the switch in the OFF (center) position, there is no complete circuit, and neither lamp lights. When the switch is in the BRIGHT position, a circuit is completed through the switch and through both lamps. With the switch in this position, the only resistance in the circuit is the resistance of the lamps. When the switch is in the DIM position, the circuit is completed through the lamps as before; but this circuit has an additional resistor in series with the lamps. This added resistance causes a decrease in current flow; therefore, the lamps glow with less intensity than before. If one of the lamps lights and the other one does not, it is not necessary to check the complete circuit to find the open. The part of the circuit up to Point A is common to both lamps, and that much of the circuit must be completed for even one lamp to light. The place to begin checking the circuit is after Point A in the affected part of the circuit. In a circuit, such as the one shown in figure 5-74, it is best to use a voltmeter to locate the trouble. If you connect the negative lead of the voltmeter to the ground and the positive lead to Point A, you will get a reading on the voltmeter scale because Point A is connected through the switch to the positive line wire. If the positive lead of the voltmeter is moved in succession from A to B to C, you are able to check the continuity of wires AB and BC. If the check at Point C reveals no voltage, this condition indicates that wire BC is open. You can make this same check with an ohmmeter, but several additional steps are required. First, remove power from the circuit by placing the circuit breaker and circuit switch in the OFF position. Next, disconnect the junction of wires at Points A and C. Then, with one ohmmeter lead placed on the loose wire at A and the other one on B, check the continuity of the wire. When this check is made and the ohmmeter indicates a low resistance, you have continuity in the wire. But if you place one ohmmeter lead on B and the other on the disconnected wire at C and you get an infinite resistance, there is an open in the wire.
Figure 5-73. - Circuit with open in the wiring.
Figure 5-74. - Troubleshooting an open circuit with a voltmeter.Continue Reading