WARNING 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 parts.
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.Continue Reading