Tracing an Open with an Ohmmeter

A modified use of the ohmmeter can also locate this type of open. If the ohmmeter is connected across the open resistor, as shown in Figure 175, an erroneous reading of continuity would be obtained. Even though the circuit switch is open, the open resistor is still in parallel with R1 and R2, and the ohmmeter would indicate the open resistor had a resistance of 15 ohms, the equivalent resistance of the parallel combination of R1 and R2.

Therefore, it is necessary to open the circuit as shown in Figure 176 in order to check the resistance of R3. In this way, the resistor is not shunted (paralleled) by R1 and R2. The reading on the ohmmeter will now indicate infinite resistance, which means the open component has been isolated.