Tracing an Open with an Ammeter
If the technician now places an ammeter in the circuit, the total current would be indicated as 2 amperes as show in Figure 10-174 instead of the calculated 3 amperes. Since 1 ampere of current should be flowing through each branch, it is obvious that one branch is open. If the ammeter is then connected into the branches, one after another, the open branch will eventually be located by a zero ammeter reading.
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 10-175, 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 10-176 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.
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