Troubleshooting the Shorting Faults in Parallel Circuit

As in a series circuit, a short in a parallel circuit will usually cause an open circuit by blowing the fuse. But, unlike a series circuit, one shorted component in a parallel circuit will stop current flow by causing the fuse to open. Refer to the circuit in Figure 10-177.

If resistor R3 is shorted, a path of almost zero resistance will be offered the current, and all the circuit current will flow through the branch containing the shorted resistor. Since this is practically the same as connecting a wire between the terminals of the battery, the current will rise to an excessive value, and the fuse will open. Since the fuse opens almost as soon as a resistor shorts out, there is no time to perform a current or voltage check. Thus, troubleshooting a parallel DC circuit for a shorted component should be accomplished with an ohmmeter. But, as in the case of checking for an open resistor in a parallel circuit, a shorted resistor can be detected with an ohmmeter only if one end of the shorted resistor is disconnected and isolated from the rest of the circuit.

Troubleshooting the Shorting Faults in Series-Parallel Circuit

Logic in Tracing an Open

Troubleshooting a series-parallel resistive circuit involves locating malfunctions similar to those found in a series or a parallel circuit. Figures 10-178 through 10-180 illustrate three points of failure in a series-parallel circuit and their generalized effects.

  • In the circuit shown in Figure 10-178, an open has occurred in the series portion of the circuit. When the open occurs anywhere in the series portion of a series-parallel circuit, current flow in the entire circuit will stop. In this case, the circuit will not function, and the lamp, L1, will not be lit.
  • If the open occurs in the parallel portion of a series-parallel circuit, as shown in Figure 10-179, part of the circuit will continue to function. In this case, the lamp will continue to burn, but its brightness will diminish, since the total resistance of the circuit has increased and the total current has decreased.
  • If the open occurs in the branch containing the lamp, as shown in Figure 10-180, the circuit will continue to function with increased resistance and decreased current, but the lamp will not light.

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