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Difficulty: MediumCircuit Faults and Troubleshooting

A technician performs electrical diagnostic checks on various direct-current (DC) circuit components. Match each multimeter measurement or symptom to the correct circuit condition or fault.

  • An ohmmeter connected across an isolated relay coil reads infinite resistance (O.L.).Open circuit fault within the coil winding
  • A voltmeter connected across an intact fuse in an active, energized circuit reads 0 V0\text{ V}.Normal operation with negligible voltage drop
  • A voltmeter across a load reads 0 V0\text{ V} while branch current surges and immediately blows the upstream protective fuse.Direct short circuit across the load
  • An ohmmeter connected across closed switch contacts measures 180 Ω180\,\Omega instead of near 0 Ω0\,\Omega.High-resistance fault due to contact pitting or corrosion

Answer

Match the open coil to infinite resistance, intact fuse to 0 V drop, shorted load to 0 V with high current surge / blown fuse, and high-resistance contact fault to elevated closed-switch resistance.
Each diagnostic measurement corresponds to a fundamental circuit state: infinite coil resistance reflects an open circuit; zero volts across an intact fuse demonstrates normal low-resistance conduction; zero volts with blown protection indicates a short circuit; and elevated resistance across closed switch contacts indicates a high-resistance contact fault.

Step-by-Step Solution

1
Analyze the isolated coil resistance test reading
An infinite resistance (O.L.) measurement signifies an internal discontinuity or open circuit.
Current cannot pass through a severed conductor, resulting in infinite measured resistance.
2
Evaluate the in-circuit voltage drop across the closed fuse
A 0 V drop across an intact conducting fuse indicates normal low-resistance operation.
According to Ohm's law (V=I×RV = I \times R), a conductor with negligible resistance produces virtually no voltage drop.
3
Identify the fault causing 0 V across a load with blown overcurrent protection
A short circuit parallel to or across the load bypasses normal resistance and draws excessive current.
Zero branch resistance eliminates voltage drop across the parallel terminals while causing current to surge past the fuse rating.
4
Interpret the elevated resistance reading across closed switch contacts
A reading of 180 Ω180\,\Omega represents an abnormal high-resistance fault.
Clean, closed mechanical contacts should read less than 1 Ω1\,\Omega; oxidation and pitting insert unwanted resistance into the path.

Key Concept

Circuit Fault Diagnosis and Multimeter Interpretation
Estimated Time:1m 15s
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Topics

Circuit Faults and Troubleshooting

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