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Safety and work practicesQuestion 620 of 1506

How does a ground fault current interrupter (GFCI) detect a fault?

  1. It senses a rise in the supply frequency of the circuit.
  2. It senses a drop in voltage across the load in the circuit.
  3. It senses a difference between hot and neutral currents.
  4. It senses heat in the metal case of the connected appliance.
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Correct answer: It senses a difference between hot and neutral currents.

In a healthy circuit, hot and neutral currents are equal. A difference means current leaks through another path, so the device opens.

Source: Lessons in Electric Circuits I: DC (Kuphaldt), 3.8: Safe Circuit Design

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Questions last reviewed: 18 October 2026