What does the earth fault loop impedance test assess?

• A. The impedance of the fault loop to ensure protective device will trip within the required time.
• B. The resistance of the power supply.
• C. The continuity of phase conductor.
• D. The continuity of neutral conductor.

Answer: (A)

It checks that, if a fault occurs to earth, the path back to the source has low enough impedance so the protective device will disconnect quickly. The earth fault loop impedance test measures the total impedance of the fault loop (from the live conductor, through the fault to earth, back to the supply) and includes the resistance of conductors, the earth path, and the source. By knowing this impedance, you can determine the fault current that would flow (I_fault = V_source / Zs). The protective device is designed to trip when that fault current exceeds a certain level within a specified time, so the test confirms there is enough fault current to guarantee disconnection within the required time. If the impedance is too high, the device may not trip fast enough, leaving a dangerous situation. This test is not about the power supply’s resistance alone, nor is it a check of continuity of just the phase or just the neutral conductors.