A worker is installing a new service entrance for a residential dwelling. Following the installation, an electrician performs a ground fault impedance test on the main panel. The test reveals a ground fault loop impedance of 0.8 ohms. The main circuit breaker is rated at 100 A (Type C). What does this impedance value indicate regarding fault clearing capability?

Question: A worker is installing a new service entrance for a residential dwelling. Following the installation, an electrician performs a ground fault impedance test on the main panel. The test reveals a ground fault loop impedance of 0.8 ohms. The main circuit breaker is rated at 100 A (Type C). What does this impedance value indicate regarding fault clearing capability?

Answer options:

• The ground fault current will be too high, potentially damaging equipment.
• The overcurrent device will clear a ground fault effectively. ✅ The impedance is too high, and the overcurrent device may not trip rapidly enough.
• The installation is likely a floating neutral system, requiring further investigation.

Correct answer: The impedance is too high, and the overcurrent device may not trip rapidly enough.

Explanation: For a 120 V circuit protected by a 100 A Type C breaker, the tripping current for instantaneous operation (short-circuit protection) ranges from 5 to 10 times the rated current (500-1000 A). For effective fault clearing, the calculated fault current (Voltage/Impedance) should exceed this trip threshold significantly. With 120 V / 0.8 ohms = 150 A, this current is too low to guarantee rapid tripping of a 100 A Type C breaker for instantaneous fault protection, which would require at least 500 A. Therefore, the impedance is too high, and the overcurrent device may not trip rapidly enough. This scenario indicates a safety concern as the fault might persist, potentially causing overheating or fire before the breaker trips.