What does a GFCI monitor and when will it trip?

• A. It monitors current on the hot conductor only and trips if current exceeds 15 A.
• B. It monitors current on the hot and neutral conductors and trips if there is a difference of 5 mA within 1/40th of a second.
• C. It monitors voltage between hot and neutral and trips if voltage difference exceeds 120 V.
• D. It monitors frequency and trips if outside 60 Hz.

Answer: (B)

A GFCI protects by watching what leaves and what returns in the circuit. It compares the current in the hot conductor with the current in the neutral conductor and will trip if there’s any leakage to ground that causes an imbalance. When the difference between those two currents exceeds about 5 milliamps, the device trips, and it does so very quickly—within roughly 1/40th of a second (about 25 milliseconds). This fast response is what helps prevent shock if a person becomes the path to ground.

This isn’t about voltage or frequency. It’s not monitoring the voltage between hot and neutral, nor is it reacting to a 60 Hz condition. It also isn’t just looking at the hot conductor’s current or at a high current like 15 amps—that would be typical for a standard circuit breaker, not a GFCI. The key idea is current balance between hot and neutral; any leakage current that doesn’t return through the neutral triggers the trip.