How fast will the GFCI interrupt the electric power?

• A. 1/40th of a second
• B. 1/4th of a second
• C. 1 second
• D. 10 seconds

Answer: (A)

GFCI devices are designed to shut off power almost instantaneously when a ground fault is detected. They continuously monitor the current in the hot and neutral conductors and look for even a small imbalance. If leakage to ground occurs—typically about 5 mA—the device trips and interrupts the circuit extremely quickly, usually within about 25 milliseconds. That 25 ms is 1/40 of a second, which is the reason this choice is correct. This rapid response helps prevent a dangerous shock by cutting off power long before the fault current can harm someone. Other times listed (like a quarter of a second, one second, or ten seconds) would be far too slow to reduce the risk in a shock scenario.