What is the approximate current at which a ground-fault detector will open a circuit?

• A. 1 mA
• B. 5 mA
• C. 15 mA
• D. 30 mA

Answer: (B)

The idea being tested is how sensitive a ground-fault detector (GFCI) is to leakage current. A GFCI watches the current in the hot and the neutral. If any portion leaks to ground, the two currents won’t match, and the device trips to cut power.

Five milliamps is used because it’s low enough to protect people from shock—early human perception and the risk of harm begin at only a few milliamps, and a threshold around 5 mA lets the device trip quickly before a dangerous situation develops. At the same time, this level is high enough to avoid nuisance trips from normal, tiny leakage currents that can occur in wiring and equipment.

Higher thresholds like 15 mA or 30 mA are used in other protective contexts but do not provide the same level of personal protection as a 5 mA trip. Very small currents (around 1 mA) are often not enough to reliably trigger a fast, protective shutoff.

So, the approximate current that will cause a ground-fault detector to open the circuit is about 5 mA.