Definition
A ground fault circuit interrupter (GFCI), also called a residual current device (RCD) or, in Korea, a nujeon chadangi (누전차단기, "leakage breaker"), is a household safety device that cuts power in a split second when electricity leaks out of a wire or appliance, preventing electric shock and electrical fires. In Korean homes, you'll usually find it inside the apartment's breaker panel, which people nickname the dukkeobi-jip ("toad house" box).
It compares the electricity going in with the electricity coming out, in real time
The electricity in your home works like a round-trip train ticket. Power flows in through one slot of an outlet, runs the appliance, and must come back out through the other slot in exactly the same amount. When everything is normal, the current going in matches the current coming out.
But say the wiring inside a washing machine gets old and the insulation wears off, or water gets into an appliance. Some of the current then escapes along the wrong path. It might run through a person's body into the ground, or travel along the metal casing of the machine. That's called a ground fault, or a leakage current.
Inside a GFCI, a small donut-shaped sensor (a zero-sequence current transformer) wraps around the wires. It compares the current going out with the current coming back, moment by moment. If it spots even a tiny gap, it knows power is leaking somewhere. The instant it notices the difference between the electricity that went in and the electricity that came back, it flips the switch and cuts the power.
Why does it cut power in just 0.03 seconds?
When current flows through the human body, it can seriously disrupt the heart. In particular, ventricular fibrillation, where the heart quivers irregularly, can become life-threatening within just a few seconds. So household GFCIs are precisely designed to trip within 0.03 seconds (30 milliseconds), in the blink of an eye, before the heart is affected that badly.
To be more precise, a GFCI doesn't cut power for every tiny difference in current. Wires and appliances always leak a very small amount even when they're working fine, and if the power cut out every time, daily life would be a hassle. So a standard household unit in Korea trips only when it detects about 30 milliamperes (mA) or more of leakage current, and then it acts like lightning.
Bathrooms and laundry rooms are wet, and wet skin makes shocks far more dangerous. So a more sensitive, high-sensitivity 15 mA breaker is sometimes installed there separately. This way, a GFCI protects your life as a last line of defense without making everyday life inconvenient.
How it differs from a regular circuit breaker, and how to test it properly
Open the breaker panel and you'll see a bunch of switches. Layouts differ from home to home, so don't go by position. Just remember: the one with a test button on its front is the GFCI. A regular circuit breaker protects against overload, when you draw too much power at once and the wires overheat, and against short circuits. A GFCI protects against electricity leaking out. That said, most Korean household GFCIs also include overcurrent protection, so they can trip when you use a lot of power, too.
On the front of a GFCI there's a small red or yellow test button. When you press it lightly with a finger, it creates an artificial leakage signal inside. If the switch drops down with a "click," that's proof the breaker can detect a dangerous situation properly.
Internal parts can wear out over time, so it's important to get into the habit of pressing the test button once a month. If you press it and the switch doesn't drop, the breaker has probably failed. Replacing a breaker inside the panel is electrical work with a risk of shock, so it's usually inspected and replaced by a professional, such as the Korea Electrical Safety Corporation or a licensed electrician.
Common misconceptions
A breaker only trips because you used too much electricity at once.
Using too much power (overcurrent) can trip it, but it also works when electricity leaks out because of damaged wire insulation or moisture (a ground fault). These are two completely different causes.
If you have a GFCI, there's no risk of electric shock at all.
A GFCI looks at the difference between the current going in and the current coming back, so it works even without a ground wire. But it works by detecting current that has already started leaking and then cutting it, so it can't prevent even the briefest shock. With proper grounding, leaking current flows into the earth before anyone touches the appliance, so the breaker trips sooner and the voltage on the appliance's casing stays lower, which is much safer.
Where you meet it
On a rainy day, rainwater gets into an outdoor air conditioner unit and the GFCI trips.
The insulation on an old hair dryer wears away, and the exposed copper wire touches the metal body or a wet hand, so electricity leaks out and the breaker trips instantly.
In one sentence
When electricity that went in doesn't fully come back and leaks out instead, a GFCI cuts the power in 0.03 seconds to prevent electric shock and fires.