Ground Fault Circuit Interrupter
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Ground Fault Circuit Interrupter is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. The presumption is that such an imbalance may represent current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A shock, possibly lethal, is likely to result from these conditions; RCDs are designed to disconnect quickly enough to prevent such shocks.
In the early 1970s most GFCI devices were of the circuit breaker type. However the most commonly used GFCIs since the early 1980s are built into outlet receptacles. The problem with those of the circuit breaker type was that of many false trips due to the poor alternating current characteristics of 120 volt insulations, especially in circuits having longer cable lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current imbalance.
One might more properly call the device a Balance Fault Interrupter (BFI), rather than GFI, because it will trip if current, for example, leaks to or from another circuit such as either the "hot" or "cold" side of a nearby 12 volt DC renewable energy system, or a nearby ethernet jack, etc. The device will trip on any balance fault, not just a balance fault to ground. However, the term "Balance Fault Interrupter" is rarely used in practice.
A GFCI is a outlet that measures current. It measures current running through one circuit, and if the other circuit's current changes, even the littlest change in current, will turn power off to that circuit, and ground all power. You reset it, by pressing the "Reset" Button. If it is bad, replace the GFCI. Test it Weekly-Monthly.
How it works
A GFCI measures the Current flowing through one outlet. If the other outlet has a change in current, it will shut off power to that outlet. GFCI's are also found plug-style on Outdoor Fans, and Hairdryers, etc. They are also found Outlet-style. Say, you had a hairdryer (120-AC Volts), a GFCI (which the Hairdryer was plugged into), and a Bucket full of water. You have turned the hairdryer on, and dropped it into the bucket of water. Then, the current through the outlet(s) become inbalanced, grounding all electrical current running to this outlet(s). You just reset it by pressing the "RESET" button.
