A ground fault circuit interrupter (GFCI), or Residual Current Device (RCD) is a device that shuts off an electric power circuit when it detects that current is flowing along an unintended path, such as through water or a person. It is used to reduce the risk of electric shock, which can cause the heart to stop or cause burns. They can also prevent some fires, like when a live wire touches a metal conduit.
Operation[change | change source]
A GFCI device uses Kirchhoff's current law. This law states that on all points on an electrical circuit the current must be exactly equal to every other point on that same circuit. House wiring normally has a "hot leg" and a "neutral leg". The GFCI device monitors the current "going out" on the hot leg and the current "coming back in" on the neutral. If there is a difference between the two currents, with as little as 4-5 milliamps, then that is an abnormal condition and then the GFCI device will trip and stop all current flow. This is because a difference between the two monitored currents on that one circuit is usually caused by a short circuit. A short circuit is when current leaves the normal current path, and takes an unintended path. This unintended path may send current through the human body, causing harm or death.
Death (electrocution) can happen when as little as 30 milliamps of current flows through the heart for just a fraction of a second. The GFCI device protects at a threshold far less than what is needed to cause harm or death.
One GFCI wall outlet can provide ground fault protection to other standard non-GFCI outlets that are wired on the same circuit after (downstream of) the GFCI device.
It is not common, or advised, to have more than one GFCI device on any one circuit.
If a GFCI device trips and after the problem is fixed, the GFCI device must be reset manually by pushing the reset button (the red button in the image shown at right). If the problem is not fixed, the GFCI device will keep shutting the circuit off.
There is also a test button next to the reset button, which shows that the GFCI works properly. GFCI outlets should be tested at least once a month.
Types[change | change source]
GFCIs are available in two types for permanent installation: the circuit breaker type that installs in an electrical panel, and the receptacle type that installs in a normal electrical wall outlet box. GFCIs that attach to appliance cords, or are built in to extension cords, are also available. Newer hair dryers may have them, too, appearing as a small box at the end of the power cord or on the handle itself.
Since a GFCI only monitors current on the hot leg compared to the neutral and it does not monitor current on a ground, the GFCI can be used to upgrade older two-prong (non-grounded) outlets to three-prong (grounded) outlets without installing any new wire. A circuit with a GFCI device without a ground is far safer than a standard 2 prong outlet without a ground. Using a GFCI without a ground is safer than using the two-to-three prong adapter, as these adapters do not connect the appliance to ground at all.
When a GFCI is installed in the electrical box without connecting the ground screw (as there is no ground wire), a label that says "No Equipment Ground" must be placed on the GFCI outlet and all downstream outlets. Several of these labels are usually included with the GFCI. Note that in some parts of the world, "ground" is called "earth".
Regulations[change | change source]
GFCIs are usually required by local laws to be installed in 2 prong receptacles, kitchens, bathrooms, unfinished basements, garages, outdoors, and anywhere near water.