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A ground fault circuit interrupter (GFCI) or Residual Current Device (RCD) is a device that shuts off an electric circuit when it detects that current is flowing along an unintended path, possibly through water or through a person. It is used to reduce the risk of electric shock. It works by measuring the current leaving the hot side of the power source and comparing it to the current returning to the neutral side. If they are not equal, this means that some of the current is flowing along an unintended path, and the GFCI shuts the power off. When the problem is corrected, the GFCI can manually be reset by pushing the reset button. There is also a test button that can be used to verify that the GFCI works. It is recommended to test GFCIs at least once a month. GFCIs are required in kitchens, bathrooms, unfinished basements, garages, outdoors, and anywhere near a sink. GFCIs are available in two types for permanent installation, the circuit breaker type that installs in the panel, and the receptacle type that installs in a normal electrical box. GFCIs that attach to appliance cords are also available. These are often found on hair dryers. Although GFCIs are designed primarily to protect from electric shock, they can also prevent some fires, in particular fires that result from a live wire touching metal conduit.
GFCIs can be used to upgrade older two-prong (non-grounded) outlets to three-prong (grounded) outlets without installing any new wire. This is safer than using the two-to-three prong adapter, as the adapter may not connect the appliance to ground at all. The 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.