A ground fault circuit interrupter (GFCI), also called Ground Fault Interrupter (GFI) 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.
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 these adapters 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. Note that in some parts of the world, "ground" is called "earth".