Residual-current device

A ground fault circuit interrupter (GFCI), or Residual Current Device (RCD) is a type of circuit breaker. This type shuts off electric power (or breaks the electrical circuit),   to protect people from an electric shock caused when some of the current travels through a person's body due to an electrical fault such as a short circuit, insulation failure, or equipment that is not working right. 

Standard  circuit breakers shut off power when the current is too high, like 10, 15, or 20 amps. This protects machinery, and is often good enough to prevent hurting people.

The GFCI / RCD breaks the circuit when it detects an imbalance of only 0.005 amps (0.030 amps in Australia and some European and Asian countries).  Even this small amount of current, 0.030 amps, going through one human body might possibly kill, by causing paralysis of skeletal muscles and stopping the heart. 

A standard circuit breaker protects the house electrical wires, receptacles and switches, and appliances and fixtures from dangerous conditions, which could contribute to overheating and possible fire.  A GFCI / RCD  protects people from shock and electrocution and is often found in bathrooms or kitchens where electrical devices may be used near water. While using electrical devices in these locations, someone's bare skin may be in contact with water or a wet floor or metal fixtures.  Any of these can provide an alternate path for current to travel in the case of an electrical fault. 

A GFCI / RCD can also prevent dangers from short circuits and other electrical faults that don't involve humans such as a low current short where the current never reaches the trigger point for a circuit breaker, e.g. a live wire falls in a tub of water or moist soil and only 1 or 2 amps of current flow.

 Requirements 

There are many types of gfci devices that includes several circuit breaker gfci devices as well as receptacle gfci devices. Typically the NES code states that any outlet within 6 feet of a sink or tub or shower enclosure must be a gfci outlet. And that they be on a 20 amp circuit for bathrooms and kitchens. Some municipalities require both a gfci circuit breaker and gfci outlets. Some others require that only the circuit breaker be gfci without necessarily having to install gfci outlets. Outlets that are not within the 6 foot parameter of a water fixture are not required to be gfci.

The National Electric Code (NEC) requires GFCI protection in dwelling on kitchen counter tops, bathrooms, unfinished basements not intended as habitable rooms, crawl spaces, garages, sinks where the receptacles are installed within 6 feet from the top edge of the bowl of the sink, boathouses, bathtubs or shower stalls where receptacles are installed within 6 feet from the edge of the bathtub or shower, laundry areas, outdoors except for receptacles that are not readily accessible and are supplied by a branch circuit dedicated to electric snow-melting. De-icing, or pipeline and vessel heating equipment shall be installed in accordance with NEC 426.28 or 427.22 as applicable.

GFCI protection is required on construction sites.

References

Category:Electrical wiring
Category:Safety
Category:Hardware (mechanical)