The ground fault senses a difference in the flow of current from the hot wire through the neutral, if that difference is about 5 milliamps or more the ground fault will trip the circuit out. It actually assumes that if the current is not flowing in the neutral it is flowing through something else. Some motor windings have sufficient losses to cause one to trip out so don't use a G.F.I. circuit for a refrigerator or washer outlet.

You should use (and the NEC requires) the use of G.F.I. protected outlets within6 feet of a sink, anywhere in a bathroom, in a garage or outside; anywhere an outlet can be reached from a water source, a wet area, or earth ground, you should use G.F.I. protection.

 

A G.F.I. receptacle has a line side (incoming power) and a load side (outgoing power). The receptacle will not work if the incoming power is connected to the load side of the receptacle. Connect the incoming power to the line marked terminals and the continuation of the circuit (the next outlet) to the load terminals. The one G.F.I. will protect all the following plugs or receptacles connected in this way. Even if you don't have a continuation of the circuit, connect the power to the line side of the receptacle.

 

G.F.I. receptacles and G.F.C.I. breakers have a test button that should cause the circuit to trip, operate the test button after installing and regularly there after to be sure it works properly.

Where do you put G.F.I.s?

Any bathroom or garage outlet within 6' of a sink must be G.F.C.I protected. The code also requires all kitchen outlets for countertop use to be G.F.C.I protected. G.F.C.I outlets must be installed in any area where electricity and water may come into contact, including basements, pools, spas, utility rooms, attached garages and outdoors. At least one G.F.C.I. outlet is required in an unfinished basement and for most outdoor outlets.
 

The are two types of G.F.C.I.s in homes, the G.F.C.I outlet and the G.F.C.I circuit breaker. Both do the same job, but each has different applications and limitations.
The G.F.C.I. outlet is actually a replacement for a standard electrical outlet. A G.F.C.I. is not dependent of a ground to function. It does not measure shorts to the ground, it measures the current difference between the hot and neutral wires. A sudden difference of 5 ma. or more, indicating that there is another path for the electricity to flow through will trip this device. The only downside to this is there may be some nuisance tripping in highly inductive loads like large motors or even fluorescent lamps or fixtures on the same circuit. But the newer models seemed to have corrected this somewhat.
 

It protects any appliance plugged into it, and can also be wired to protect other outlets that are connected to it. In reality, G.F.I. protection is designed to protect persons and as a result, items plugged into those outlets are also protected.

The G.F.C.I. circuit breaker controls an entire circuit, and is installed as a replacement for a circuit breaker on your home's main circuit board. Rather than install multiple G.F.C.I. outlets, one G.F.C.I. circuit breaker can protect the entire circuit. There is a test button and a reset button on these units. If you press the test button the reset should pop out. To reset, just push the reset button in.
 

It is not a good idea to put lights on G.F.C.I. protected circuits so you aren't left in the dark if the circuit trips. Generally, equipment such as refrigerators, freezers and sump pumps that cannot go without electrical power for an extended period of time without causing costly losses or property damage should not be placed on a G.F.C.I. protected circuit. G.F.C.I.s are very sensitive and are subject to nuisance tripping. G.F.C.I. receptacles don't last outdoors even under the best of conditions. Be sure to test the device using the "test" button before you use one.

 

Can you use a G.F.I. in an ungrounded outlet?

The N.E.C. allows a G.F.C.I. to be used in an outlet with a two wire ungrounded cable, but this might not be that great an idea for several reasons.
First: The ground lug on a G.F.C.I. receptacle might give the false impression that the outlet has a grounding wire.
Second: The surge suppressors used for computers and other electronic equipment require a properly installed grounding wire to work correctly.
There are several other ways to ground outlets. Here is a couple of them. First...Find out if the metal box that holds the receptacles has a proper ground, if it does you can either run a pigtail from the threaded screw in the box to the ground on your receptacle, or install a self grounded type receptacle. Second...And possibly the best and safest way is to run grounded cable back to your panel for each branch circuit. Whichever way you decide to go, these outlets should be grounded for your own safety.

Click on the link below for a chart that will tell you when each requirement for G.F.I. protection was added to the electric code courtesy of  Jerry Peck and the late Norm Sage.

G.F.I. timeline chart

 From the 2005 NEC (National Electric code)

210. Ground-Fault Circuit-Interrupter Protection for Personnel.

FPN: See 215.9 for ground-fault circuit-interrupter protection for personnel on feeders.
(A) Dwelling Units. All 125-volt, single-phase, 15- and 20-ampere receptacles installed in the locations specified in (1) through () shall have ground-fault circuit-interrupter protection for personnel.
(1) Bathrooms
(2) Garages, and also accessory buildings that have a floor located at or below grade level not intended as habitable rooms and limited to storage areas, work areas, and areas of similar use
Exception No. 1 to (2): Receptacles that are not readily accessible.
Exception No. 2 to (2): A single receptacle or a duplex receptacle for two appliances located within dedicated space for each appliance that, in normal use, is not easily moved from one place to another and that is cord-and-plug connected in accordance with 400.8(A)(6), (A)(8), or (A)().
Receptacles installed under the exceptions to 210.(A)(2) shall not be considered as meeting the requirements of 210.52(G)
(3) Outdoors
Exception to (3): Receptacles that are not readily accessible and are supplied by a dedicated branch circuit for electric snow-melting or deicing equipment shall be permitted to be installed in accordance with 426.2.
Receptacles installed under the exceptions to 210.(A)(5) shall not be considered as meeting the requirements of 210.52(G).
(4) Crawl spaces — at or below grade level
(5) Unfinished basements — for purposes of this section, unfinished basements are defined as portions or areas of the basement not intended as habitable rooms and limited to storage areas, work areas, and the like
Exception No. 1 to (5): Receptacles that are not readily accessible.
Exception No. 2 to (5): A single receptacle or a duplex receptacle for two appliances located within dedicated space for each appliance that, in normal use, is not easily moved from one place to another and that is cord-and-plug connected in accordance with 400.8(A)(6), (A)(8), or (A)().
Exception No. 3 to (5): A receptacle supplying only a permanently installed fire alarm or burglar alarm system shall not be required to have ground-fault circuit-interrupter protection.
(6) Kitchens — where the receptacles are installed to serve the countertop surfaces
(8) Laundry, utility, and wet bar sinks — where the receptacles are installed within 1. m (6 ft) of the outside edge of the sink
() Boathouses
(B) Other Than Dwelling Units. All 125-volt, single-phase, 15- and 20-ampere receptacles installed in the locations specified in (1) through (5) shall have ground-fault circuit-interrupter protection for personnel:
(1) Bathrooms
(2) Commercial and institutional kitchens — for the purposes of this section, a kitchen is an area with a sink and permanent facilities for food preparation and cooking
(3) Rooftops
(4) Outdoors in public spaces—for the purpose of this section a public space is defined as any space that is for use by, or is accessible to, the public

Exception to (3) and (4): Receptacles that are not readily accessible and are supplied from a dedicated branch circuit for electric snow-melting or deicing equipment shall be permitted to be installed in accordance with the applicable provisions of Article 426.
(5) Outdoors, where installed to comply with 210.63
(C) Boat Hoists. Ground-fault circuit-interrupter protection for personnel shall be provided for outlets that supply boat hoists installed in dwelling unit locations and supplied by 125-volt, 15- and 20-ampere branch circuits.

Click on the links to the left for more important information.