What is a Ground-Fault Circuit Interrupter?
A ground-fault circuit interrupter (GFCI or GFI) is a safety device that is designed to protect either a complete 120-volt, two-wire circuit or a single receptacle against ground-fault currents.
There is an additional level of protection that has been very successful since its use became widespread in the 1960s. It is the ground-fault circuit interrupter (GFCI). With increased usage in the decades that followed, nonutility fatalities from electric shock have declined dramatically, particularly in the home. This simple-looking device has saved the lives of countless thousands.
Ground fault circuit interrupters (GFCI) are used to prevent people from being electrocuted. They work by sensing the amount of current flow on both the ungrounded (hot) and grounded (neutral) conductors supplying power to a device. In theory, the amount of current in both conductors should be equal, but opposite in polarity.
How Does a Ground Fault Circuit Interrupter Work?
If there is a fault between the hot wire and the frame of a defective tool or appliance held by a person, the ground-fault circuit interrupter will open the circuit quickly enough to keep the shock from being dangerous, even if it will be felt.
A ground fault occurs when a path to the ground, other than the intended path, is established. Assume that a person comes in contact with a defective electrical appliance. If the person is grounded, a current path can be established through the person’s body. In the example shown in the below figure, it is assumed that a current of 0.1 amps is flowing through the person.
This means that the hot conductor now has a current of 10.1 amps, but the neutral conductor has a current of only 10 amps. The GFCI is designed to detect this current difference to protect personnel by opening the circuit when it detects a current difference of approximately 5 milliamperes (0.005 amps). Section 210.8 of the National Electrical Code lists places where ground fault protection is required in dwellings.
Present product standards require a GFCI to trip the circuit if the fault current reaches 4 to 6 mA. The allowable tripping time varies inversely with the magnitude of the current.
Another principle of operation is shown in Figure C. In a properly installed two-wire, 120-volt circuit, the amount of current flowing in the grounded wire is precisely the same as in the hot (ungrounded) wire. But if there is a fault in a tool or appliance, part of the current (the fault current) will flow through the grounding conductor, if present. Some will flow through the body of the operator. Therefore the amount of current in the grounded conductor will be less than in the hot conductor. The GFCI senses this difference and opens the entire circuit or the ungrounded wire to the receptacle that it protects.
An ordinary GFCI will not protect a three-wire circuit or receptacles connected to two of the wires of a three-wire circuit. But if a three-wire circuit is divided into two 2-wire circuits, a GFCI can be used to protect one of those two-wire circuits or to protect one or more receptacles on one of those circuits. Be sure the GFCI has the same ampere rating as the circuit, and be sure it is listed for the type of protection for which you are using it. Two-pole GFCI circuit breakers are also available to protect an entire three-wire circuit, or a straight 240-volt, two-wire circuit with no neutral required.
Ground-Fault Circuit Interrupter Types
Ordinary GFCIs are available in various types. One form is a separate device installed to protect a receptacle or circuit. More popular are combination circuit breakers and GFCIs, which are installed in a panelboard to protect an entire 120-volt circuit.
To install the device, run the coiled white wire to the neutral bus bar in the panel, and terminate both the ungrounded and grounded circuit conductors at the appropriate points on the breaker. The breaker needs to sense the current on both sides of the line in order for its GFCI function to operate. The circuit breaker provides ground fault protection for an entire circuit, so any device connected to the circuit is ground fault protected. The second method of protection, ground fault receptacles (Figure E) provides protection at the point of attachment.
If you only need the GFCI protection and not the receptacle (or Code rules prohibit installing a receptacle on the circuit you need to protect), you can use the device in Figure F.)
The receptacle in Figure E is a “feed-through” device, which means it has LINE and LOAD markings. It protects downstream loads connected to its load terminals. Do not mix up the line and load sides of these devices. If you wire the device backward, downstream loads will be protected, but appliances plugged into the receptacle will have no protection. The test button will trip, apparently functioning normally, but the ungrounded receptacle slots will still be hot.
Figure F is essentially a GFCI feed-through receptacle as depicted in Figure E but without any receptacle slots. If the test and reset buttons have additional OFF and ON markings, the device has been evaluated as a manual motor controller and can be used as a switch.
Where Ground-Fault Circuit Interrupters are Required?
The NEC requires GFCI protection for 15- or 20-amp 125-volt receptacles as follows (there are many other required locations, but this list covers most of the common examples):
- In residential properties, for all receptacles in the following locations: outdoors including balconies (except those for rooftop snow-melting equipment, for which special rules apply), bathrooms, garages, serving kitchen countertops, within 6 ft of any sink, bathtub, shower stall, laundry areas, crawl spaces, unfinished basements, and boathouses. The GFCI may be installed to protect individual receptacles, or the entire branch circuit supplying the receptacles.
- In other occupancies, for all receptacles in bathrooms and on rooftops and outdoors, again with an exception for rooftop de-icing equipment for which special rules apply. The requirement also applies to all nonresidential kitchen receptacles (“kitchen” being defined for this purpose as an area with a sink and permanent facilities for food preparation). The NEC also applies this requirement to receptacle outlets within 6 ft of sinks, with only two exceptions that cover applications beyond the scope of this book. In addition, the NEC requires coverage of indoor wet locations, garages and service bays and the like (but not exhibition halls and showrooms), and locker rooms if they have associated showering facilities.
- Formerly just for construction sites, now for all 15-, 20-, and 30-amp, 125-volt, single-phase receptacles that are used for “construction, remodeling, maintenance, repair, and demolition” activities, whether or not on a construction site. An exception applies for some industrial occupancies with expert supervision, but only in cases where a random nuisance trip would create a greater hazard, or in cases where the powered equipment can be shown to be incompatible with GFCI devices.
- Dwelling unit dishwashers and boat hoists, even if hardwired.
If you replace a receptacle, and it is in a location where the NEC now requires GFCI protection for that receptacle, even if no GFCI protection was required at the time the original receptacle was installed, you must provide GFCI protection for the replacement. If you replace a receptacle and there is an equipment grounding conductor available at the outlet, the replacement receptacle must be of a grounding configuration and properly grounded whether or not the receptacle needed to be grounded at the time it was originally installed.
However, if you replace a receptacle at an outlet with no equipment grounding conductor present, you have three options, assuming you don’t plan to rewire the branch circuit. You could (1) use a new, non-grounding receptacle, (2) use a GFCI receptacle (they only come in grounding configurations) with a NO EQUIPMENT GROUND marking on the faceplate, or (3) use a conventional grounding receptacle fed from a GFCI protective device and mark the receptacle location with both NO EQUIPMENT GROUND and GFCI PROTECTED.
Ground-Fault Protection of Equipment (GFPE)
There is a form of protection closely related to the GFCI that is used to prevent very low-level arcing faults from causing fires. It is typically available in specialized circuit breakers. Imagine circuitry identical to that of a GFCI, but with the trip set in the neighborhood of 30 mA. This would not provide adequate shock protection, but the NEC does require this protection for frost protection systems run for snow-melting purposes (NEC 426.28) and along with piping systems (NEC 427.22). Experience has shown that these cables have so much resistance that faults will not trip an overcurrent device, but they will sputter for long periods, eventually starting fires. Although GFPE in this sense has a far different “feel” than the industrial systems, with their current imbalance trigger points set for hundreds of amperes, the principles are the same. Arcing faults usually produce a current flow outside of normal circuit paths; that difference can be detected, and the circuit is automatically opened. Be sure you understand the differences so you can order and apply the appropriate protective devices. Do not confuse any of these devices with the new AFCIs, also designed to clear low-level arcing faults. AFCI devices use different technology to provide a more comprehensive approach to addressing these faults, which may involve line-to-line faults or even arcing across two damaged ends of a single conductor.