Technical Article

An Introduction to Ground-Fault Circuit Interrupters (GFCIs)

August 16, 2021 by Alex Roderick

This short series will touch on the basics and the differences between GFCIs and AFCIs. This article will focus on ground-fault circuit interrupters.

GFCIs are designed to protect a person from being shocked or electrocuted when they come in contact with a device that is accidentally touching a circuit. AFCIs, on the other hand, are designed to protect a home from catching fire due to arcing in defective conductors or appliances (see Figure 1).

Figure 1. GFCIs are designed to protect a person from being shocked when a circuit is accidentally touching a device that a person comes in contact with. AFCIs are designed to protect a home from catching fire due to arcing in defective conductors or appliances. Image courtesy of Eaton


Electrical Shock

An electrical shock is a shock that results anytime a body becomes part of an electrical circuit. Electrical shock kills over 1000 people a year in the United States, according to the National Safety Council (NSC). Electrocution can take place when any electrical current entering the human body is strong enough to stop the natural rhythm of the heart. Water with impurities is an excellent conductor of electricity, and the human body is 70% water. When a person touches an energized bare electrical wire or a faulty appliance, the electricity will use the body as the shortest path to the ground (see Figure 2).

Figure 2. Electrical shock is a dangerous occurrence that results in any time a body becomes part of an electrical circuit.

The severity of an electrical shock depends on the amount of electric current (in mA) that flows through the body, the length of time the body is exposed to the current flow, and the path the current takes through the body. The amount of current that passes through a circuit depends on the voltage and resistance of the circuit. During an electrical shock, a person’s body becomes part of the electrical circuit. The resistance a person’s body offers to the flow of current can vary. Sweaty hands have less resistance than dry hands. A wet floor has less resistance than a dry floor. The lower the resistance, the greater the current flow. The greater the current flow, the greater the severity of shock.

GFCIs have a trip level of 6mA, which is the amount of electrical current a human body can withstand without serious harm. The physical effects will differ depending on whether the individual is male or female and if they are an adult or an infant. Typically, at about 10mA, a person may not be able to let go if they touch an energized conductor. In small children, current flow as low as 30 mA may cause breathing difficulties and heart problems. A current over 2000mA (2 A) can cause heart paralysis and tissue burning.



A ground-fault circuit interrupter (GFCI) is an electric device that protects personnel by detecting ground faults and quickly disconnecting power from the circuit (see Figure 3). GFCIs were created due to the limitations of commonly used circuit breakers, which were designed to trip only when large currents are present (short or overload). Individuals have been electrocuted by equipment in electrical systems where the fault current was not great enough to trip a standard circuit breaker. Currents present from deteriorated insulation and minor equipment damage do not produce enough current flow to open a standard circuit breaker. GFCIs were specifically designed with such leakage currents in mind and can react to current as small as 5⁄1000 (0.005) of an amp in a fraction of a second.

Figure 3. GFCIs are designed to protect a person from being shocked or electrocuted when they come in contact with a device that is accidentally touching a circuit. GFCIs can react to a current as small as 5⁄1000 (0.005) of an ampere in a fraction of a second. Image courtesy of Family Handyman


GFCI Operation

A sequence of events takes place when a fault current is detected by a GFCI. Typically, a load (for example, an electric shaver, drill, or garden tool) has the same amount of current flowing to it through the black or red (hot) wire as flowing away from it through the white (neutral) wire. However, in the event of a ground fault, some of the current, which normally returns to the power source through the white wire, is diverted to the ground.

Because a current imbalance exists between the hot and neutral wires, the sensing device detects the current difference and signals an amplifier. When the amplified signal is large enough, the amplifier activates the interrupting device. Once the interrupting device is activated, the circuit is opened, and the current to the load is shut off.

After the ground fault has been discovered and repaired, the GFCI is reset and is ready to begin the process again. GFCIs have a test circuit built into the unit so that the GFCI can be tested on a monthly basis without a ground fault condition.


GFCI Required Locations

The National Electric Code® requires ground-fault circuit interrupters for protection in many areas. It is always a good idea to check the NEC requirements as well as local building codes. Required residential GFCI locations include outdoor receptacles, bathrooms, residential garages, kitchens (countertop areas), unfinished basements, and crawl spaces.

Additional locations that require GFCI receptacles include construction sites as well as the outdoor receptacles and bathrooms of mobile homes and mobile home parks. GFCIs must also be installed nearby swimming pools and fountains. These locations include receptacles near pools, lighting fixtures and lighting outlets near pools, underwater lighting fixtures over 15V, electrical equipment used with storable pools, fountain equipment operating at over 15V, and cord-and plug-connected equipment for fountains.


GFCI Installation

GFCIs are installed in standard receptacle boxes indoors and in weatherproof boxes outdoors. GFCI receptacles are typically installed individually. GFCIs may also be installed to protect several standard receptacles in one continuous circuit. GFCIs are considered easy to install, and most manufacturers supply an installation kit to aid in installation. In addition to being available as receptacles, GFCIs are also available as circuit breakers to protect entire circuits.


Plug-in GFCIs

Plug-in GFCIs are the most convenient type of GFCI device to use. Through the simple use of a plug-in GFCI in any grounded receptacle, any tool plugged into the GFCI device and its operator are protected (see Figure 4).


Figure 4. Plug-in GFCIs are the most convenient GFCI device to use. Image courtesy of Cable Organizer


Individuals should typically keep a plug-in GFCI unit in a toolbox and use the device whenever working with power tools outdoors, especially when conditions are wet.