GFCI and AFCI protection

An ordinary breaker protects the wiring: it opens when current exceeds the circuit rating long enough to overheat the conductors. It does nothing for a person who touches a live conductor while standing on a wet floor, because a current that can stop a human heart is a tiny fraction of an ampere, far below any breaker rating. Nor does it reliably detect the arcing of a damaged cord or a loose connection, which can start a fire while drawing less current than the circuit rating. Two devices fill these gaps: the ground-fault circuit interrupter (GFCI), which protects people from shock, and the arc-fault circuit interrupter (AFCI), which protects buildings from fires caused by arcing. Together with tamper-resistant receptacles and smoke alarms, they are among the most effective safety improvements in residential wiring of the past fifty years.

Electric shock

The effect of current on the body depends on its magnitude, path and duration. Approximate thresholds for 60 Hz alternating current flowing hand to foot are:

The current is set by the voltage and the body's resistance, which is dominated by the skin. Dry skin may present tens of thousands of ohms; wet or broken skin, a few hundred to a thousand. At 120 V, a wet body path of 1000 Ω passes 120 mA, which can kill. The danger rises sharply in bathrooms, kitchens, outdoors, basements and anywhere a person may be wet or in contact with earth.

How a GFCI works

In a healthy circuit, all the current that leaves on the hot conductor returns on the neutral. A GFCI passes both conductors through a small current transformer (a toroidal core). As long as the currents are equal and opposite, their magnetic fields cancel and the transformer's sense winding sees nothing. If some current leaks to ground, through a person or through damaged insulation, the outgoing and returning currents differ, the core sees a net field, and the electronics trip a set of contacts that open the circuit.

Class A GFCIs, the type required for personnel protection in the US, are listed to UL 943 and trip at a ground-fault current of 6 mA or less (and must not trip below 4 mA), within times defined by an inverse time-current curve: the higher the fault current, the faster the trip, typically within about 25 milliseconds for large faults.

A GFCI does not limit the current that flows through the person at the instant of contact; it limits the duration, which is what determines whether the heart fibrillates. It also cannot protect against a shock between hot and neutral, since that current returns on the neutral and causes no imbalance; for that reason GFCIs complement, rather than replace, insulation and enclosures. A GFCI works without an equipment grounding conductor, so a GFCI is an accepted way to protect old two-wire circuits where a ground cannot be added (the receptacles are then marked "No Equipment Ground" and "GFCI Protected").

Types of GFCI

• GFCI receptacle: protects its own outlets and, if wired from its LOAD terminals, every receptacle downstream on the circuit. Line and load must not be reversed; modern devices will not reset if they are.
• GFCI breaker: protects the whole circuit from the panel, useful where the first receptacle is hard to reach or the circuit supplies a hard-wired load such as a hot tub.
• Portable and cord-mounted GFCIs, for temporary power and tools.
• Deadfront GFCIs: a GFCI without receptacle slots, mounted in a wall box to protect downstream outlets from a convenient location.

GFCIs made since about 2015 must monitor themselves automatically and either trip or signal at end of life. All GFCIs should still be tested with the TEST button regularly (monthly is the classic advice) and after installation.

Where GFCI protection is required

In US dwellings, NEC 210.8(A) (2023 edition) requires GFCI protection for 125 V through 250 V receptacles supplied by single-phase branch circuits of 150 V or less to ground in: bathrooms; garages and accessory buildings; outdoors; crawl spaces; basements; kitchens (all receptacles, not only countertop); areas with sinks, within 6 ft of the bowl edge; boathouses; within 6 ft of bathtubs and showers; laundry areas; and indoor damp and wet locations. Specific appliances, such as dishwashers, also require GFCI protection (210.8(D) and 422.5), and outdoor outlets such as those for heat pumps were added in 2020–2023 (210.8(F)).

Arc faults and AFCIs

An arc fault is an unintended electric arc in wiring or cords. Arcs reach thousands of degrees and can ignite insulation, wood framing and dust, and many residential electrical fires start this way. Two kinds matter:

• Series arcs occur within one conductor, at a loose terminal, a broken strand in a cord that has been pinched under furniture, or a nail through a cable that has nicked a conductor. The arc is in series with the load, so the current is no more than the load current, sometimes only a few amperes, and an ordinary breaker cannot see it.
• Parallel arcs occur between conductors (hot to neutral or hot to ground) through damaged insulation. The current can be high but intermittent, in short bursts that may not last long enough to trip a thermal-magnetic breaker.

An AFCI monitors the current waveform with electronics that recognize the signature of arcing: the irregular, high-frequency noise and the characteristic shoulders and gaps in the current each half-cycle as the arc strikes and extinguishes. It must distinguish these from normal waveforms of motors with brushes, dimmers and switching power supplies, which also produce noise; manufacturers' detection algorithms are proprietary and are tested against standard arc and masking-load tests (UL 1699 in the US, CSA C22.2 No. 270 in Canada). A combination-type AFCI detects both series arcs (down to about 5 A) and parallel arcs, and is the type required for most residential use.

NEC 210.12(A) requires AFCI protection for all 120 V, single-phase, 15 and 20 A branch circuits supplying outlets or devices in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas and similar rooms or areas. Protection is normally a combination-type AFCI breaker; an outlet-branch-circuit AFCI at the first outlet is allowed under conditions, for example where the wiring from the panel to that outlet is in metal raceway or armored cable. Extensions and modifications of existing circuits in those areas must also be protected (210.12(D)).

Installing and troubleshooting

Combination AFCI and dual-function (AFCI and GFCI in one device) breakers have a pigtail neutral that connects to the panel's neutral bar, and the circuit neutral lands on the breaker, not on the bar. Because the device monitors the circuit's neutral, two practical rules follow:

• Neutrals must not be shared between circuits. A neutral that carries current from another circuit, or a neutral that has been connected to a different circuit's neutral in a junction box, makes the currents appear unbalanced and trips GFCI and dual-function devices. Multiwire branch circuits need two-pole AFCI or GFCI breakers designed for them.
• Neutral-to-ground contact downstream of the device, such as a neutral touching a metal box or a bare bonding conductor, gives the return current a path that bypasses the sensor and causes trips (and is a wiring fault in its own right).

When an AFCI trips repeatedly, the cause is usually real: a loose terminal, a staple driven through a cable, a damaged cord or appliance, or a shared or grounded neutral. Many AFCI breakers display or blink a code identifying the type of trip (arc, ground fault, overcurrent), which speeds diagnosis. Some loads, such as older treadmills and vacuum cleaners with worn brushes, genuinely produce arc-like noise; replacing the appliance, not removing the protection, is the remedy.

Related protection

Several other devices round out residential protection:

• Tamper-resistant receptacles prevent children from inserting objects into a single slot (Lesson 5).
• Surge protective devices limit transient overvoltages from lightning and switching.
• Smoke alarms and carbon monoxide alarms are required by the residential building code, usually hard-wired and interconnected; the NEC allows them on AFCI-protected circuits, and many jurisdictions require their circuit to be one that is regularly used (such as a lighting circuit) so that a tripped breaker is noticed.
• Since the 2020 NEC, a surge protective device is required at the service of every dwelling unit (230.67).
• Equipment ground-fault protection (GFP), typically tripping at hundreds of amperes, protects large services from destructive arcing ground faults; it is not personnel protection and is not used in houses.

Lesson 11 shows how GFCI protection allows three-slot receptacles on older ungrounded circuits, and when extending old circuits triggers current AFCI requirements.