Electrical · Lesson 6 of 13Intermediate11 min read

GFCIs, AFCIs and Other Protection

A breaker protects the wires, but it can’t protect a person: the current that can kill you is far too small for a breaker to notice. Two smarter devices fill the gap. The GFCI guards against shock and the AFCI guards against fires from arcing. This lesson shows how each one works, along with the other safety devices in a modern home.

The gap a breaker leaves

A 15-amp breaker doesn’t trip until well over 15,000 milliamps flows. OSHA lists 50 to 150 milliamps through the body as enough to cause extreme pain and respiratory arrest, with death possible (Lesson 8). So a person can be badly hurt by a current the breaker will never notice.

A ground-fault circuit interrupter (GFCI) closes that gap. OSHA defines it as a device for protecting people that shuts off a circuit when the current to ground goes over a set value that is less than what it takes to trip the breaker.

How a GFCI works

In a healthy circuit, every bit of current that goes out on the hot wire comes back on the neutral. A GFCI keeps checking that. Both wires pass through a small ring-shaped coil. When the two currents are equal and opposite, their magnetic fields cancel and the coil feels nothing. If some current leaks out another way, through a wet hand, a person or a faulty tool to ground, the two no longer match. The leftover field induces a voltage in the coil, and the GFCI’s circuit opens the contacts.

It reacts to tiny leaks. OSHA says a GFCI shuts off the power if the difference exceeds 5 milliamps, and can interrupt the current within as little as 1/40 of a second to prevent electrocution. NIOSH gives the trip level as 4 to 6 milliamps, and CPSC says as little as 6 milliamps. A GFCI also trips if the leak goes safely to ground through a ground wire, which forces you to find the problem.

A GFCI does not prevent you from feeling a shock. It doesn’t limit the current, and it can’t help if you touch the hot and neutral wires at the same time, because then the current out still equals the current back. What it does is cut the power fast, before a ground-fault shock lasts long enough to be deadly.

Body resistance figures are NIOSH’s (dry skin 100,000 ohms or more, wet skin as low as 1,000 ohms); effects are from OSHA’s table. The trip level of 5 milliamps and time of 1/40 second are OSHA’s. The animation is slowed down so you can watch it.

Where GFCIs go and how to test them

CPSC lists the places the National Electrical Code has required GFCI protection, added over the years starting in 1968: bathrooms, garages, kitchens, outdoor outlets, crawl spaces and unfinished basements, wet bar sinks, laundry and utility sinks, and underwater pool lighting. On construction sites, OSHA requires GFCI protection (or a special grounding program) for temporary 120-volt, 15- and 20-amp outlets.

There are three types. A GFCI breaker protects a whole circuit. A GFCI receptacle replaces an ordinary outlet and also protects the outlets wired after it. A portable GFCI plugs into an outlet, or is built into an extension cord, for temporary use such as outdoor tools.

Test each GFCI at least once a month, after a power failure, and as the maker directs: press TEST and the power should go off; press RESET and it should come back. Being an Adult 101 has the steps.

AFCIs: stopping fires before they start

An arc is a glowing discharge of electricity across a gap. Problems in home wiring, such as loose connections, a cord crushed under furniture or wires damaged by age, can make arcs, and arcs can start fires. The U.S. Fire Administration found that electrical arcing was the heat source in 74 percent of the home electrical fires started by powered equipment in 2014 to 2016.

Ordinary breakers don’t respond to early arcing. CPSC staff explain why. A series arc, at a loose connection or a broken wire in a cord, is in line with the load, so the load limits its current to well below the breaker’s rating. A parallel arc, such as a hot wire arcing to a neutral or ground through damaged insulation, may flicker on and off too erratically to heat the breaker’s bimetal. By the time a breaker trips, a fire may already have started.

An arc-fault circuit interrupter (AFCI) uses electronics to tell normal arcing from the dangerous kind, which has an irregular pattern. When it sees a dangerous one, it shuts the circuit off. The 1999 National Electrical Code required AFCIs on bedroom circuits starting in 2002. Combination devices include both AFCI and GFCI protection in one unit. Like GFCIs, AFCIs have a test button and should be tested monthly. CPSC says to have a qualified electrician install AFCIs, and not to attempt the work yourself.

Series arcloose connection or broken strand: the load limits the currentbreak in one wire, in line with the loadloadParallel archot to neutral or ground through damaged insulationarc between the wiresloadNeither may draw enough steady current to trip an ordinary breaker.An AFCI watches for the irregular pattern of a dangerous arc instead.
Source: U.S. Consumer Product Safety Commission staff, “New Technology for Preventing Residential Electrical Fires: Arc-Fault Circuit Interrupters.”

Surges

A surge is a brief spike of voltage, from causes such as lightning or switching on the power system. NIST, which published a booklet for homeowners on the subject, calls surges unavoidable. Ready.gov recommends surge protection devices, and during a power outage it advises unplugging appliances and electronics to avoid damage from surges.

A power strip doesn’t add capacity to a circuit. The U.S. Fire Administration says not to overload power strips and to use ones with internal overload protection.

Tamper-resistant outlets

Tamper-resistant (TR) receptacles have built-in shutters behind the slots. The shutters open only when both blades of a plug push in together, so a child poking an object into one slot can’t reach the live metal. TR receptacles are required in some locations to protect children, and the U.S. Fire Administration recommends installing them if you have young children.

Smoke and CO alarms

Smoke and carbon monoxide (CO) alarms are part of a home’s safety system too. The U.S. Fire Administration says to put smoke alarms inside and outside each bedroom and sleeping area, on the ceiling or high on the wall, and to test them monthly and replace them every 10 years. Interconnected alarms all sound when one does. Only qualified electricians should install hardwired smoke alarms, which usually have a backup battery to replace once a year. CPSC recommends CO alarms with battery backup outside sleeping areas and on each floor. When a smoke alarm sounds, you may have less than 2 minutes to get everyone out.

See Test Smoke and CO Alarms in Being an Adult 101 for how to test them and what to do when one goes off.

Parts and what they do
PartWhat it doesSigns of trouble
GFCI receptacleTrips on a few milliamps of leakage; protects outlets after it on the circuit.Doesn’t cut power when you press TEST, or won’t reset.
GFCI breakerGives GFCI protection to a whole circuit from the panel.TEST doesn’t trip it.
Portable GFCIPlug-in protection for outdoor or temporary use.Damaged cord or housing.
AFCI breakerShuts off a circuit when it detects dangerous arcing.Trips often; TEST doesn’t trip it.
Combination AFCI/GFCIBoth protections in one device.Same as each one alone.
Surge protective deviceDiverts voltage spikes away from equipment.Damage after a storm; some have a status light.
Tamper-resistant receptacleShutters block objects pushed into one slot.Plugs won’t go in unless pushed straight.
Smoke and CO alarmsWarn of fire and carbon monoxide.Chirping (low battery), failing a test, or over 10 years old (smoke).

You canWhat you can safely do

StopWhen to call an electrician

If a GFCI doesn’t cut the power when you press TEST, or RESET won’t bring the power back, don’t count on that outlet for protection: have a qualified electrician fix or replace it. Adding GFCI or AFCI protection, replacing a GFCI, and installing hardwired smoke alarms or whole-house surge protection are jobs for an electrician.

Check yourself

Tap an answer to check it, or open “Show the answer.”

  1. What does a GFCI compare?

    Show the answer

    The current going out on the hot wire with the current coming back on the neutral. If some current is missing on the way back, it’s leaking another way, maybe through a person.

  2. According to OSHA, at what difference does a GFCI shut off the power?

    Show the answer

    More than 5 milliamps. OSHA says a GFCI trips if the difference exceeds 5 milliamps, within as little as 1/40 of a second.

  3. Why doesn’t an ordinary breaker catch a series arc at a loose connection?

    Show the answer

    The arc current is limited by the load, so it stays below the breaker’s rating. A series arc is in line with the load, so its current is no bigger than the load’s normal current.

  4. A GFCI’s light stays on when you press TEST. What does that mean?

    Show the answer

    It isn’t protecting you; have a qualified electrician fix or replace it. A GFCI can keep delivering power after it stops protecting. That’s why you test monthly.

  5. What do tamper-resistant outlets do?

    Show the answer

    Use shutters so objects pushed into one slot can’t reach live parts. The shutters open only when both blades of a plug push in together.

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