Quick answer
An arc fault is an unintended electrical arc in a circuit, where current jumps across a gap or through damaged insulation and produces intense heat at that spot.
UL 1699 defines it as an unintentional arcing condition in a circuit 1. Arcs can char nearby wood and insulation and start fires hidden inside walls.
Quick facts
The key facts about Arc Fault, with sources:
Key takeaways
- An arc fault is sparking where wires are loose, broken or damaged 1.
- The arc can be hot enough to ignite nearby material even when current is low 1.
- Ordinary breakers often do not see early arcing 2.
- AFCIs were designed to detect arc faults and cut power 2.
- Buzzing outlets, flickering lights and scorch marks are warning signs.
What an arc fault is
Think of a spark plug, but in your wall. When a wire breaks, a terminal screw loosens or insulation cracks, current can jump the gap. That jump is an arc. It makes light, sound and a lot of heat in a tiny area.
NEMA explains that arcing creates high-intensity heating at the arc point. It throws off burning particles that can, over time, ignite material around it 1. The arc may come and go for weeks before a fire starts.
How an arc fault starts a fire
In a series arc, the current path is broken along one conductor, often at a loose connection. The arc sits in series with the load, so the current stays at normal levels or lower. A breaker sees nothing wrong.
In a parallel arc, current jumps between two conductors, such as hot to neutral through crushed insulation. These arcs can draw bursts of high current, but often too briefly for a standard breaker to trip 1.
Either way, the heat is local. Wood framing, dust, old cloth insulation or a carpet over a cord can char and then ignite. That is why CPSC warns that by the time a fuse or breaker opens, a fire may already have begun 2.
- Connection loosens or insulation is damagednail, crushed cord, aging insulation
- Current jumps the gapan arc forms
- Heat builds at one pointburning particles are thrown off
- Nearby material charswood, insulation, dust
- Ignitionunless an AFCI cuts power first
Series vs parallel arc faults
| Type | Where the arc is | Common causes | Breaker sees it? |
|---|---|---|---|
| Series arc | In line, along one conductor | Loose terminal, broken wire, backstabbed connection | Rarely; current stays normal |
| Parallel arc, line to neutral | Between hot and neutral | Crushed or pierced cable, damaged cord | Sometimes, if current is high long enough |
| Parallel arc, line to ground | Between hot and ground | Damaged cable in a metal box | Sometimes; a GFCI may also trip |
Example: what arcs cost in U.S. homes
Published fire data shows why arc faults get so much code attention.
| Measure | 2003 to 2014 yearly average | Source |
|---|---|---|
| Home electrical system fires | 25,366 | NEMA 1 |
| Civilian deaths | 378 | NEMA 1 |
| Civilian injuries | 1,290 | NEMA 1 |
These totals include all home electrical system fires, not only arcs. Earlier CPSC research put fires from home electrical distribution at over 40,000 a year in 1992 to 1996 3, and that concern is what led to the AFCI.
Where arc faults tend to happen
- Loose screw terminals on outlets and switches.
- Cords crushed under furniture or rugs; NEMA lists these as arcing risks 1.
- Cords near hot air ducts or in sunlight, where insulation degrades 1.
- Cables hit by a nail or screw during a project.
- Old wiring with brittle insulation, and some aluminum wiring connections.
- Backstab connections that have loosened.
Why the term matters to owners
When an AFCI trips, it is telling you it saw an arc. Knowing that changes how you respond. You look for a damaged cord or loose outlet rather than blaming the breaker.
It also helps you spot early clues. A warm switch plate or a faint crackle at an outlet is worth a call, even if nothing has tripped.
The term also shapes how you talk to an electrician. Saying "the AFCI in the bedroom trips when the space heater runs" points them toward a cord, a loose outlet or a shared circuit. Saying "the power keeps going out" does not. Clear details shorten the visit.
CPSC research on fires in home wiring is what led to arc-fault detection in the first place 3. Each new code edition has widened the list of rooms that need it 1.
What makes arc faults hard to catch
Arcs can be intermittent. They show up when a cord is bent a certain way, or when a heater runs and the terminal heats up. An inspection on a quiet day may miss them.
They also hide. Most wiring is behind drywall. Thermal cameras and AFCIs help, but no single tool finds every arc. Codes lean on AFCIs because they watch the circuit every second it is on.
Some arcs are normal. A light switch makes a tiny arc each time it opens, and so do some motors with brushes. An AFCI has to tell those harmless arcs from dangerous ones.
UL 1699 tests for both: catching real arc faults and not tripping on normal ones 1. Older or cheaper electronics can still confuse some AFCIs, which is one reason for nuisance trips.
What drives repair costs after an arc fault
We do not publish prices. Costs depend on where the arc is and how much damage it did. A loose outlet connection is a short visit.
A cable damaged inside a finished wall means opening and patching. Widespread brittle insulation can lead to whole-house rewiring. Adding AFCIs adds the device cost and may require a compatible panel.
See how electricians price this work in our cost guides.
How an electrician handles an arc fault
- Ask what tripped, when, and whether you smelled or heard anything.
- Check outlets, switches and fixtures on the circuit for heat damage and loose terminals.
- Inspect cords and plugged-in equipment.
- Test the circuit wiring and look for pierced cables near recent work.
- Repair connections or replace damaged cable; add AFCI protection where needed and allowed.
Lowering arc fault risk
- Do not run cords under rugs or pinch them behind furniture 1.
- Replace cords with cracked, cut or heat-damaged insulation.
- Avoid loose-fitting plugs; an outlet that will not grip a plug should be replaced by a pro.
- Do not overload power strips.
- Test AFCI breakers as the maker directs.
Signs of arcing
- Buzzing, sizzling or crackling at an outlet or switch.
- A warm or discolored outlet or switch plate.
- A burning plastic smell with no clear source.
- Lights that flicker when you bump a wall or switch.
- An AFCI that trips again and again.
Treat signs of arcing as urgent
Turn off the breaker for that circuit if you can do it safely, and do not use the outlet. If there is smoke or a hot smell that keeps growing, leave and call 911. Do not open outlets or panels yourself; call a licensed electrician.
Codes and standards for arc faults
UL 1699. UL published this AFCI standard in 1999. It defines an arc fault and sets how AFCIs must detect arcs and avoid false trips 1.
NEC (NFPA 70). The 1999 NEC first required arc-fault protection for bedroom circuits, effective in 2002 3. By 2014 coverage reached most living areas, kitchens and laundry areas 1. NEC 2023 210.12 continues the rule and adds some sleeping areas in other occupancies 4.
Each state adopts its own NEC edition, with or without changes, so the rule at your address may differ.
Arc fault vs other faults
| Fault | What happens | Main risk | Protection |
|---|---|---|---|
| Arc fault | Current jumps a gap | Fire | arc-fault breaker |
| Short circuit | Hot meets neutral or hot, very high current | Fire, equipment damage | Breaker or fuse |
| Ground fault | Current leaks to ground | Shock | GFCI, grounding |
| Overload | Too much load on a sound circuit | Overheated wire | Breaker |
Arc fault myths
- Myth If the breaker has not tripped, there is no arcing.
- Reality Early arcing often does not trip a standard breaker 2.
- Myth Arc faults only happen in old houses.
- Reality New homes get them too, from nails, crushed cords and loose terminals.
- Myth An arc is just a harmless spark.
- Reality Arcs make intense local heat and can ignite surrounding material 1.
- Myth A GFCI stops arc faults.
- Reality GFCIs address shock. AFCIs address arc-related fire 2.
How local codes change AFCI coverage
Whether a given room needs arc-fault protection depends on the NEC edition your state or city adopted and its local changes 5. A remodel or new circuit usually has to meet the current local rule. Look up your state before you plan work.
When to act on arc faults
Act now if you notice any sign in the list above. Plan ahead if your home is older, has known wiring issues, or has bedrooms without AFCIs.
Next step: use the electrical safety check, then book an electrical inspection. If wiring is worn throughout, read about knob-and-tube wiring and rewiring options.
Arc fault vs arc flash
An arc fault is a small, often hidden arc in branch wiring that can start a fire. An arc flash is a violent arc explosion at energized equipment, mainly a hazard for workers.
Questions about Arc Fault
What causes an arc fault?
Common causes are loose terminal screws, broken wires, nails or screws through cables, cords crushed under furniture or rugs, and insulation damaged by heat, sunlight or age. Any of these can leave a gap the current jumps across, creating an arc.
Is an arc fault dangerous?
Yes. An arc creates intense heat at a tiny spot and can throw off burning particles. Over time it can ignite wood, insulation or dust inside a wall. Many arcs draw too little current for a normal breaker to notice.
What does an arc fault sound like?
Arcing can make a buzz, sizzle or crackle at an outlet, switch or fixture. Not every arc makes noise, though. Pair any sound with other signs, like heat, a burning smell or flickering lights, and call an electrician.
How do you find an arc fault?
An electrician checks the circuit an AFCI keeps tripping, looks for heat damage at outlets and switches, inspects cords and appliances, and tests wiring. Recent work, like hanging pictures or shelves, is a good clue. Owners should not open boxes or panels.
What is the difference between a series and parallel arc?
A series arc happens along one wire, such as at a loose connection, and current stays normal. A parallel arc jumps between two wires, such as hot to neutral, and can cause brief high-current bursts. Combination AFCIs detect both.
Will a regular breaker trip on an arc fault?
Often not. CPSC says typical fuses and breakers do not respond to early arcing. A breaker reacts to too much current, and many arcs stay below that level. That gap is why AFCIs exist.
Can an extension cord cause an arc fault?
Yes. Cords that are pinched, crushed under furniture, run under rugs or exposed to heat can crack inside. That damage can lead to arcing. Replace damaged cords and do not use cords as permanent wiring.
What should I do if my AFCI keeps tripping?
Unplug everything on the circuit, reset once and add items back one at a time. If one item trips it, stop using that item. If it trips with nothing plugged in, leave it off and call an electrician, because the arc may be in the wiring.
Can arc faults happen in new homes?
Yes. A nail driven through a cable, a loose connection from installation, or a crushed cord can all arc in a new house. Newer homes usually have AFCIs, which improves the odds of catching it before a fire.
Are old homes more likely to have arc faults?
Often, yes. Insulation can get brittle with age, connections loosen over decades, and older homes usually have no AFCI protection. CPSC suggests owners of older homes consider adding AFCIs for that reason. An electrical inspection can show where wiring is worn or connections are loose.
Is an arc fault the same as an arc flash?
No. An arc fault is a small, often hidden arc in home wiring that can start a fire. An arc flash is a violent explosion of energy at energized equipment, a hazard mainly for electrical workers.
Sources
- NEMA, Arc Fault Circuit Interrupters: Using Advanced Technology to Reduce Electrical Fires (SE 1512), retrieved .
- U.S. CPSC, Preventing Home Fires: Arc Fault Circuit Interrupters (Publication 5133), retrieved .
- Lee, Trotta and King (U.S. CPSC), New Technology for Preventing Residential Electrical Fires: Arc-Fault Circuit Interrupters, retrieved .
- Leviton Captain Code 2023, NEC 2023 210.12(D) arc-fault requirements, retrieved .
- NFPA, NEC enforcement maps, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definition, arc types, causes and fire figures from NEMA SE 1512; breaker behavior from CPSC Publication 5133; code history from a CPSC technical paper and Leviton's quotation of NEC 2023 210.12. Read 10 October 2026.
Suggest a correction. We fix errors and say what changed.