Quick answer
An arc flash is an electrical explosion that happens when current jumps through the air between conductors or to ground, releasing intense heat, light, pressure and molten metal in a fraction of a second.
OSHA notes arc temperatures can exceed 35,000°F, nearly four times the heat of the sun's surface, and that most items within 3 feet can burn, melt or vaporize 1.
Quick facts
The key facts about Arc Flash, with sources:
- Arc temperature
- Can exceed 35,000°F (19,400°C) 1
- Burn zone
- Most items within 3 ft (0.9 m) can burn, melt or vaporize 1
- Arc flash boundary (NFPA 70E)
- Where incident energy equals 1.2 cal/cm² (5.02 J/cm²) 1
- OSHA clothing trigger (power generation and distribution)
- Estimated incident energy above 2.0 cal/cm² 2
- Estimated frequency
- Up to 10 arc flash explosions a day in the U.S. (NFPA estimate) 7
Key takeaways
- An arc flash is an explosion of energy from an electrical arc, not a shock 1.
- Temperatures can top 35,000°F; most burns come from clothing catching fire 1.
- The arc flash boundary is the distance where heat drops to 1.2 cal/cm² (NFPA 70E) 1.
- It is mainly a workplace hazard at panels, switchgear and service equipment.
- Homeowners should never open live panels or meter bases; this is where arc flash risk lives at home.
What an arc flash is
An arc is current flowing through air. Air normally insulates, but if conductors are close enough, or a tool bridges them, the air breaks down and the current flows through a ball of super-heated gas and metal vapor.
OSHA describes it as an electrical explosion with a bright flash, temperatures above 35,000°F, deafening noise, supersonic concussive forces and super-heated shrapnel. The energy vaporizes the metal conductors 1.
An arc flash is different from a shock. In a shock, current passes through you. In an arc flash, you can be badly burned without touching anything, just by standing too close.
How an arc flash starts and grows
Common triggers are a dropped tool, a loose part falling across bus bars, a test probe slipping, dust or corrosion, or equipment failing as it is switched. Once the arc forms, it sustains itself until the circuit protection opens it.
The energy released depends mainly on the available fault current and how long it flows. A big service with high fault current and slow protection can release far more energy than a small branch circuit.
Employers must estimate the incident heat energy a worker could face. OSHA 1910.269(l)(8)(ii) requires a reasonable estimate for each employee exposed to electric arc hazards in power generation, transmission and distribution work 2.
- Triggertool contact, failure, contamination
- Air breaks downcurrent flows through plasma
- Explosionheat above 35,000°F, light, pressure, molten metal
- Clothing ignitesthe source of most burn injuries
- Protection opensbreaker or fuse ends the arc; duration sets the energy
Source: [1]
Arc flash hazards
| Hazard | What it does | Source |
|---|---|---|
| Heat | Burns skin; ignites clothing; most burns come from burning clothing | 1 |
| Light | Intense flash can injure eyes | 1 |
| Pressure (arc blast) | Supersonic concussive forces can throw workers | 1 |
| Sound | Deafening noise | 1 |
| Molten metal and shrapnel | Vaporized and molten conductors spray outward | 1 |
| Fire | Can start structural fires | 1 |
Example: the three approach boundaries
OSHA's fact sheet summarizes NFPA 70E's three boundaries around exposed energized parts. Their distances vary with voltage and the work being done 1.
| Boundary | Protects against | Who may cross |
|---|---|---|
| Limited approach boundary | Shock | Unqualified workers only if supervised and escorted by a qualified worker |
| Restricted approach boundary | Shock; highest likelihood inside | Never unqualified workers; qualified workers only with proper PPE and insulated tools |
| Arc flash boundary | Second-degree burns from an arc flash (1.2 cal/cm²) | Unqualified workers only with proper PPE and close supervision |
- 35,000°F+arc temperatureOSHA FS-4474 (2024)
- 3 ftradius where most items burn, melt or vaporizeOSHA FS-4474
- 1.2 cal/cm²incident energy at the arc flash boundaryNFPA 70E, cited by OSHA
- 2.0 cal/cm²OSHA 1910.269 trigger for arc-rated clothing29 CFR 1910.269(l)(8)
The arc flash boundary has no fixed position relative to the other two. It can sit inside or outside them 1. When both apply, workers use both.
Where arc flash happens
- Switchgear, switchboards and motor control centers in commercial buildings.
- Service equipment and large panelboards.
- Utility transformers and distribution equipment.
- At home: inside the main panel, meter base and service disconnect, which are live even with the main breaker off on the line side.
- Any time energized parts are exposed for testing or troubleshooting.
Why arc flash rules exist
The aim of every arc flash rule is to keep people out of the energy. The best control is to de-energize equipment and verify it is dead before work. OSHA 1910.333(a)(1) requires live parts to be de-energized before employees work on or near them, unless de-energizing creates greater hazards or is infeasible 4.
When work must be done live, boundaries, training and arc-rated PPE limit injury. OSHA's December 2024 guidance gives employers ways to fold arc flash into their safety programs 7.
Limits of arc flash protection
PPE reduces burns; it does not make live work safe. Arc-rated clothing is rated for a set energy, and an arc can exceed it. Blast pressure and shrapnel are hard to guard against.
Estimates have limits too. Incident energy depends on fault current and clearing time, which change when the utility or the equipment changes. OSHA allows broad estimates across system areas if the assumptions are reasonable 2.
The frequency of arc flash incidents is uncertain. AIHA reports NFPA's estimate of up to ten arc flash explosions a day in the U.S. 7.
What drives arc flash program costs
We do not list prices. For businesses, cost drivers include an engineering study of incident energy, equipment labeling, arc-rated PPE, training for qualified workers, and upgrades that cut clearing time. For homeowners, the relevant cost is simply hiring a licensed electrician rather than opening live equipment.
Commercial owners can start with commercial electrical services.
How workplaces control arc flash risk
- Plan to de-energize first, as OSHA 1910.333 requires unless infeasible 4.
- Lock out and tag out the energy sources under OSHA 1910.147 or 1910.333(b) 5.
- Verify absence of voltage with rated test equipment.
- If work must be live, estimate incident energy and set the arc flash boundary.
- Select arc-rated clothing and PPE with a rating at or above the estimated energy 2.
- Allow only qualified workers within the boundaries; supervise anyone else who must cross 1.
Maintenance that lowers arc flash risk
- Keep electrical rooms clean and dry; dust and moisture can start arcs.
- Keep covers, dead fronts and doors closed and secured.
- Have connections checked and tightened by qualified workers.
- Maintain breakers so they clear faults at their rated speed.
- Keep working clearance in front of equipment clear 6.
Warning signs at electrical equipment
- Buzzing, crackling or popping from a panel or switchgear.
- Burn marks, melted plastic or a hot smell.
- Rust or water inside an enclosure.
- Missing covers or exposed bus bars.
- Breakers that trip with flashes or loud reports.
Never open a live panel
Homeowners should never remove a panel cover, open a meter base or test inside live equipment. The service conductors stay energized even with the main breaker off. If you see burning, smoke or sparks at a panel, leave the area and call 911, then a licensed electrician.
Arc flash rules: authority, standard, edition
OSHA 29 CFR 1910.269(l)(8) (power generation, transmission and distribution). Employers must assess hazards from flames and electric arcs, estimate incident heat energy for exposed employees, and provide protective clothing with an arc rating at least equal to the estimated heat energy where it exceeds 2.0 cal/cm² 2.
In force as published in the current eCFR, read 10 October 2026.
OSHA 29 CFR 1910.335(a)(1)(i). Employees in areas with potential electrical hazards must be provided with, and use, electrical protective equipment for the body parts to be protected 3.
OSHA 29 CFR 1910.333(a)(1). De-energize live parts before work unless that creates more hazard or is infeasible 4.
NFPA 70E, Standard for Electrical Safety in the Workplace. A consensus standard, not a law, that defines approach boundaries and the 1.2 cal/cm² arc flash boundary. OSHA cites it in its 2024 fact sheet FS-4474 1 8. NFPA revises 70E periodically, so confirm which edition your employer or contractor follows.
These rules govern employers and workers. They apply to existing equipment in workplaces now; they are not building codes for homes.
Arc flash vs arc fault vs shock
| Term | What happens | Where | Main harm |
|---|---|---|---|
| Arc flash | Explosive arc through air at energized equipment | Panels, switchgear, service | Burns, blast injury |
| Arc fault | Small arc in branch wiring or cords | Walls, outlets, cords | Fire |
| Electrical shock | Current through the body | Any live part | Heart rhythm, burns |
| Short circuit | Low-resistance fault path | Any circuit | Can trigger an arc flash |
Arc flash myths
- Myth You have to touch something to be hurt.
- Reality Arc flash burns people who are only nearby; most items within 3 ft can burn 1.
- Myth Low-voltage equipment cannot arc flash.
- Reality Boundaries vary with voltage, but arc flash can occur at common service voltages; the fault current and clearing time decide the energy.
- Myth Turning off the main breaker makes the panel safe.
- Reality The line side of the main stays live from the utility.
- Myth PPE makes live work safe.
- Reality PPE reduces injury; de-energizing is the first control 4.
State plans and local rules
Federal OSHA rules apply nationwide, and some states run their own OSHA programs, which OSHA says must be at least as effective as the federal one 1. Licensing for electricians, who do this work, varies by state. Check who may work on service equipment where you live.
What arc flash means for you
For homeowners, the rule is short: do not open panels, meter bases or disconnects. Leave all service work to a licensed electrician who de-energizes and verifies before touching anything.
For business owners, ask whether your equipment has incident energy labels and whether your staff or contractors follow NFPA 70E. Next step: read about hiring an electrician and see our electrical safety resources.
Arc flash vs arc fault
An arc fault is a small arc in home wiring that can start a fire and is caught by an AFCI. An arc flash is an explosion at energized equipment that injures people nearby.
Questions about Arc Flash
What causes an arc flash?
An arc flash starts when current jumps through air between conductors or to ground. Common triggers are dropped tools, slipping test probes, loose parts, dust, corrosion, moisture and equipment failure during switching. The released energy depends on fault current and how fast protection opens.
How hot is an arc flash?
OSHA says arc temperatures can exceed 35,000°F (19,400°C), nearly four times the heat of the sun's surface. At those temperatures most items within about 3 feet, including skin and flammable clothing, can burn, melt or vaporize.
What is the arc flash boundary?
NFPA 70E defines it as the distance from exposed energized parts where incident energy equals 1.2 cal/cm². At that boundary, a person without proper PPE could receive second-degree burns. The distance varies with the equipment and the work.
Is arc flash the same as electric shock?
No. Shock is current passing through the body. Arc flash is an explosion of heat, light and pressure from an arc. You can be badly burned by arc flash without touching anything. Both hazards can occur at the same equipment.
Can an arc flash happen in a house?
Arc flash can occur at a home's panel, meter base or service equipment, especially if someone opens them while live. That is why homeowners should never remove panel covers or work on service equipment. Leave it to licensed electricians.
What PPE protects against arc flash?
Arc-rated clothing, face shields or hoods, gloves and hearing protection, chosen for the estimated incident energy. OSHA 1910.269 requires clothing with an arc rating at least equal to the estimated heat energy above 2.0 cal/cm² for covered work.
How common are arc flash incidents?
Exact frequency is uncertain. AIHA, reporting on OSHA's 2024 guidance, cites an NFPA estimate of up to ten arc flash explosions a day in the United States.
What is NFPA 70E?
NFPA 70E is the Standard for Electrical Safety in the Workplace. It sets practices such as approach boundaries, arc flash risk assessment and PPE selection. It is a consensus standard that OSHA references; it is not itself a federal law.
Why do most arc flash burns happen?
OSHA notes most arc flash burn injuries come from the arc igniting flammable clothing, not from the arc itself. Arc-rated clothing exists to stop that ignition.
Does turning off the main breaker remove arc flash risk in a panel?
No. The incoming service conductors and the line side of the main breaker stay energized from the utility. Only the utility can de-energize those. That part of the panel can still arc flash.
Sources
- OSHA Fact Sheet FS-4474, Establishing Boundaries Around Arc Flash Hazards (November 2024), retrieved .
- OSHA, 29 CFR 1910.269 Electric power generation, transmission, and distribution, retrieved .
- OSHA, 29 CFR 1910.335 Safeguards for personnel protection, retrieved .
- OSHA, 29 CFR 1910.333 Selection and use of work practices, retrieved .
- OSHA, 29 CFR 1910.147 The control of hazardous energy (lockout/tagout), retrieved .
- OSHA, 29 CFR 1910.303 General requirements (Subpart S), retrieved .
- AIHA, OSHA Releases Arc-Flash Guidance (5 December 2024), retrieved .
- NFPA, NFPA 70E Standard for Electrical Safety in the Workplace, standard development page, 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: Hazard description and boundaries from OSHA FS-4474 (2024); regulatory text from OSHA 1910.269, 1910.333 and 1910.335 on osha.gov; frequency estimate via AIHA's summary of OSHA guidance. Read 10 October 2026. Held for review by a qualified electrical safety reviewer before indexing.
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