Arc Flash & NFPA 70E
An arc flash is an electrical explosion. In a fraction of a second it can reach temperatures several times hotter than the surface of the sun, throw molten metal, and create a pressure wave that can knock you down.
- Electrical
- General Industry
- Construction
What causes arc flash
- Dropping a tool or touching a probe across energized parts
- Racking breakers in or out, opening covers on energized equipment
- Dust, corrosion, rodents or failed insulation
- Improper testing or using the wrong meter
NFPA 70E basics
- Electrically safe work condition first: the goal is to de-energize, lock out and verify absence of voltage. Energized work is allowed only when justified, usually with an energized electrical work permit.
- Approach boundaries: the arc flash boundary (where an arc could cause a second-degree burn), and the limited and restricted approach boundaries for shock. Unqualified people stay outside the limited approach boundary unless escorted.
- Equipment labels show the hazard, boundaries and required PPE.
- Arc-rated PPE: clothing and face shields rated in cal/cm², chosen by an incident energy analysis or the PPE category method (Categories 1 to 4).
- Training: only qualified persons may work within the boundaries on exposed energized parts.
What an arc flash does to the body
- Burns: severe burns to skin, even through ordinary clothing. Synthetic clothing can melt into the skin.
- Eyes: intense light can damage vision; molten metal can blind.
- Hearing: the blast can rupture eardrums.
- Lungs: hot gases and vaporized copper can be inhaled.
- Blast injuries: the pressure wave can throw a worker or cause a fall from a ladder.
PPE categories
When an employer uses the NFPA 70E table method instead of a calculated study, equipment is assigned a PPE category. Each category requires arc-rated clothing with at least the rating shown:
| PPE category | Minimum arc rating | Typical clothing |
|---|---|---|
| 1 | 4 cal/cm² | Arc-rated shirt and pants or coverall, face shield, safety glasses, hearing protection, leather gloves or rubber gloves with protectors |
| 2 | 8 cal/cm² | Arc-rated shirt and pants or coverall, arc-rated face shield and balaclava (or hood) |
| 3 | 25 cal/cm² | Arc flash suit with hood, arc-rated gloves |
| 4 | 40 cal/cm² | Heavier arc flash suit with hood, arc-rated gloves |
Many equipment labels instead list a calculated incident energy in cal/cm² at a working distance. Your arc-rated PPE must meet or exceed that number. See Flame-Resistant Clothing.
Reading an arc flash label
- Nominal voltage
- Arc flash boundary distance
- Incident energy and working distance, or the PPE category
- Shock approach boundaries (limited and restricted)
- Date of the study (labels should be reviewed when the system changes and at least every 5 years)
Energized work permits
When energized work is justified (for example, de-energizing would create a greater hazard, or testing and troubleshooting require power), NFPA 70E generally requires a written permit that includes:
- Description of the equipment, its location and the work
- Why the work must be done energized
- Shock and arc flash risk assessment results, boundaries and PPE
- How unqualified people will be kept out of the area
- Job briefing confirmation and approval signatures
If you’re not qualified: don’t open electrical panels, switchgear or motor control centers, and stay outside the marked boundaries when others are doing energized work.
Standards & references
- NFPA 70E: Standard for Electrical Safety in the Workplace
- 29 CFR 1910.335: Safeguards for personnel protection
- See also: Flame-Resistant Clothing
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