An arc flash is one of the most violent events in electrical work. In a fraction of a second, an electric arc can release a fireball, a pressure wave, molten metal and intense heat, and workers have been seriously burned by arc flashes on ordinary 480-volt panels. NFPA 70E, the Standard for Electrical Safety in the Workplace, is the US standard that tells qualified workers how to recognise this hazard and protect themselves.
What Is an Arc Flash?
An arc flash happens when electric current jumps through the air between conductors, or between a conductor and ground. It can be triggered by a dropped tool, a loose connection, damaged insulation or a mistake during testing. The arc is a plasma channel that can reach about 35,000 degrees Fahrenheit, hotter than the surface of the sun. It can melt copper, cause severe burns, hearing damage and eye injury, and the pressure wave that follows, the arc blast, can throw a worker across a room. The danger is measured as incident energy, in calories per square centimetre (cal/cm2): the higher the fault current and the longer the arc lasts, the higher the incident energy.
The Arc Flash Boundary
NFPA 70E defines the arc flash boundary as the distance from the arc source at which the incident energy falls to 1.2 cal/cm2, the level at which unprotected skin can receive a second-degree burn. Anyone inside this boundary must wear arc-rated personal protective equipment (PPE) matched to the incident energy there. The boundary comes from an arc flash study, usually calculated with the IEEE 1584 method, or from the PPE category tables in NFPA 70E.
The ISO 7010 W042 arc flash warning sign, also shown on equipment labels.
Shock Boundaries: Limited and Restricted
NFPA 70E also sets shock protection boundaries around exposed energised parts. The limited approach boundary is the outer line: unqualified people must stay outside it unless escorted by a qualified person. The restricted approach boundary is closer in: only a qualified person with the right shock protection may cross it. These distances grow with voltage. As a rule, any circuit at 50 volts or more is a possible shock hazard.
PPE Categories 1 to 4
When no arc flash study is available, NFPA 70E lets workers use the PPE category method. Each task is assigned a category, and each category sets a minimum arc rating:
- Category 1: 4 cal/cm2. Arc-rated shirt and pants or coverall, face shield, hard hat, safety glasses, hearing protection and leather gloves.
- Category 2: 8 cal/cm2. Arc-rated clothing plus a face shield with balaclava or an arc-rated hood.
- Category 3: 25 cal/cm2. An arc flash suit with hood, plus rubber insulating gloves with leather protectors.
- Category 4: 40 cal/cm2. A full multi-layer arc flash suit with hood.
Remember the hard limit: NFPA 70E does not allow energised work above 40 cal/cm2. At that level the equipment must be de-energised first, because no standard PPE can protect you.
A typical breaker panel. Arc flash can occur in panels like this during switching or testing, so the label on the panel, not its size, decides the PPE.
The First Rule: Make It Safe
The most important idea in NFPA 70E is the electrically safe work condition. Before you touch anything, the equipment should be de-energised, locked out and tagged out (LOTO), and tested to prove it is dead. Test your meter on a known live source, test the equipment, then test the meter again: this is live-dead-live testing. Energised work needs an energised work permit, a risk assessment and the correct PPE.
Key Takeaways
- An arc flash is an explosive release of energy that can cause severe burns and blast injuries.
- The arc flash boundary is where incident energy drops to 1.2 cal/cm2, the second-degree burn threshold.
- PPE categories 1 to 4 need arc ratings of 4, 8, 25 and 40 cal/cm2. Above 40 cal/cm2, de-energise.
- Limited and restricted boundaries protect against shock and grow with voltage.
Objective Questions
1. What is an arc flash?
a) A slow overheating of a wire
b) Electric current jumping through the air between conductors, or between a conductor and ground, releasing a fireball, pressure wave and intense heat
c) A tripped circuit breaker
d) A blown fuse
Answer: (b) Electric current jumping through the air between conductors, or between a conductor and ground, releasing a fireball, pressure wave and intense heat
Explanation: An arc flash is a plasma channel that releases a fireball, a pressure wave, molten metal and intense heat in a fraction of a second.
2. What is the arc flash boundary?
a) The distance from the arc source at which the incident energy falls to 1.2 cal/cm2
b) The edge of the building
c) The length of the power cable
d) The distance to the nearest exit
Answer: (a) The distance from the arc source at which the incident energy falls to 1.2 cal/cm2
Explanation: The arc flash boundary is where the incident energy drops to 1.2 cal/cm2, the level at which unprotected skin can receive a second-degree burn.
3. What is the minimum arc rating for PPE Category 2?
a) 4 cal/cm2
b) 8 cal/cm2
c) 25 cal/cm2
d) 40 cal/cm2
Answer: (b) 8 cal/cm2
Explanation: Category 2 needs a minimum arc rating of 8 cal/cm2, with arc-rated clothing plus a face shield with balaclava or an arc-rated hood.
4. What is the hard limit for energised work under NFPA 70E?
a) Energised work is not allowed above 40 cal/cm2; the equipment must be de-energised first
b) Energised work is allowed up to 100 cal/cm2
c) There is no limit
d) Energised work is not allowed above 4 cal/cm2
Answer: (a) Energised work is not allowed above 40 cal/cm2; the equipment must be de-energised first
Explanation: NFPA 70E does not allow energised work above 40 cal/cm2, because no standard PPE can protect a worker at that level.
5. What is live-dead-live testing?
a) Testing the meter on a known live source, testing the equipment, then testing the meter again
b) Testing only after the work is done
c) Testing with two meters at once
d) Testing the equipment while it is live
Answer: (a) Testing the meter on a known live source, testing the equipment, then testing the meter again
Explanation: Live-dead-live testing proves the meter works, proves the equipment is dead, and then proves the meter still works.
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