Tools & Safety

Arc Flash and NFPA 70E: Why We Turn It Off First

An arc flash can release intense heat and pressure in a split second. Here is what it is, how approach boundaries work, and why de-energizing is the first choice.

By IES team, Innovative Electrical Services (IES), Auburn, CA · Reviewed November 2026 · 2 min read

Key takeaways
  • An arc flash is an explosive release of energy through the air between conductors.
  • NFPA 70E is the consensus standard for electrical safety in the workplace; OSHA rules require protection from these hazards.
  • The arc flash boundary is the distance where an unprotected person could receive a second-degree burn (1.2 cal/cm²).
  • The first choice is an electrically safe work condition: shut off, lock out, and test before touching.

Shock is the hazard most people think of with electricity. For electricians, the other big one is arc flash: when current jumps through the air between conductors, often after a dropped tool, a failed part or a mistake. The arc can reach temperatures hotter than the surface of the sun for a fraction of a second, with a blast of pressure, molten metal and blinding light.

OSHA and NFPA 70E

OSHA’s electrical rules (for example 29 CFR 1910.333) require employers to protect workers from electrical hazards and to de-energize parts before work unless that is infeasible or would create a greater hazard. NFPA 70E, the Standard for Electrical Safety in the Workplace, is the consensus standard that spells out how to do this in practice. OSHA has said that following 70E is a recognized way to meet its requirements.

Approach boundaries

NFPA 70E defines boundaries around exposed energized parts:

  • Limited approach boundary: a shock-protection boundary. Unqualified people stay outside unless escorted by a qualified person.
  • Restricted approach boundary: closer in, where the shock risk is higher. Only qualified people with the right protection and plan may cross it.
  • Arc flash boundary: the distance at which the incident energy from an arc would be 1.2 cal/cm², about the onset of a second-degree burn on bare skin. Inside it, arc-rated clothing and PPE are required.

The shock boundaries are based on voltage. The arc flash boundary depends on how much fault current is available and how fast the protective device clears it, so it can be much farther out than the shock boundaries, or closer in.

Arc flash boundary: 1.2 cal/cm²(where an unprotected person could get a second-degree burn)Limited approach boundaryqualified people, or escortedRestricted approach
Approach boundaries around exposed energized parts. The shock boundaries come from tables based on voltage; the arc flash boundary comes from the risk assessment and can be inside or outside them.

Labels on commercial equipment

Commercial and industrial equipment that is likely to be worked on energized is field-marked with arc flash warnings (NEC 110.16), and many facilities add labels from an arc flash study that show the boundary and required PPE. Those labels are only accurate while the system matches the study, so changes to the utility transformer or protective devices mean the study should be updated.

The electrically safe work condition

The safest arc flash is one that can’t happen. Before working on equipment, 70E’s process is, in short:

  1. Identify all sources of supply (don’t forget solar, batteries and generators).
  2. Open the disconnects and verify the blades are open where possible.
  3. Apply lockout/tagout devices.
  4. Test for absence of voltage with a properly rated meter, checking the meter on a known live source before and after.
  5. Apply temporary grounds where stored or induced energy is possible.

Testing is itself treated as energized work, so PPE is worn for that step. See meter CAT ratings for choosing the tester.

For homeowners

Home panels have less available energy than big commercial gear, but arcs still happen. Removing a panel cover exposes live parts even with the main breaker off: the service conductors above the main stay energized. Please leave that to a licensed electrician.

Sources and further reading

  1. OSHA, Electrical safety-related work practices (OSHA 4472)
  2. OSHA standard interpretation on NFPA 70E (2006-11-14)
  3. NFPA 70E, Standard for Electrical Safety in the Workplace
  4. NFPA free access to codes and standards (read the NEC online)

Local rules decide. These articles explain general ideas and point to the code sections we use. They are not legal or engineering advice and don’t replace the code itself. California adopts the NEC with its own amendments, cities and counties can add more, and the local inspector (the AHJ, or authority having jurisdiction) has the final say on any job. Read the code free at NFPA’s free access page.