Arc Flash vs Arc Blast: Two Hazards, One Event

An arcing fault produces two distinct hazards at the same instant. Arc flash is the thermal hazard: intense radiant energy that causes severe burns without any physical contact. Arc blast is the mechanical hazard: the pressure wave, sound, and flying debris from the same event. They come from one fault, but they injure differently, they are measured differently, and only one of them is the number on your equipment label.

Last reviewed: July 18, 2026.

Arc flash: the hazard that gets the number

The flash is the radiant thermal energy of the arc. It is quantified as incident energy in cal/cm² at a working distance — the basis of everything else in the system:

• IEEE 1584-2018 computes incident energy and the arc-flash boundary (the distance where energy falls to 1.2 cal/cm², the second-degree-burn threshold).
• NFPA 70E puts the incident energy, boundary, and PPE level on the equipment label (§130.5(H)).
• Arc-rated PPE is chosen so its arc rating exceeds the label's cal/cm² value.

This burn hazard without contact is what Ralph Lee's landmark 1982 paper, "The Other Electrical Hazard: Electric Arc Blast Burns" (IEEE Transactions on Industry Applications), put on the map: his model treated the arc as an expanding sphere and asked whether the energy at the worker's distance caused curable or incurable burns. Modern incident-energy analysis descends directly from that framing.

Arc blast: the pressure wave

The blast is the mechanical side of the same event. An arc heats surrounding air and vaporizes conductor material almost instantaneously, and that rapid expansion drives a pressure wave — plus molten metal spray, shattered enclosure parts, and a sound pulse. These effects are real and they injure: barotrauma, impact wounds, hearing damage.

But here is where the honest version matters more than the dramatic one.

The "copper expands 67,000 times" figure. If you have read anything about arc blast, you have seen this number. It traces to Lee's early work on vaporized copper, and it has been repeated hundreds of times across papers, training decks, and posters. A review presented in the 2024 IEEE IAS Electrical Safety Workshop proceedings documents that repetition — and that later research, including laboratory pressure-wave measurements by Drouet and the critical review by Hoagland, found the 67,000× expansion figure inaccurate as commonly applied. The review is blunt about the number's use as a scare tactic. The blast hazard does not need inflation: a pressure wave with shrapnel at arm's length is serious at any exact expansion ratio.

Why the distinction matters for labels and PPE

Three practical consequences:

1. Your label number covers the flash, not the blast. A 8 cal/cm² label tells you the thermal dose and the PPE for it. It does not certify the enclosure against the pressure event.
2. The controls differ. Thermal exposure is managed with arc-rated clothing and boundary discipline. Blast exposure is managed upstream: maintenance of protective devices, remote switching and racking, arc-resistant equipment designs, and hearing protection (which NFPA 70E garment systems include).
3. One mitigation helps both. Clearing time. The faster the upstream device opens, the less total energy the event dumps into both heat and pressure — which is why maintenance-mode trip settings and current-limiting devices show up in both conversations.

For the thermal math, arc flash boundary explained walks a worked IEEE 1584-2018 example; PPE category method explained separates category results from incident-energy selection; and how to read an arc flash label covers every field on the label itself. Run your own panel in the free arc flash calculator to see the incident energy and boundary for your inputs.