Arc-Flash Boundary

The arc-flash boundary is the distance at which incident energy has fallen to the threshold for a second-degree burn on bare skin — conventionally taken as 1.2 cal/cm², a figure that comes from the Stoll burn-injury curve and is quoted throughout the public NFPA and NIOSH literature. Inside that distance, an arc would blister you; outside it, it would not. Because incident energy obeys the inverse-square law, you never need to redo the whole calculation: if a study reports E at a working distance D, the boundary is simply D scaled by the square root of the energy ratio. Eight cal/cm² at 450 mm becomes 1.2 cal/cm² at 450 × √(8/1.2) ≈ 1160 mm.

Do not confuse this boundary with the shock approach boundaries. They answer different questions and are set by different physics: the limited and restricted approach boundaries are about contact and flashover to a person and depend on system voltage, while the arc-flash boundary is about radiant heat and depends on energy and clearing time. A 120 V panel can have a shock boundary of centimetres and an arc-flash boundary of metres, or the reverse. All of them appear on the equipment label, and all of them have to be respected independently.

The honest caveat is the same as on the incident-energy page: this scaling is only as good as the study that produced E and D in the first place. It also assumes the same source, the same clearing time and the same geometry at both distances — change the upstream device's settings and every number moves. And when the boundary comes out smaller than the working distance, which happens on small, fast-clearing systems, that is a genuine result and not an invitation to stand closer: the shock approach limits and the equipment's own working clearances still apply.