Arc flash — risk, energy and PPE
§6.3 — Arc flash: risk, energy and protection
An arc flash is a short-circuit arc that jumps through the air between live parts, or between phase and earth. The arc ionises the air into a plasma of ~20,000 °C — hotter than the surface of the sun — and turns metal explosively into vapour. The accompanying pressure wave (the arc blast) can blow doors open, damage hearing and throw a person off a ladder.
How an arc starts
- Switching under fault or pulling a loaded disconnector.
- A forgotten tool or a loose wire bridging two phases.
- Insulation failure from moisture, contamination, vermin or ageing.
- A measurement error: the wrong range setting on a board with a high prospective short-circuit current.
Incident energy and PPE
The incident energy is expressed in cal/cm² and depends on the prospective short-circuit current, the protection's clearing time and the distance to the arc. The slower the protection trips, the more energy the arc dumps into the body. The calculated value sets the arc-rated PPE (category per IEC 61482 / NFPA 70E):
| Incident energy | Minimum arc-rated PPE |
|---|---|
| ≤ 4 cal/cm² | arc-rated coverall + face shield |
| ≤ 8 cal/cm² | heavier arc-rated clothing |
| ≤ 25 cal/cm² | arc-flash suit + hood |
| > 40 cal/cm² | work prohibited — de-energise first |
Ordinary cotton workwear or a hi-vis vest offers no protection and may ignite and melt to the skin.
What NEN 3140 / EN 50110 require
Working on or near live parts is only allowed after a risk assessment (LMRA). The hierarchy is strict:
- De-energising is the norm — apply the five safety rules and LOTO. No voltage, no arc.
- Where dead working is demonstrably impossible: arc-rated PPE, insulated tools, a defined work zone and the four-eyes principle.
- Determine the arc-flash boundary — the distance within which the incident energy exceeds 1.2 cal/cm² (the second-degree burn threshold).
Common mistake
Opening a live distribution board "just to take a quick measurement" without arc-rated PPE. It is exactly these routine actions — measuring, swapping a fuse, a visual check — that cause most arc incidents, because the protection in that part of the installation trips slowly while the short-circuit current is high.