§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 (the thresholds are an orientation following the US NFPA 70E approach, which is not part of NEN 3140. In Europe arc-flash clothing and face shields are tested and classified under IEC 61482-1-1 and IEC 61482-1-2):

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:

1. De-energising is the norm — apply the five safety rules and LOTO. No voltage, no arc.
2. Where dead working is demonstrably impossible: arc-rated PPE, insulated tools, a defined work zone and the four-eyes principle.
3. 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.

Current through the body — why small values are already dangerous

These values explain why a 30 mA RCD was chosen for personal protection: 30 mA is above the let-go threshold but below the fibrillation threshold (about 40 mA for exposure of a few seconds), and the RCD trips within a fraction of a second, so the exposure stays short.

The severity of a shock does not depend on current alone, but on current × time × path through the body. A hand-to-hand current path is more dangerous than hand-to-foot, because the former runs straight across the heart; with hand-to-foot, a larger share of the current bypasses the heart. Body resistance itself is not a fixed value: dry, calloused skin can offer several kΩ up to tens of kΩ, while moist or sweaty skin drops to just a few hundred Ω — the same voltage then drives a much higher current through the body. This is exactly why the safe touch voltage is set lower in wet or conductive environments (outdoor work, agricultural premises, damp rooms) than the standard 50 V AC.

What to do in the event of an electrical accident

1. Do not touch the victim directly until you are certain the source is de-energised — otherwise the rescuer becomes a victim too.
2. De-energise via the main switch, or push the victim away with a dry, non-conductive object (a wooden handle, dry clothing) if switching off isn't immediately possible.
3. Call 112 and start resuscitation / use an AED as soon as the area is safe, if needed.
4. Always have the victim checked by a doctor, even without visible injury — internal damage (heart rhythm, muscle damage, kidney function from breakdown products) can appear later.
5. Report the incident — an electrical accident with (near-)injury is subject to the reporting obligation to the Dutch Labour Inspectorate (Arbeidsinspectie).

What to do in case of fire caused by an electrical fault

• Use only a CO₂ or powder extinguisher — never water or foam on an installation that may still be live.
• Ventilate the room after using CO₂: CO₂ displaces oxygen and is itself a suffocation risk in a small, enclosed space.
• Don't just open a burning distribution board — the sudden inrush of oxygen can cause the arc to reflash.

First aid for an arc-flash burn

• Cool the burn for 10 to 20 minutes under lukewarm (not ice-cold) running water.
• Do not pull off burnt clothing that is stuck to the skin — this increases the injury.
• For an arc close to the face, always check for eye injury (the intense light can damage the retina) and have it examined medically.