What is an Arc Flash?
Plasma physics, sustaining conditions, and how a bolted fault becomes an arc.
A bolted fault is a metallic short — copper to copper, no air gap, near-zero impedance. Current is huge, the protective device clears in a cycle or two, the equipment may be damaged but the fault itself doesn’t do anything beyond the current flow.
An arc fault is what happens when the path between the conductors is plasma instead of metal. Ionized air carrying tens of kiloamps. The arc itself is the hazard — it radiates light, heat, sound, pressure, and molten metal vapor. It can sustain for a long time if nothing forces it to clear. And until it clears, it pumps energy into anyone close enough.
How an arc fault starts
Three common ways:
- Tool dropped across busbars. A screwdriver, a wrench, an arc chute mounting plate — anything conductive briefly bridges phase-to-phase or phase-to-ground.
- Insulation failure. A cable nicked during pulling, a contaminated bus barrier, a creep path on a porcelain insulator — tracking can develop into an arc.
- Improper racking. Withdrawing a breaker partially out of a switchgear cell while energized. Done wrong, the stabs make intermittent contact with the bus, arcing as they break.
In all three cases the initiating event makes brief metallic contact that might clear itself, might extinguish into open air, or might ionize enough of the surrounding air that the arc continues between the original contact points (or somewhere nearby).
What sustains the arc
For a 480 V arc to sustain, two conditions have to hold:
- The driving voltage has to be enough to maintain ionization. Roughly 60–200 V per cm of arc gap, depending on current. For a typical 480 V switchgear bus with a 32 mm conductor gap (about 3 cm of arc path), the system can comfortably drive an arc. Below ~200 V the system can’t reliably sustain — that’s part of why control-circuit voltage doesn’t get treated the same way.
- Available fault current has to be enough to keep the plasma channel above its thermal threshold. Once the arc is established, the arcing current is typically 50–65 % of the bolted Isc — the arc voltage drop steals back some current that would otherwise flow. But that arcing current is still tens of kiloamps, more than enough to keep the channel hot.
When both conditions hold, the arc continues until something opens the circuit upstream — typically the nearest protective device that sees the fault.
Where the energy goes
The instantaneous power dissipated in the arc is roughly:
P_arc = V_arc · I_arc (a few hundred volts × tens of kiloamps)For a 480 V bus, a 25 kA bolted fault might produce ~15 kA of arcing current at about 130 V of arc drop — about 2 MW dissipated in a few-cubic-inch volume of plasma. That energy doesn’t all stay there; it radiates outward as light, sound, heat, and pressure — and as metallic vapor at thousands of degrees that condenses into burning droplets on any nearby surface.
A worker standing 18 inches away absorbs a fraction of that radiated energy. How big a fraction, and for how long, is what the IEEE 1584 model in the next lesson predicts. The two main variables under your control as an engineer are the arcing current (set by the system, but reducible) and the clearing time — how long until the upstream protective device opens the circuit.
A 3-cycle (~50 ms) arc at 25 kA on a 480 V bus delivers about 2 cal/cm² at 18 inches — uncomfortable but survivable in PPE Cat 1 clothing. The same arc lasting 60 cycles (1 s) delivers ~45 cal/cm² at the same distance — beyond PPE Cat 4 and into the DANGER zone, where no PPE category exists and energized work isn’t permitted.
The time difference between a 3-cycle clear and a 60-cycle clear is the protective device’s coordination settings.
That’s the bridge from arc flash to the TCC tutorial. Module 3 of this tutorial will land squarely on it.
Bolted fault vs arc fault — a side-by-side
| Bolted fault | Arc fault | |
|---|---|---|
| Path | Metallic, near 0 Ω | Plasma + air, ~10–50 mΩ |
| Current | Full Isc (tens of kA) | 50–65 % of Isc at LV |
| Voltage across fault | ~0 V | 60–200 V per cm of gap |
| Power dissipated in fault | Near 0 (no V × I) | MW scale |
| Primary hazard | Equipment damage | Worker injury + equipment damage |
| Used for | Equipment ratings, breaker AIC | Worker PPE, AFB, NFPA 70E compliance |
What’s next
The next lesson is the calculator that turns Isc + clearing time + geometry into a number on a label. You’ll see how each input moves the needle — and which ones the engineer has any control over.