An arc flash happens when fault current jumps through ionised air, producing an arc of around 20,000 degrees Celsius, molten copper spray, a pressure wave and intense ultraviolet light. Incident energy at a working distance, quoted in cal/cm2, depends on fault current, arcing time and enclosure geometry and is calculated with methods such as IEEE 1584. The result sets the arc flash boundary and the arc rating of the clothing workers must wear.
Arc flash label on that MCC says 38 cal/cm2 at 18 inches, nobody opens it live, we isolate and test.
Faster clearing cuts incident energy more than almost anything else, which is why maintenance-mode switches that temporarily lower relay settings are common.
Do not mix it up with
- Flashover: Flashover names the insulation breakdown event; arc flash is the hazard to people from the resulting arc.
Related terms
Fault current
Abnormally high current that flows when insulation fails and a short circuit or earth fault forms.
Permit to work
Formal written authority to work on equipment that has been isolated, earthed and declared safe.
Rack out
To wind a withdrawable breaker out of its connected position in metal-clad switchgear to isolate it.