Most people picture an arc flash as an especially bad electrical fire. It isn't. When current jumps through ionized air between two conductors - or from a conductor to ground - the energy releases in microseconds and forms an arc plasma that can reach roughly 35,000°F. That's about four times the surface temperature of the sun, packed into a fraction of a second, close enough to hurt someone who never touched anything energized.
The number that actually matters: incident energy
Voltage tells you how much pressure is behind the system. It doesn't tell you how badly you'll get burned. The value that does is incident energy - the amount of thermal energy a worker's body could absorb at a specific working distance during an arc flash event, measured in calories per square centimeter (cal/cm²). Every arc flash hazard analysis produces an incident energy value at a defined working distance, and that single number drives every PPE decision downstream of it.
Working distance - the distance from a worker's face and chest to the likely arc source - matters more than most people expect. The relationship between distance and incident energy isn't linear: roughly doubling the distance cuts incident energy by a factor of four. Standing an extra foot back can be the difference between a PPE category most people already own and one that requires a full arc-rated suit.
Flash vs. blast - two different things happening at once
The thermal component is what causes burns, and it's what the cal/cm² system is built around. But there's a second, separate hazard: the arc blast, the pressure and shock wave from the explosion. Both are dangerous. Exam questions and real-world PPE selection focus on the thermal side because that's what the rating system measures, but don't mistake "PPE-rated" for "blast-proof."
What actually starts one
Arc flashes aren't random. Common initiating events include a dropped conductive tool landing across bus bars, racking a device while it's still energized, a blown fuse causing a re-strike, insulation failure from tracking or contamination, and even rodents bridging conductors inside a panel. How much energy gets released depends on the available fault current from the utility and system impedance, and on how fast the upstream protective device clears the fault - which is why fast-acting protection matters as much as PPE.
Incident energy, working distance, and the Arc Flash Protection Boundary calculated from them are covered in more depth in the Study Guide, along with practice questions in the Exam Companion.
Standard reference: NFPA 70E 2024.