An arc is produced when electric current flows through an ionized air path instead of a metal conductor. During an equipment fault, high current arcs may develop between conductors of different phases or between conductors and the metal enclosure of the equipment. These arcs produce heat, light, and sound just like the low power arcs used in welding, and that is where the hazards exist for workers who are exposed to them.
The crackling sound associated with a welding arc is the result of rapid expansion of vaporized metal and heated air. Vaporized copper expands in volume 67,000 times, and heated air expands roughly proportionally to the temperature. In fault arcs, this expansion produces an explosive effect and pressures that can be high enough to propel a worker across the room. Even at lower pressures, the blast may "knock the wind out" of the worker who, struggling to recover, is then likely to inhale a cloud of vaporized metal and superheated air, severely burning the trachea and lungs.