Arc-flash reduction methods must be considered prior to working on energized electrical equipment for personnel safety. The idea behind energy reduction is to place energized electrical equipment in a state where less arc energy is available when an arcing fault occurs. These techniques range from the obvious (de-energize!) to more complex and costly engineering solutions. NEC Article 240.87-2011, initially titled …

An electric arc or arc flash occurs when a high voltage causes ionization of a gas — for example, air. Ionization turns an ordinarily nonconductive gas into conductive gas that can carry a sustained current. The color of a typical arc flash in air is very distinctive. This is because air is composed of 21% oxygen and 78% nitrogen; oxygen …