An arc flash is, very simply, an electrical short circuit through air. It doesn’t matter whether it occurs from phase to ground or phase to phaser. What does matter for it to occur is that enough energy is present to perpetuate the arc. Both lightning and an electric arc welding machine are examples of an arc flash. The difference between these two examples is that lightning discharges itself in a very short period of time, whereas an arc welding machine sustains the arc indefinitely. Either way, you don’t want to get near either of them.
Arc flashes, like lightning, can cause electrical equipment to vaporize, resulting in an arc-plasma fireball. Temperatures may exceed 35,000°F. As a comparison, the surface of the sun is 9,000°F. These high temperatures also cause rapid heating of the surrounding air, and extreme pressures join together to produce the arc blast. The arc blast will further vaporize electrical equipment, including solid copper conductors, which can expand up to 67,000 times their original volume. The result of this violent event is usually destruction of the equipment involved, fire, and severe injury or death to any people who are nearby.
NFPA 70E contains the requirements to perform a shock and arc flash hazard assessment, which involves a calculation performed by a professional engineer to determine the incident energy found at each location. This risk assessment determines the various arc flash boundaries, as well as the PPE that must be used when approaching each boundary.
When determining what the arc flash boundaries are, the assessment will include three separate items. First is the flash protection boundary. This is the distance at which the incident energy from the live part is equal to 1.2 cal/cm2 (calories per square centimeter), which is the limit for a second-degree burn on bare skin. Persons must not cross this boundary unless they are wearing appropriate protective clothing and are under the close supervision of a qualified person.
The second item is the limited approach, which is the distance at which barriers should be placed to protect unqualified personnel from an electrical hazard. Only qualified persons and escorted unqualified persons are allowed to enter a limited space.
The third is the restricted approach. This is the distance at which only qualified personnel are allowed, with appropriate protective clothing and other PPE for the associated hazard. No unauthorized conductive material and no unqualified persons are permitted to cross a restricted boundary. Further, a documented, management-approved plan, known as an energized electrical work permit, is required to enter a restricted space.