An ars flash is very simply,an electrical short circuit through air.

Whether it occurs from phase to ground or phase to phase,doesn't matter. What does matter is that enough energy is present to perpetuate the arc. Lightning an electric arc welding machine are both examples of an arc flash is that lightning discharges itself in a very short period of time.

The difference between these two examples

Whereas an arc welding machine sustains the arc indefinitely either way you don't want to get near either of them. Arc flash like lightning can cause electrical equipment to vaporize,resulting in an arc plasma fireball.Temperatures may exceed 35000 degrees fahrenheit,as a comparison the surface of the sun is 9 000 degrees fahrenheit, These high temperatures also cause rapid heating of the surrounding air,and extreme pressures joined together to produce the arc blast.The arc blast will further vaporize electrical equipment,including solid copper conductors which can expand up to 67000 times their original volume. The resut 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 are  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 the following items. First is the flash protection boundary,this is the distance at which the incident energy from the live part this isthe distance at which the incident energy from the live part, 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 is a 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 nauthorized conducted 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.