An arc flash is a complex phenomenon gaining increased understanding and attention in the electrical safety world. Arcs are basically ionized gases caused when electricity travels through air. "arc flash", "arc blast" and "electric arc" are really all different names for the same basic phenomenon.
There are three primary issues in electrical arcs from an electrician's point of view:
Electricity begins to travel through air because of a breakdown of insulation from air or another insulator. This causes air to ionize, spinning off the outer shell electrons from air. This ionization has two effects: blast and radiation release. The blast is a function of fault current, container size and other parameters including magnetic forces. The pressure wave is not as well documented as the thermal effects but it does occur. Standards do not tell us what the cutoff is for arc blast but good understanding is key.
If you have greater than a 40 cal/cm2 exposure, you should seek engineering guidance to see if it is safe to de-energize or work live. Remember de-energizing can cause an arc if equipment fails or is not designed to be safely de-energized in the manner you are using. The thermal portion of the arc is very well understood and standards like NFPA 70E, NESC, and CAN/UL S801 have reasonable guidelines for determining arc flash energies and some guidelines on clothing worn.
In the most prevalent writings on the matter, it is common to read about two types of arcs, most predominantly considered in calculations and standards. However, there are actually four types of arcs, two of which are rarely considered in arc hazard assessments and only briefly addressed in the arc flash calculations.
Only the first two are considered in the calculations and standards. The open air arc is well understood.
In lab testing, we control movement of the arc for the sake of replication, but in reality, from 480V higher, the open air arc can quickly turn into an ejected arc or a tracking arc. The tracking arc is most common at very high voltages or during an electrical contact.
Arc flashes are not the most common killer of electrical workers. Arc flash training must include full electrical safety principles and solid risk analysis.