The switch arc
Basically, an electrical arc exists during a self-sustaining gas discharge between two electrodes. If the open contacts are not far enough from each other during the switching operation, it only takes little electric field strength to consistently ionize the air. The result is the forming of an arc, just like lightening during a thunderstorm. In alternating current, the arc extinguishes at the zero crossing of the sinusoidal current wave. If the contacts are not far enough apart upon re- tensioning, the arc ignites again. The goal of various systems is to avoid this from happening and to quickly extinguish the arc. For example, compressed air could blow out the arc or various switching methods could with high dielectric strength, in a vacuum or with hexafluoride gas.
An arc fault that is not quickly extinguished can cause great damage such as fire and explosion, which can occur if the arc fault lasts up to 300 milliseconds. Depending on the design of the switchgear, personnel can be seriously injured.