An electric arc is a continuous electric discharge of high current between conductors, generating very bright light and intensive heat.
"This video clip shows you an extreme electrical arc. It was captured by Neil Brady, the maintenance foreman of the 500 kV Eldorado Substation near Boulder City, Nevada in the US. Due to a faulty switcher which failed to interrupt when the isolation switched open, it created the huge arc.
The arc stretches upward, driven by rising hot gases and writhing from small air currents, until it easily exceeds 100 feet in length. The arc continues to elongate until the voltage was not sufficient to support the resulting arc impedance due to the arc length.
Switching arcs usually terminate long before reaching this size since they normally flash over to an adjacent phase or to ground. Once this happens, the phase-to-phase fault current will cause an upstream circuit breaker to trip, disconnecting the circuits."
The incident heat energy that an electric arc can produce is determined by the amount of the arc current, the duration of the arc, the distance between the worker and the arc and the configuration of the conductors and the surrounding environment.
In order to choose adequate protective clothing, the potential incident heat energy (in units of energy per square surface - cal/cm² or kJ/m2) caused by an electric arc and to which a worker may be exposed, needs to be calculated in a thorough risk assessment.