Term to Remember

Incident Energy

The amount of energy impressed on a surface, at a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared (cal/cm2).

The working distance is the distance from where the worker stands in relation to the arc flash location. The minimum distance for incident energy calculations on low voltage equipment (<600 volts) is 18 inches. For equipment operating at greater than 600 volts, the minimum distance is 36 inches. Incident energy is a function of system voltage, available short-circuit current, arc current, and the time required for circuit protective devices to open.

Informative Annex D of NFPA 70E provides references to standard methods for calculating incident energy and the arc flash boundary.