Arc Flash – Incident Energy at Working Distance

With the given values:
Cf = 1.5 for voltages at or below 1 kV.
En = 5.26 J/cm2
En = 4.4126 J/cm2 (reduced incident energy)
t = 0.025 s (from Table 1, See note 1)
x = 1.641 (from Table D.4.2)
D = 455 mm See Table 3.


For the reduce incident energy


Now that the incident energy has been determined, the Arc Flash Boundary can be calculated. Arc Flash Boundary is the distance at which a person is likely to receive a second degree burn. The onset of a second degree burn is assumed to be when the skin receives 5.0 J/cm2 of incident energy.

For the empirically derived equation,


For the theoretically derived equation,


DB = distance (mm) of the arc flash boundary from the arcing point
Cf = calculation factor
Cf = 1.0 for voltages above 1 kV
Cf = 1.5 for voltages at or below 1 kV
En = incident energy normalized
t = time, sec
x = distance exponent from Table D.4.2
EB = incident energy in J/cm2 at the distance of the arc flash boundary
V = system voltage, kV
Ibf = bolted three-phase available short-circuit current

The above equations could be used to select personal protective equipment (PPE), to ensure that it is adequate to prevent thermal injury at a specified distance in the event of an arc flash.

For our example, using the reduced arcing current and equation [3], the calculated arc flash boundary (AFB) is 506.4 mm requiring Category 1 PPE (see Table 130.7(C)(15)(A)(b)).

1. This need to taken from the time-current curve of the protective device. In the absence of such information, Table 1 can be used. Latest models of MCCB can clear faults in 0.5 - 1 cycle. As per NFPA 70E, 2 seconds is a reasonable maximum time for calculations.