Arc Flash – Incident Energy at Working Distance

 
With the given values:
C_f = 1.5 for voltages at or below 1 kV.
E_n = 5.26 J/cm²
E_n = 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.

J/cm²
cal/cm²

For the reduce incident energy

J/cm²
cal/cm²

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,

.....[3]

For the theoretically derived equation,

.....[4]

where:
D_B = distance (mm) of the arc flash boundary from the arcing point
C_f = calculation factor
C_f = 1.0 for voltages above 1 kV
C_f = 1.5 for voltages at or below 1 kV
E_n = incident energy normalized
t = time, sec
x = distance exponent from Table D.4.2
E_B = incident energy in J/cm² at the distance of the arc flash boundary
V = system voltage, kV
I_bf = 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)).

Notes:
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.