NFPA 70E arc flash calculations method

The procedure used for determining incident energy and arc flash protection boundary in the NFPA 70E online arc Flash calculator is based upon NFPA70E 2000. It is important to note that the flash protection boundary is dependent upon the available bolted short-circuit current or the let-through current if the overcurrent protective device is current-limiting, and the opening time of the overcurrent protective device t. These formulas were derived from a broad base of empirical test data and were state of the art when introduced.

Flash Protection Boundary Calculation

where:
D_c = distance in feet for a "just curable" burn
D_f = distance in feet for an "incurable burn"*
MVA_bf = bolted three phase MVA at point of short-circuit
MVA_bf = 1.73 x VOLTAGE_L-L x AVAILABLE SHORT-CIRCUIT CURRENT x l0^-6
t = time of exposure in seconds

*Not included in NFPA 70E.

Calculation of Incident Energy Exposure for a Flash Hazard Analysis.

The following equations are used to predict the incident energy produced by a three-phase arc on systems rated 600 volts and below. As stated in NFPA70E-2000, the results of these equations might not represent the worst case in all situations. It is essential that the equations be used only within the limitations indicated in the definitions of the variables shown under the equations. The equations must be used only under qualified engineering supervision. (Note: Experimental testing continues to be performed to validate existing incident energy calculations and to determine new formulas.)

Arc in a Cubic Box (20 inch. cubic box). The estimated incident energy for an arc in cubic box (20 in. on each side, open on one end) is given in the following equation. This equation is applicable to arc flashes emanating from within switchgear, motor control centers, or other electrical equipment enclosures.

where:
E_MB = Maximum 20 in. Cubic Box Incident Energy (cal/cm²)
D_B= Distance from Arc Electrodes, inches (for distances 18 in. and greater)
t_A = Arc Duration, seconds
F = Short Circuit Current, kA [for the range of 16kA to 50kA]