Calculating Flash Protection Boundary Distances

Time-Current Curve

In NFPA 70E, the short-circuit symmetrical ampacity from a bolted 3-phase fault at the transformer terminals is calculated with the following formula:
I_{sc} = {{MVA_{Base} * 10^6} / {sqrt{3}*V}} * {100 / %Z}
Isc = Amperes,
V = Volts,
%Z = transformer MVA.

A typical value for the maximum power (in MW) in a 3-phase arc can be calculated using the following formula:
P = MVA_{bf}*{0.707}^2~MW
MVAbf = maximum bolted fault, MVA

The Flash Protection Boundary distance is calculated in accordance with the following
P = 1.732*V*I_{sc}*10^6*0.707^2
D_c = {2.65*MVA_{bf}*t}^{1/2}
D_c = {53*MVA*t}^{1/2}
Dc = distance in feet of person from arc source for a just curable burn (i.e., skin temperature remains less than 80 degrees)
MVAbf = bolted fault MVA at point involved
MVA = MVA rating of transformer. For transformers with MVA ratings below 0.75 MVA, multiply the transformer MVA rating by 1.25.
t = time of arc exposure in seconds