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


  • A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia, and Professional Electrical Engineer (PEE 2574 - 1st Place April 1991) - Philippines with extensive experience in concept selection, front-end engineering, HV & LV detail design, construction, and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and wastewater pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).


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