arc-flash analysis of utility power systems - Michigan Technological ...

" : ) % & 6 7
=9 " (3.7)
<
where
E = incident energy (cal/cm 2 )
V = system voltage (kV)
t = arcing time (seconds)
D = working distance (mm)
I a
= bolted fault current (kA)
The next step in the calculations is to determine the flash protection boundary.
This is an approach distance at which a worker will receive a second-degree burn if an
arc flash would occur; this is the level of 1.2 cal/cm 2 . (3.8) provides the equation to
determine this distance
> " " 4 5 & 6 7
9 8 6:8; 9A
2 @
B
;
(3.8)
where
D b
C f
E a
= distance of flash protection boundary from arc (mm)
= calculation factor of 1.5 for voltages less than or equal to 1 kV
= normalized incident energy
E b = incident energy in J/cm 2 at the flash protection boundary: typically 5
J/cm 2 which is equal to 1.2 cal/cm 2
t = arcing time (seconds)
X = distance exponent
610 = normalized distance of 24 inches converted to millimeters
0.2 = normalized 0.2 second clearing time
(3.8) is based on the current being interrupted by a non-current limiting device.
Additional equations are needed if these devices are present.
14