Here's some "Quick 'N Dirty" SCA calc formulas, using the basic "Point-to-Point" method.
Available short-circuit symmetrical RMS current is found using the 6 steps described below:
Step 1; Determine the Transformer's Full-Load Amperes (I·fl):
3Ø Transformer
I·fl = KVA × 1000 / E·l-l × 1.732
1Ø Transformer
I·fl = KVA × 1000 / E·l-l
Step 2; Find the Transformer multiplier (M·tr):
M·tr = 100 / (%Z × 0.9)
where: %Z is the Transformer's Impedance.
Step 3; Determine the Transformer let-thru short circuit current (I·sc):
I·sc = I·fl × M·tr
FYI: Induction Motor contribution may be added to the above figure at this point.
An estimate for Motor contribution to SCA is:
4 to 6 × FLA of Motor(s).
LRA may also qualify.
Step 4; Calculate the "F" factor:
3Ø Faults:
F = 1.732 × L × I·3ø / C × E·l-l
1Ø L-L Faults on 1ph center tapped transformer:
F = 2 × L × I·l-l / C × E·l-l
1Ø L-N Faults on 1ph center tapped transformer:
F = 2 × L × I·l-n / C × E·l-n
where:
"L" = Length (feet) of conductor to the fault.
"C" = Constant from "Table C" of the Bussman manual (will add this later - big database!).
"I" = Available short circuit current at beginning of circuit.
"L-L" = Line-to-Line.
"L-N" = Line-to-Center Tapped "Neutral" conductor.
*Note: L-N fault current is higher than the L-L fault current at the secondary terminals of a 1Ø center tapped transformer. The short circuit current available for this case in step 4 should be adjusted at the transformer terminals as follows:
I·l-n = 1.5 × I·l-l at transformer terminals.
Step 5; Calculate "M" (multiplier):
M = 1 / 1 + F
Step 6; Calculate the available short circuit symmetrical RMS current at the point of fault (I·afc):
I·afc = I·sc × M
I'll post a few example SCA scenarios later when posting the "C" database.
Scott35
