HELP: Single phase SC Calculation w/ SKM
HELP: Single phase SC Calculation w/ SKM
(OP)
Can anyone help me with a single phase short-circuit calculation as follows?
It's a airport runway ILS job. We need to run single phase from a 250KVA 13.8KV-240/120v transformer to a 25KVA 240v-2400v step-up transformer, then about 4000' #6 single phase wire to 25kva 2400-120-240V step-down transformer to feed the load about 20KVA.
We are using SKM software to do the short-circuit calculation, but just cannot get any result. Does anyone have the experience on this?
thanks!
James
It's a airport runway ILS job. We need to run single phase from a 250KVA 13.8KV-240/120v transformer to a 25KVA 240v-2400v step-up transformer, then about 4000' #6 single phase wire to 25kva 2400-120-240V step-down transformer to feed the load about 20KVA.
We are using SKM software to do the short-circuit calculation, but just cannot get any result. Does anyone have the experience on this?
thanks!
James






RE: HELP: Single phase SC Calculation w/ SKM
RE: HELP: Single phase SC Calculation w/ SKM
My problem is that when I start the "Unbalanced/Single Phase Study Setup" dialog window, I cannot find report file name next to "Comprehensive Fault (SC)" item. So how can I start the SC study and view the result?
Also, I am a little confused with the "Compoment Editor" for single-phase transformers. How can I setup the transformer connection correctly? Right now, the following is my configuration of the transformers:
XFMR Primary Secondary TYPE Phase
XF1 13800v(L-L) 120v(L-N) Single Phase Pri:AB
XF2 240v(L-L) 1200v(L-N) Single Phase Pri:AB
XF3 1200v(L-N) 120v(L-N) Single Phase Pri:A
Does it look right to you?
thanks,
James
RE: HELP: Single phase SC Calculation w/ SKM
RE: HELP: Single phase SC Calculation w/ SKM
thanks,
James
RE: HELP: Single phase SC Calculation w/ SKM
RE: HELP: Single phase SC Calculation w/ SKM
However;
25 KVA / 2400 Volts= 10.4 Amps
10.4 Amps / 3% Impedance voltage = 347 amps available fault current symetrical.
25 KVA / 240 Volts= 104 Amps
104 Amps / 3% Impedance voltage = 3472 amps available fault current symetrical.
Check your actual impedance voltages and redo the arithmetic.
Hope this helps
RE: HELP: Single phase SC Calculation w/ SKM
RE: HELP: Single phase SC Calculation w/ SKM
For instance, a breaker rated for 5000 Amps available fault current will withstand the maximum stresses imposed by the much greater asymetrical currents that may be present.
It has been my understanding that the codes use the phrase "Available shortcircuit current" to describe the current flowing in a steady state condition, after the asymetrical currents have decayed to zero.
If you are co-ordinating protection for a distribution utility you must calculate the actual fault current.
I do agree with you that calculations of the actual fault current are a lot more involved.
However, as I understand the original post, I think the poster is trying to select panels breakers and/or switches
that are suitable for the available short circuit current.
The last time that I had to submit calculations for available fault current calculations to an inspection authority prior to a permit being issued, the figures I showed were the accepted calculations.