Short Circuit Calculation
Short Circuit Calculation
(OP)
Hi This is my first post and am working as an EIT. I am working on a 4.16 kV swgr spec which is supplied by two transformers in parallel. Both transformers are old and will be replaced in the near future. I want to take a conseravtive approach and make sure that the new switchgear will be able to hanlde short circuit.
My swgr specs. calls for swgr rated at 31.5 kA RMS Symmetrical at 4.16 kV.
This is what I have considered;
TR1- 27.6/4.16 Kv 5%z 5 MVA
Substation TR1 Unit
Size 5 MVA
Vol. 4.16 kV
%Z(One in Service) .05 P.U
%Z(Two in Service) 0.025 p.u
I Fault(P.U. one in service) 20 P.U.
%Z(Two XFRM in Parallel Impedance) .025 p.u
I(Fault P.U.) two in Service 40
I Fault(1) One in Service only 13895.06 Amps
I Fault(2) Both in service 27790.13 Amps
Based on this , Is a 31.5 kA rating would work. Just wondering if I missed anyting.
My swgr specs. calls for swgr rated at 31.5 kA RMS Symmetrical at 4.16 kV.
This is what I have considered;
TR1- 27.6/4.16 Kv 5%z 5 MVA
Substation TR1 Unit
Size 5 MVA
Vol. 4.16 kV
%Z(One in Service) .05 P.U
%Z(Two in Service) 0.025 p.u
I Fault(P.U. one in service) 20 P.U.
%Z(Two XFRM in Parallel Impedance) .025 p.u
I(Fault P.U.) two in Service 40
I Fault(1) One in Service only 13895.06 Amps
I Fault(2) Both in service 27790.13 Amps
Based on this , Is a 31.5 kA rating would work. Just wondering if I missed anyting.






RE: Short Circuit Calculation
RE: Short Circuit Calculation
RE: Short Circuit Calculation
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RE: Short Circuit Calculation
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RE: Short Circuit Calculation
RE: Short Circuit Calculation
1) Fault contribution from the Utility( Ultimate vs present day)
2) Any synchronous motors attached on 4.16KV bus
3) 31.5KA is the short circuit interrupting rating, and not the bracing rating of the equipment.
Please correct if I am wrong. Thanks
RE: Short Circuit Calculation
on utility fault contribution - this amounts to adding a source impedance so the calculated results would be lower than what was already shown.
motor contribution - this would add some current to what was already calculated.
switchgear rating - short circuits have both thermal effects and forces that result from the current level. In order to meet a particular interrupting rating, I would expect that the bracing is already taken care of by the manufacturer.