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Switchgear Short Circuit Fault Level

Switchgear Short Circuit Fault Level

Switchgear Short Circuit Fault Level

(OP)
Hello All,

I am buying a 4.16 KV swgr fed by two incoming XFRM's rated 5 MVA, 27.6 kV/4.16 kV. Both will operate in parallel with no tie.

  Voltage= 4.16 kV
 TR1= 5 MVA  %Z1= 5
 TR2= 5 MVA  %Z2=5

  Short Circuit Contribution MVA(TR1)=  20 P.U.=13895 A
  Short Circuit Contribution MVA(TR2)=  20 P.U.=13895 A

 Total Contribution= 40 P.U.

 S.C Contribution(Total from TR1 & TR2)= 27790 A

My switchgear is rated at 31.5 kVA rms symettrical. I just wanted a second opinion if I am missing anything in calculating maximum short circuit contribution towards 4.16 kV switchgear.
....

Appreciate your input !

RE: Switchgear Short Circuit Fault Level

You might have to consider the X/R ratio since you are close to the rating.  The rating is based on an X/R ratio of 17.  See thread238-254443: Breaker Test X/R Ratio
 

RE: Switchgear Short Circuit Fault Level

your math is correct.

RE: Switchgear Short Circuit Fault Level

If the switchgear is rated at 31.5 kVA, you definitely have a problem.  looking around

If it's actually 31.5 kA, refer to jghrist's response.  

Also, you need to consider any motor contribution that may be coming from feeders connected to the switchgear.  In a large facility, these cannot be ignored.  

David Castor
www.cvoes.com

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