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 Post subject: Short Circuit magnitude through UPS
PostPosted: Sat Jan 05, 2013 2:32 pm 
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I have (4) 160kVA UPS feeding the same bus. On the line side of the (4) UPS the short circuit magnitude is 24kA. However, the short circuit mag. on common bus on the load side of the (4) UPS is significantly less; 0.615kA. So my question is should I be seeing such a significant drop in my short circuit magnitude?


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PostPosted: Sat Jan 05, 2013 4:34 pm 
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bruinfan wrote:
I have (4) 160kVA UPS feeding the same bus. On the line side of the (4) UPS the short circuit magnitude is 24kA. However, the short circuit mag. on common bus on the load side of the (4) UPS is significantly less; 0.615kA. So my question is should I be seeing such a significant drop in my short circuit magnitude?


On the inverter side - Yes. Typically the short circuit current from the inverter is around 2 times the full load current. If there is a static and / or maintenance bypass, then you would want to consider Murphy. i.e. his law and the possibility of the arc flash occurring in bypass mode.

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PostPosted: Mon Jan 07, 2013 8:26 am 
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Additionally, if you are performing a short-circuit analysis on equipment downstream of the UPS then you do not want to use the value as-calculated with the UPS online when there is a static switch involved. This is because in the event of a short-circuit downstream of the inverter the static switch will remove the load from the inverter almost immediately and place it into bypass mode. At that point the short circuit current would then only be reduced by the impedance of the bypass circuit.


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