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Fault current on finite bus

  1. Aug 13, 2013 #1
    so calculating FC on an infinite bus is easy but how do you calculate it by hand for a finite bus?

    here is the concept i am having an issue understanding:

    let say i have a transformer that is 225KVA with 5%z and 3 phase 480 to 208.

    with infinite bus i have 12491A available on the secondary of this transformer IF 12291A is flowing on the secondary then based on the voltage ratio and OHMs law 5412.8 A is flowing on the primary.

    If this is true then what does it matter if i have infinite amps available on the primary or i have 5412.9 amps available 12491A should still flow in my mind.

    I have run this in SKM and i see that it does matter but i just dont understand why.
  2. jcsd
  3. Aug 13, 2013 #2


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    Gold Member

    Let's talk in an ideal world. If you have no load, no current will flow. As you add a load to the secondary, current will flow based on the secondary voltage. Since power in == power out, then equal power will flow in the primary and you can calculate the primary current based on the primary voltage. So the key is power in == power out. Your source merely needs to be able to supply the required current.

    I'm not sure where ohms law comes in since you aren't talking resistance yet. I don't quite understand your infinite vs finite either. Am I missing some basic assumption?
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