Fault Current Calculations

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I've received a letter from a utility giving me two different AFC's.

One is Symmetrical Fault Current of 24,000 amps and the other is Ultimate 81,000 symmetrical.

What in the world do I do with this, other than divide by 1.414?

Which value do I use?
 
Re: Fault Current Calculations

I have never heard the word "ultimate" used in this context. I might suspect it is intended to mean the same thing as "assymetrical," but I would not expect that much of a difference between the two numbers.

I do know, however, that the numbers are already in terms of RMS values. The division by 1.414 has already taken place (i.e., to covert from peak value to RMS value). So you don't divide by 1.414 again.
 
Re: Fault Current Calculations

Never heard of ultimate short circuit current either. Maybe it's short circuit current on steroids. ;) Give 'em a call and ask what they mean.
 
Re: Fault Current Calculations

I haven't heard of it either but I would guess that the ultimate would be the lowest impedance transformer that they have on their system. That would be in case they had to replace the bank. The other guess would be the next size transformer in case they had to replace the bank with the next size.

IMO, that is weird terminology. We all have weird terminology though. :D
 
Re: Fault Current Calculations

It's probably an asymetrical value, that is a peak that occurs within 1/2 cycle rising considerable more above the horizontal axis than it goes below for the next few cycles.
As I understand it this condition occurs in circuits containing reactance which are short-circuited at some time other than when the current is passing through point zero in the cycle. The reactance effect superimposes a DC component on top of the AC current causing asymmetry which decays within a short time after the occurance of the fault to the symetrical value.
 
Re: Fault Current Calculations

I agree with David, ask'em. But, since we're otherwise speculating, it may be the maximum value system planners anticipate at that location at some future date and they are committed not to exceed it.
 
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