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 Post subject: Utility provided system impedancesPosted: Wed Jun 03, 2015 12:29 pm

Joined: Tue Jun 02, 2015 11:32 am
Posts: 2
Good Day,

We were provided utility data, but it only included system impedances (R1,X1,R0,X0-PU on 100MVA Base). The utility company considers providing the fault current values a liability. What is the recommended method, convert to SC amps (reference Jim Phillips article, Making Sense of the Numbers, Utility Company Short-circuit Data For Arc Flash Studies) or input directly into software as ‘Zpu’ fault duty units, or other method. Will each method yield the same results? Thanks - DBL

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 Post subject: Re: Utility provided system impedancesPosted: Wed Jun 03, 2015 6:05 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1635
Location: Scottsdale, Arizona
DBL wrote:
Good Day,

The utility company considers providing the fault current values a liability.

That is a pretty interesting statement from the utility. The impedance data and the short circuit data are pretty much the same thing - just need a few calculations to go from one to the other. The article that you reference did not have a case with Z0. That is used when there is a ground/non-phase path so....

For a line-to-ground fault:
I line-to-ground = (3xV) / (Z1 + Z2 + Z0) in your case (and in almost every case Z2 is the same as Z1)
I line-to-ground = (3xV) / (2xZ1 + Z0)

I show everything as a magnitude Z to make it simple. You have the better format of individual R and X values.

The utility indirectly gave you the fault current. It is a direct function of the equivalent impedance which they gave you. Many utilities simply shroud this information in legal disclaimers and go ahead and provide it.

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 Post subject: Re: Utility provided system impedancesPosted: Mon Jun 08, 2015 4:10 am

Joined: Tue Jun 02, 2015 11:32 am
Posts: 2
Thanks Jim.
To confirm, one can obtain Z with individual R and X values, using Z=SQRT[R squared + X squared].

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 Post subject: Re: Utility provided system impedancesPosted: Mon Jun 08, 2015 7:04 am

Joined: Mon Nov 10, 2014 11:01 am
Posts: 23
For some programs you have to enter per unit impedance. I developed a spreadsheet to convert to Per Unit impedance. I tried to paste the spreadsheet
but it was unreadable. I will be happy to provide it.

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 Post subject: Re: Utility provided system impedancesPosted: Mon Jun 08, 2015 7:13 am
 Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 178
I would rather have the impedance values, most of the time I only get a fault current.

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