Accounting for X/R in Point-to-Point SCC Calculations

[MyCleveland]

Using the Point-to-Point calculation method, is there a method that would allow taking into account the X/R ratio.

Also, reading that if X/R is not provided from utility, acceptable to use value of 12 as basis of design. Agree or Disagree ?

 

[NewtonLaw]

Using the Point-to-Point calculation method, is there a method that would allow taking into account the X/R ratio.

Also, reading that if X/R is not provided from utility, acceptable to use value of 12 as basis of design. Agree or Disagree ?

There is a number of software programs out there that make use and allow you to accumulate the X/R ratio from point to point along the system route to the short circuit point. Look at SKM, EasyPower, CYME and others. If you are doing the calculations by hand, the impedance calculations you are coming up with gives you the X/R at any point of interest.

Also, every Utility I have worked with to date has given me the available fault current and X/R ratios for the three phase, and single phase faults at the point of contact (POC). If requested, I have also received the symmetrical components for impedance at the POC.

I hope I was able to answer your question since I may not understand just what you needed.

 

[shortcircuit1]

Using the Point-to-Point calculation method, is there a method that would allow taking into account the X/R ratio.

Also, reading that if X/R is not provided from utility, acceptable to use value of 12 as basis of design. Agree or Disagree ?

You have to remember one crucial point that the higher X/R ratio higher the available fault current.X/R ratio plays a very important role in determining your momentary SC rating which you will see mostly on MV/HV equipment. If X/R ratio is higher there will be a higher speak asymmetrical fault current at the onset of the fault and it takes more cycles to restore to steady state. Mostly yes to use X/R ratio as 12.I think its in one of the IEEE standards but if you request utility company they can provide you with X/R ratio as well...

 

[MyCleveland]

NewtonLaw & shortcircuit1
Thank you both for the input.

I am trying to develop an adjustment factor for precisely what you stated....higher asymmetrical component when your X/R is above the equipment TESTED X/R.

The 12 number was taken from one of many papers on the subject, but I have not been able to pin down the origin. HELP appreciated.

Yes the utility should be providing an X/R, but I do not receive it in every case so I am trying to cover that base from a data entry solution if not provided.