Determining avaliable L-G fault current from utility fault contribution

[mull982]

If a utility has given its avaliable 3 phase fault current for a facility to be used in a short circuit study is it possible to find or estimate the avaliable L-G fault current from the utility? Can you simply divide the 3phase fault current by 1.73 to get an estimate or must you know the zero sequence impedance from the utility?

Someone was trying to tell me that you can simply divide the 3phase fault current by 3 as a rough estimate but I dont believe that is correct?

 

[zog]

L-G fault levels can vary greatly depending on different conditions, I have a note here (I do not recall the source) that a L-G can be up to 74% of 3 phase BF value.

 

[kingpb]

There are instances where a L-G fault can be greater than a 3-ph fault.

In lieu of the correct L-G info, it is a good practice to assume 3ph and SLG are the same.

 

[RoberteFuhr]

There are instances where a L-G fault can be greater than a 3-ph fault.

In lieu of the correct L-G info, it is a good practice to assume 3ph and SLG are the same.

I agree. This is what we do as well. You may have better luck by sending a form to the utility and let them fill it in for you. I have tried to upload it to this site. However, I keep getting an error message. Send me an email if you are interested in having a copy of our form.