Arc Flash info for calculation

[chelin]

We are starting with an arc flash analysis and we use SKM PTW but we have some concerns on the input information for the utility available fault.
1. the utility company only gives us the three phase fault current at the meter and the %Z. SKM ask for Line to ground contribution also and for X/R for both not %Z, where can I find this values or can I caluate them?
2. in our oneline diagram we define the utility with the values given for secondary as shown and it came up with category #1 PPE for the Switch Board

Utility--------Line--------PD--------SB

but when we run it with the utility and the utility transf (assuming a greater fault current at the utility) it comes up with cat #4 PPE

Utility--------transf------line------PD--------SB

Is this because the transf contribute with the fault and what is the best way to do it?

I hope somebody can help me!

Thanks...

 

[kingpb]

Technically yes, but not without more effort, time, expense, and expertise then you are willing to put into it.

There are rules of thumb assumptions that you can make for the values needed. However, the utility has the information you need, they just need to be persuaded to give it up.

For Q: #2, you have to know where the fault location is that they gave you. If the utility owns the transformer, then the fault current they gave you may be on the LV side. In which, you do not need the transformer info in the one-line. if the SC value provided is on the high voltage side of the transformer, then you need to include it in the one-line and you will need the pertinent xfmr data.

I doubt you will get a PPE of 1 for a switchboard, the 4 sounds more reasonable. Remember, fault current goes up as voltage goes down. But fault current is only one part of the equation, it is also time dependent.

 

[bob]

Fault Current Caculations

Fault Current Caculations

Chelin
Check this discussion. It may help.
http://www.eng-tips.com/viewthread.cfm?qid=160384&page=1

 

[chelin]

Kingpb and Bob your replies have been very helpful...

For the utility transf I'll try to get the more information I can, but the utilities do not cooperate much with it. I just don't want to make many assumptions in something so important.

Thank you!