Flat region in GroundMat 3-D graph of Grid Rise and Touch Potential

[Duluth-01]

Hello,

I am a new user of SKM GroundMat software and have seen a strange-looking flat area extending across relatively large regions of the 3-D graph of ground potential rise and touch potential. I have emailed SKM help desk and submitted my model and requested an explanation but their answer was only that the software was working correctly. I would like to know why the flat area exists. Is it because the software converges to a constant value (V=Fault-I X R-earth) in those regions?

Best Regards,
Chuck in Duluth, MN

 

[Ingenieur]

Please post the image

 

[Duluth-01]

Example of plateau in Earth Potential Rise during fault

Example of plateau in Earth Potential Rise during fault

Earth model: 5.9 inches of 3k-Ohm-M surface gravel, 2nd layer: 4 feet of 35 Ohm-M fat clay, 3rd layer to infinite depth: 32 Ohm-M fat clay. 9k-Amp SLG fault current. User Defined Box checked. Split Factor = 1.0, One transmission line & One distribution neutral.
I have seen this plateau on 2 separate models. It seems that low values of earth resistivity promote the plateau (R-earth < 35 Ohm-M). The plateau is so prominent that if the graph is included in a design report to a Client, they will surely ask about it. And, I don't have a good explanation for them. That is what I am seeking here.

 

[drcampbell]

It might be an artifact of the graph itself. If the data exceeds the upper limit of the graph's range, it might display that way.

 

[tkb]

To me it looks like the max of the range.

 

[Ingenieur]

Could be the iterative calculation doesn't have a differential greater than specified
so it assigns the last value or is indeterminate and uses ladt value

as mentioned above could be the graphing function

is there a setting for tha highest allowable value?

 

[Duluth-01]

If I change the earth resistivity to higher values: Layer 1= 100 Ohm-M & Layer 2= 80 Ohm-M, with all other settings the same, the earth potential rise doubles & the scaling appears to follow. There is no flat area. So, I don't think there is a scaling or value-limiting function occurring. The .pdf file I generated is 7kb over the limit (153kb), so I must trim it down somehow to provide an example.

 

[Ingenieur]

If I change the earth resistivity to higher values: Layer 1= 100 Ohm-M & Layer 2= 80 Ohm-M, with all other settings the same, the earth potential rise doubles & the scaling appears to follow. There is no flat area. So, I don't think there is a scaling or value-limiting function occurring. The .pdf file I generated is 7kb over the limit (153kb), so I must trim it down somehow to provide an example.

do the same halving your original values

 

[Duluth-01]

I agree, fiddling with the values will likely reveal a bad combination. I found a way to reduce the file size of the 100 & 80 Ohm-M, layer model. It is attached.