Thermal Resistivity of Soil

[VTer]

The geotech reports typically include a dry out curve and tables with multiple values of thermal resistivity for a soil sample based on the percentage of water content. My understanding is that the 0% water content values are oven dry soil and not practical. Is there an industry standard for selecting the thermal resistivity value for the ampacity calculations.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[7anoter4]

See: Improving the Under-Ground Cables Ampacity by using Artificial Backfill Materials
This is not a standard but it is an extract from a publication in
Proceedings of the 14th International Middle East Power Systems Conference (MEPCON’10), Cairo University, Egypt, December 19-21, 2010, Paper ID 110.

 

[VTer]

Thanks 7anoter4. Interesting and helpful document but I could not find any discussion on the recommended soil thermal resistivity values based on the soil dry-out curves.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[davidbeach]

Ground grid design needs to work under the driest practical conditions, but ground protection has to work across the whole spectrum of values. If the difference between wet and dry is significant ground overcurrent will struggle while ground distance will have less difficulty dealing with the variations.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dpc]

If this is for a duct bank with cables in fiber or plastic ducts and possibly concrete encasement, you might want to do a sensitivity analysis to see how much difference the native soil rho actually makes. There are multiple factors in the calculation. Have you asked the geotechs for a suggestion on the expected range of soil moisture? You might also have to consider frozen soil.

 

[VTer]

dpc,

Yes this is for ampacity calculations but for direct buried cables. I can run a sensitivity analysis but was just curious if anyone is familiar with any applicable standards or industry practice for use of soil thermal resistivity values for various moisture content.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[dpc]

You might discuss with Okonite tech support. They have been helpful in the past when determining ampacity deratings.

 

[jghrist]

For direct buried cables, you should consider the effect of soil drying from the cable heat.

 

[VTer]

Thank you all for your responses. I also reached out to the cable ampacity software company and it appears that there is no standards or guides on this at this time. We will run a few iterations and see the impact. It is interesting that there seems to be more of a push from the industry to do these types of calculations but there is still so much left in the open for engineering judgments and assumptions, especially around the areas of soil conditions which are not the expertise of a typical electrical engineer.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[7anoter4]

I agree with jghrist. And in order to appreciate the earth drying influence see IEC 60287-1-1 chapter 1.4.2 Buried cables where partial drying-out of the soil occurs
However, this method shall be applied to a single isolated cable or circuit only laid at conventional depth. A note inserted here says that the choice of suitable soil parameter is under consideration and in the meantime values may be agreed between manufacturer and purchaser.
I was “manufacturer” once, but still I don’t know the answer.
See also:

https://www.metergroup.com/en/meter...esistivity-real-rho-values-professional-power