Soil Resistivity(Grounding) 1

[rezaa]

We know generally, soil that has settled become more compact has a greater conductivity. My question is that when soil is freezing do we assume the freezing soil as uniform soil and then its resisitivity as function of tempreature? Do we consider parameteres such as moisture, chemical content and compactness while the soil is freezing? Any study or resources on this topic are welcome.

Thanks,

Reza

 

[jghrist]

According to IEEE Std 80-200 Section 12.4:

"The effect of temperature on soil resistivity is nearly negligible for temperatures above the freezing point. At 0 °C, the water in the soil starts to freeze and the resistivity increases rapidly."

A set of curves is given that shows the relationship of resistivity to % salt, % moisture, and temperature (not to be used for calculation purposes).

 

[rezaa]

In order to model soil resisitivity for temperatures above the freezing point, what parameters do we use? Is the model based on only field test for temperatures above the freezing point?

 

[jghrist]

The model is based on field tests at whatever soil temperature the tests were made at. Field tests taken at probe spacings larger than the frost depth would only be marginally affected by the frozen soil.

I guess you could test the resistivity at small probe spacings and large spacings during freezing weather and determine a multilayer soil model that would have a shallow high resistivity upper layer. This may not result in a more conservative design than measuring the resistivity without any frozen soil.

SES recently published a paper showing that neither very low nor very high soil resistivities represented the worst case for human safety. Correlation between Worst Case Safety Conditions and Soil Resistivity under Power System Fault Conditions, J. Ma and F. P. Dawalibi, Proceedings of the 3rd IASTED International Conference POWER AND ENERGY SYSTEMS, September 3-5, 2003, Marbella, Spain, pp. 695-699. An abstract of the paper is at

http://www.sestech.com/newsletter/index.asp?9

but I don't have access to the full paper.

 

[cuky2000]

_{I am assuming that your question is associated with common of grounding for power systems application not in permafrost region.}

……..We know generally, soil that has settled become more compact has a greater conductivity. My question is that when soil is freezing:

QUESTION 1: Do we assume the freezing soil as uniform soil and then its resistivity as function of temperature? Soil for application in grounding system typically modeled ass uniform soil or more common as two layers or multilayer uniform soil depending on the soil characteristics and accuracy of the calculation.

QUESTION 2: Do we consider parameters such as moisture, chemical content and compactness while the soil is freezing? A good grounding system design should consider the worst-case scenario in the seasonal variation of the soil resistivity. See the comment below.
COMMENT:
The upper layer resistivity varies significantly with the season where low temperature and low moisture increase the soil resistivity. The lower layers within a few feet below grade are les vulnerable to seasonal changes.

The high resistivity of the upper soil layer help somehow to increase the allowable step and touch potential. On the other hand, most of the current circulate in the lower layer do to the lower resistance path.

This two effect combined help to mitigate the seasonal variation of the upper soil layer.[/i]

QUESTION 3: Any study or resources on this topic are welcome. In addition to other suggestions posted above, check also IEEE Std 81 and NETA. Also check the following article and skech below with selected info from different sources.

http://www.dranetz-bmi.com/pdf/groundtesting.pdf

 

[rezaa]

Winter Soil Model Based on Soil Temperature in winter and Resistivity Measurements in summer and fall.
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For shallow depth soil resistivities, I heared that there is a theoretical values based on SES’s research work that shows soil temperature as a function of depth. The theoretical derivation is based on the air temperature, annual amplitude of the surface soil temperature, frozen soil depth, and thermal diffusivity of the soil. This theoretical values can be incorporated into summer and fall measurments for soil model to come up with a reseanable winter soil Model.

 

[rezaa]

Gustafson, R.J. Pursley, R.Albertson, V.D. "Seasonal grounding resistance variations on distribution systems"
Ohio State Univ., Columbus, OH;IEEE Transactions on Power Delivery, Vo. 5, No. 2, April 1990
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You will find the following statement in conclusions of the above paper.

The freezing of the surface soil and temperature varations over the period of the time have a major effect on values for those soil types in which the upper layer of soil is more conductive than lower layeres. Conversely, if the uppor layer of soil has poor conductivity campared to lower layers, the effects of freezing of the surface layer results in smaller seasonal fluctions of the resistance parameters.

 

[jghrist]

Cuky2000,

The graph of seasonal ground rod resistance variation is interesting. The 10' rod varies from about 10 ohms to 30 ohms (300%), but the 3' rod only varies from about 20 ohms to 40 ohms (200%). This doesn't jive with your comment "The upper layer resistivity varies significantly with the season where low temperature and low moisture increase the soil resistivity. The lower layers within a few feet below grade are les vulnerable to seasonal changes."