Soil Resistivity(Grounding)
Soil Resistivity(Grounding)
(OP)
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
Thanks,
Reza






RE: Soil Resistivity(Grounding)
"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).
RE: Soil Resistivity(Grounding)
RE: Soil Resistivity(Grounding)
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.
RE: Soil Resistivity(Grounding)
……..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.
[sub]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
RE: Soil Resistivity(Grounding)
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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.
RE: Soil Resistivity(Grounding)
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.
RE: Soil Resistivity(Grounding)
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."