Ground Grid Design 1

[stretched2thin]

If any of you do ground grid design, I'm just curious how you treat the top layer of soil during winter conditions.

I can't find any definitive information on how to treat the soil and the extent to which the soil freezing affects resistivity.

All indications are that the resistivity will go up by about 300% (at a minimum) and the freezing depth will depend on geographical location.

Can anyone offer any guidance on this?

 

[cuky2000]

During winter-frozen condition, the allowable step and touch potential increase with the increase on resistivity from both the topsoil layer and surfacing layer (typ. crushed rock).

On the other hand, the fusing characteristics of the grounding conductors favor to withstand better SC during frozen conditions since both, conductor resistance is reduced helping to generate less heat by Joule effect and the lower ambient temperature help to the heat transfer of the conductor.

Therefore, the worst-case scenario does not happen during the coldest season.

 

[stretched2thin]

I'm currently only concerned about safety considerations. I'm wondering how people typically account for this in their designs.

 

[jghrist]

Typically, the change in resistivity of the frozen soil above the frost line is not considered in the design. I would make sure that there are enough ground rods to carry the fault current if the main grid is installed above the frost line. As cuky2000 indicated, if the resistivity of the surface layer increases, the allowable touch potential increases because of increased resistance of the contact between feet and soil. I don't think the lower ambient temperature would have a significant effect on the conductor resistance because the conductor resistance is usually negligible compared to soil resistance and is usually not considered in the design.

I doubt that there would be a real big effect. The grid is normally installed about 1.5 feet below grade. The frost line in northern USA is 3-4 feet, so there is only 1.5-2.5 feet of frozen soil between the grid and the unfrozen soil.

You could measure the soil resistivity during a hard frost and use a soil model with an extra layer representing the frozen soil. SES grounding software allows you to specify the depth of a layer when calculating layer resisistivities from test data. You could then analyze a grid both with and without frozen soil to see what difference there is in touch potentials. If you do, please publish your results.

 

[stretched2thin]

Thanks to all of you for your input.