Grounding Grid Placement

[Powerman5002]

Is it necessary to place the Grounding Grid below the frost line of the soil to ensure the resistivity of the soil will remain constant and also to prevent heaving?

Thanks in Advance!

 

[gordonl]

I'm in canada, and the code keeps the ground grid between 6" and 24". I beleive if you put it below the frost you may have problems with step-touch potentials. I've dug up a couple of stations installed in this fashion and not seen damage due to frost. Ground rods are installed through the frost, and I haven't personnally seen them pushed out, but perhaps others have. I beleive it is important to get the rods below the frost level for the reason you've stated, but not the grid. I would suggest a review of IEEE 80, "IEEE Guide for Safety in AC Substation Grounding".

 

[dpc]

I would agree with gordonl. The ground rods must be long enough to be below the frost line. In Minnesota, we used 30 foot ground rods. The portion above the frost line is not useful in reducing impedance to earth when the ground is frozen.

The grounding grid, however, should probably be relatively close to the surface to provide adequate step/touch potential reduction. The frozen soil has a very high resistance.