Ground Grid outside or inside building?

[Paulpower]

I work on a project near Arctic circle. Because of high
resistivity of permafrost, some engineers suggest that ground profile be laid inside building below grade. However, the electrodes are still buried outside the building. Grounding resistance mainly depends on the number, size, length of electrodes. So what is the benefit of ground grid inside building?

 

[7anoter4]

Ground resistance depends indeed also upon the vertical electrodes but also on horizontal electrodes-the horizontal
cable. The fault current spread into the ground radial from the grounding vertical rod or grounding horizontal electrode.
According to "Designing for a low resistance Earth Interface" [LEC, Inc] published by EES [UK] see:

http://www.ees-group.co.uk/downloads/LEC-Green- Grounding Manual.pdf

there is a critical cylinder with a radius of 1.1 height of the grounding rod .If we consider under the building
a cylinder of diameter equal to building width and the height equal to h=width/2.2 then we can calculate this
cylinder resistance as grounding resistance of a rod.
Let say, there is a total substation area [included between fences] of 100 *150 ft and a building total area of
40*50 ft and ,in a permafrost condition, earth resistivity of 10^5 ohm.m .Under the building the resistivity may be
100 ohm.m [if no permafrost will be for at least 50 ft depth].
In the substation yard are 20 grounding rod of 20 ft long and a grounding grid of 15*15 ft parallel copper cables.
The outside [in the yard] total resistance will be: 1160 ohm
Calculated as a grounding rod the "under the building grid" will be approximately 850 ohm and the total resistance
[the two resistances parallel] will be 450 ohm.
I have no experience with permafrost but, as LEC company declares, one may improve the grounding rod resistance
by replacing all or part of the soil with highly conductive backfill and it will facilitates the achievement of a low-resistance ground connection. See:

http://lecglobal.com/info/product-sheets/gaf-grounding-augmentation-fill.pdf

 

[jghrist]

Bonding the foundation rebar to the grid (Ufer ground) will help, particularly if there are piers that extend below the permafrost. If there is a steel reinforced concrete floor that is also bonded, then step- and touch-voltages within the building will be minimal.

The only advantage I can see for an inside the building would be if you need a very good equipotential ground below a computer floor to prevent noise on sensitive electronic equipment. A grid buried beneath a reinforced concrete floor would not be an improvement over bonding the concrete floor rebar.

 

[ykee]

Ufer grounding may or may not be helpful. Construction of buildings on permafrost often includes a layer of insulation below the slab if it is poured on grade, if this is the case, Ufer grounding will not be effective. Piles are also a popular method, and if these are used as the electrodes, they have a lot of surface area.
Also, do a search here on grounding in permafrost, there have been a couple of threads on grounding in permafrost.

 

[davidbeach]

What the Ufer ground, and/or a ground grid under the building, will accomplish is to minimize step and touch potentials within the building. The building may achieve a rather high potential relative to "remote earth".

 

[Adamito]

I might be wrong but it looks simple to me. Why not just put those rods deeper so it is below permafrost. I don`t know how deep this permafrost is, but if it is easy to get underneath it, you'll have as much efficiency as anywhere else(I am guessing the conductivity underneath is good) .