Question on Pile Grounding

[ThePunisher]

We have a project that has buildings with pile foundations with a depth of 25 meters from grade level. We intend to use these piles as a grounding electrode and will provide ground clamps and associated grounding conductors embedded in the concrete pile.

We are caught in a dilemna as to how may piles and which piles are we going to use as a grounding electrode since there are numerous piles that will be used in the building. As a default, we take the piles at the building corners.

For the piles between the corners, is there an industry practice as to what spacing of piles we will take to select a pile as a grounding electrode?

 

[davidbeach]

Certainly no closer together than the 25m depth. Less than that and you gain very little benefit.

 

[jghrist]

If you tie the anchor bolts to the rebar, then all of the foundations will be connected together into one electrode through the building steel. The NEC only requires that one per building or structure be bonded into the grounding electrode system.

 

[7anoter4]

I agree with davidbeach. Here are the remarks of Erico CO.
From "Grounding Principles" Erico.Co. Fig 8 page 6:

http://www.erico.com/public/library/fep/LT0359_1.pdf

"Grid with Ground Rods
It may be advantageous to add ground rods to the grid. In doing so, it may be possible to access a low resistivity soil layer. Care must be taken to ensure each ground rod is spaced at least twice the installation depth."
So, the minimum distance has to be 50 m. If your building base is quadrilateral then the corner piles will be the only good grounding electrodes.