Helical Anchor Grounding

[amendez]

I have an application for a light rail platform where all metallic objects within 15 feet of the centerlline of the nearest rail shall be grounded. Due to the constructability of the platfor, the platform is composed of a precast concrete platform with poured inplace concrete pile caps. The pile caps are then anchored using galvanized steel 30 foot helical anchors.

We are driving ground rods along the length of the platform with #2/0 GEC which will be used so that benches, light poles, columns, etc are grounded to it with a #6 GEC. My concern is, will we create any type of potential difference if the helical anchors are not connected to the ground grid created by the #2/0 and the ground rods? More details can be given upon request.

 

[charlie b]

I have an application for a light rail platform where all metallic objects within 15 feet of the centerlline of the nearest rail shall be grounded.

Are you sure that the requirement is talking about connecting these objects to planet Earth ("grounding"), rather than providing a low impedance path for fault current to return to the source ("bonding")? I can't see any purpose that a ground rod could serve.

 

[barbeer]

I would think that a helical anchor, especially a 25' one would be grounded by default!?. Are they not screwed into dirt? If you are concerned about difference in potential as Charlie says, bond them together.

 

[amendez]

The reference I spoke of is based on a design guideline for Overhead Contact System light rail transit:

"The ground grid shall be designed per IEEE 80 to limit touch potentials and
step potentials to safe values in the event of a fault in the vehicle electrical
systems, or broken or dropped OCS conductor.
The resistance to remote earth shall not exceed 5 ohms."

My only concern was whether the helical anchors would in effect create a potential difference between them and the grounding system created from the ground rods IF the helical anchors were not bonded together. In effect, I agree that the helical anchors, being driven into earth between 25-30 feet, act as a grounding electrode, but they are currently not bonded together. The main question is: is it a violation of the code if they are not bonded? An additional note, the helical anchors are not exposed to the public.