Separate source

[Razzap]

Our manufacturing plant often uses 480 V 3 Ph transformers to feed new machines. Since this is a separate source, I need to run a grounding electrode conductor to a ground rod. Problem is, the floor is 18 inches of concrete, on top of 24 inches of gravel. Under that is dry soil. A ground rod or two will have very high resistance, in any case. The columns are concrete, the underground utilities are non-metalic, and all other pipes are above ground and fastened to the nearest concrete. No building steel is available anywhere. What will a high-resistance ground rod accomplish? Using the floor rebar is out of the question. These in-house machines get moved on a regular basis. The machines we build for customers will leave when they are completed. Cutting up the floor to access the rebar is not allowed, and even if it were, it would look like Swiss cheese in a few months due to machines moving in and out. Any suggestions?

 

[iwire]

You might have to run them back to the buildings grounding electrode system. But you could take advantage of this.

2011 NEC

250.30(A)(6) Grounding Electrode Conductor, Multiple Separately
Derived Systems. A common grounding electrode
conductor for multiple separately derived systems shall be
permitted. If installed, the common grounding electrode
conductor shall be used to connect the grounded conductor
of the separately derived systems to the grounding electrode
as specified in 250.30(A)(4). A grounding electrode
conductor tap shall then be installed from each separately
derived system to the common grounding electrode conductor.
Each tap conductor shall connect the grounded conductor
of the separately derived system to the common
grounding electrode conductor. This connection shall be
made at the same point on the separately derived system
where the system bonding jumper is connected.

We have had to do this when updating old mill buildings constructed of brick and lumber.

 

[tom baker]

. What will a high-resistance ground rod accomplish?

The grounding electrode system for a SDS is to stablize the voltage and protect against lightning and line surges.
Since the transformer is inside I wouldn't be concerned with the soil resistance. The same rules apply regardless of the transformer location. We are not clearing a fault with the ground rod so the soil resistance is not a concern.
The rules for grounding a SDS start with the nearest available metal water pipe, structural metal, and then you go to rebar and last a ground rod. But often its easier to go hit the building grounding sytem.

 

[petersonra]

I am not sure you can just add a nearby ground rod and be in compliance.

I thought it had to be building steel or water pipe, or you had to go back to the source GE.

 

[Smart $]

I am not sure you can just add a nearby ground rod and be in compliance.

I thought it had to be building steel or water pipe, or you had to go back to the source GE.

I am more infinitely more confident that is so :roll:

IMO, OP'er is making it sound like the trannies are getting moved around. Whatever the case may be, I'd just run a GEC with the feeders (the only question is whether a separate EGC is required under 250.121).