supplemental ground rods.

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novemberaudi

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Wired a press brake at a machine shop, 3 phase 20 amp 460 volt unit. Basically a hydraulic pump. It is a very large unit bolted to the floor. The machine shop had drive a ground rod behind the machine and had me connect a wire to the frame. The machine was used and did not have installation literature.
 
It does no good, but also no great harm.
Just recognize that you must run a properly sized EGC and bond it to the motor whether the supplemental electrode is there or not.

Tapatalk...
 
One example:
If a nearby lightning strike or lost primary neutral caused a substantial voltage gradient across the building footprint, the extra electrode would cause current to flow through the machine to the EDC. In the absence of an EDC it would put the machine at a potentially dangerous voltage relative to other nearby metal.

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GoldDigger said:
One example:
If a nearby lightning strike or lost primary neutral caused a substantial voltage gradient across the building footprint, the extra electrode would cause current to flow through the machine to the EDC. In the absence of an EDC it would put the machine at a potentially dangerous voltage relative to other nearby metal.

Tapatalk...
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just the concrete floor would do the same thing through the frame of the machine that is sitting on the floor.
 
The lateral resistance of a slab of concrete containing bonded rebar can be a lot lower than the lateral resistance of the surface layer of dirt under it, and will reduce potential gradients.
If a rod electrode goes through a hole in the concrete to deeper earth, it could introduce a higher potential difference.

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GoldDigger said:
The lateral resistance of a slab of concrete containing bonded rebar can be a lot lower than the lateral resistance of the surface layer of dirt under it, and will reduce potential gradients.
If a rod electrode goes through a hole in the concrete to deeper earth, it could introduce a higher potential difference.

Tapatalk...
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I do not necessarily disagree that just about anything "could" happen, but in general adding a supplemental rod is not going to make any difference at all. The concrete floor is a pretty good overall conductor specially with the steel in it and it has a lot of contact with the soil so it may well create an effective equipotential area. However, that would still exist even if you drilled a hole through it to install a rod. the rod goes thru this zone where it is at an equal potential, is attached to the machine frame in some way, and the machine sits on the concrete. I just do not see how if done correctly that it creates an issue that would not otherwise exist.

I agree that if the rod is not connected correctly to the incoming egc that there is potential for trouble.
 
GoldDigger said:
The lateral resistance of a slab of concrete containing bonded rebar can be a lot lower than the lateral resistance of the surface layer of dirt under it, and will reduce potential gradients.
If a rod electrode goes through a hole in the concrete to deeper earth, it could introduce a higher potential difference.

Tapatalk...
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You don't see this as trying to find a way it could make a difference for the sake of being able to point one out?

How about the fact the conduits to a macine are often secured to the steel building structure which itself is another path to the earth?
 
Just paraphrasing some of the comments in Mike's video on grounding.
His ideal, which is more constrained than code requirements, is all electrodes bonded together and then connected at a single point to the EGC network.
As you point out, that ideal is hard to achieve in a steel framed building when using the raceway as your EGC.

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