Number of Grounding Electrodes

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travish

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Central North Carolina
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Electrician
In an industrial plant with CNC machining centers, is it OK to have grounding electrodes driven at each machine? I know they cannot be connected to the grounded conductor beyond the service disconnect (only 3 phases and ground pulled to each machine), and that it is fine to have more than one at the service to get the resistance below 25 ohms. but is there a limit where or how many can be? I am not sure there there is an advantage to having them at the machine, a service guy suggested it at a plant here in town.

Thanks
Travis
 
ceb58

ceb58

Senior Member
Location
Raeford, NC
travish said:
In an industrial plant with CNC machining centers, is it OK to have grounding electrodes driven at each machine? I know they cannot be connected to the grounded conductor beyond the service disconnect (only 3 phases and ground pulled to each machine), and that it is fine to have more than one at the service to get the resistance below 25 ohms. but is there a limit where or how many can be? I am not sure there there is an advantage to having them at the machine, a service guy suggested it at a plant here in town.

Thanks
Travis
Click to expand...

Service guy needs to stick to service.
 
acrwc10

acrwc10

Master Code Professional
Location
CA
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Building inspector
travish said:
In an industrial plant with CNC machining centers, is it OK to have grounding electrodes driven at each machine? I know they cannot be connected to the grounded conductor beyond the service disconnect (only 3 phases and ground pulled to each machine), and that it is fine to have more than one at the service to get the resistance below 25 ohms. but is there a limit where or how many can be? I am not sure there there is an advantage to having them at the machine, a service guy suggested it at a plant here in town.

Thanks
Travis
Click to expand...

As long as you connect them to the equipment ground only, you can put in as many as you want.
 
infinity

infinity

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New Jersey
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This is a common requirement by some equipment manufacturers as a signal reference ground, even if it accomplishes nothing. Also you cannot use this auxiliary ground for grounding the equipment. Here's an older graphic from before the name changed:

1004224701_2.gif
 
renosteinke

renosteinke

Senior Member
Location
NE Arkansas
I do not agree with the artwork as drawn.

I'm willing to agree that you need not run a GEC to tie in with the building grounding network, but there MUST be at least an 'indirect' connection via the EGC (the green wire or conduit that accompanies the circuit that powers the machine.)
 
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petersonra

Senior Member
Location
Northern illinois
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engineer
You would be surprised at how good the connection between the concrete floor and the steel in the machine is. Making that connection lower impedance by drilling a hole in the concrete and pounding in a supplementary rod won't help any, but it won't hurt anything either. It is often the first thing demanded by CNC technicians, and it is usually best to just do it and get past it because they will make you do it if there is a problem claiming that is why the machine is not working correctly.
 
K8MHZ

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
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Electrician
When I worked as a maintenance tech an engineering firm suggested we have a separate rod driven for each CNC or CNC type machine. I spent hours drilling through the concrete floor and driving in ground rods. No one could ever tell me how the rod would be any better than the metal frame bolted to the concrete. So, in between calls I drilled holes and pounded rods.

Whatcha gonna do?
 
acrwc10

acrwc10

Master Code Professional
Location
CA
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Building inspector
It funny that art. 250 is so hard for people to grasp.
I had a neighbor 17 years ago who worked for the phone company as an installer/line man and he would say, "ground is ground the world around". He was clueless about grounding and bonding. In his house he ran a green wire from each receptacle (K&T) through the outside wall and had a bunch of short ground rods driven, one for each receptacle and none tied to the servive ground. USELESS and dangerous.
 
roger

roger

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Fl
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Retired Electrician
renosteinke said:
I do not agree with the artwork as drawn.
Click to expand...
What part do you not agree with?

renosteinke said:
I'm willing to agree that you need not run a GEC to tie in with the building grounding network, but there MUST be at least an 'indirect' connection via the EGC
Click to expand...
There is since both are bonded to the machines conductive parts, but there is nothing in the illustration that says it isn't or can't be.


Roger
 
tom baker

tom baker

First Chief Moderator
Staff member
Location
Bremerton, Washington
Mike Holt has another graphic on this subject based on a study done by the EPRI. Installing a ground rod a CNC machine
Sometime helps
Sometimes makes is worse
sometimes does nothing.
His graphic illustrates that the GR at the machine introduces another path for lightning energy to the CNC.

this type of GR is called an auxiliary ground rod. Not required, code rules are minimal. I call them time and material GRs
 
kwired

kwired

Electron manager
Location
NE Nebraska
renosteinke said:
I do not agree with the artwork as drawn.

I'm willing to agree that you need not run a GEC to tie in with the building grounding network, but there MUST be at least an 'indirect' connection via the EGC (the green wire or conduit that accompanies the circuit that powers the machine.)
Click to expand...
Artwork does not indicate exactly what it is connected to on machine end.

petersonra said:
You would be surprised at how good the connection between the concrete floor and the steel in the machine is. Making that connection lower impedance by drilling a hole in the concrete and pounding in a supplementary rod won't help any, but it won't hurt anything either. It is often the first thing demanded by CNC technicians, and it is usually best to just do it and get past it because they will make you do it if there is a problem claiming that is why the machine is not working correctly.
Click to expand...
And after you install this rod on a machine not working correctly it still doesn't work correctly - they just can't find any other reason at the time so they want to try something simple first. The next step from these technicians will want is to when it still doesn't work is disconnect the equipment grounding conductor - there must be some kind of ground loop and that will isolate it:thumbsdown:
 
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