Grounding Triad

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DIG_84

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Houston, TX. USA
A client is requesting an Isolated Ground Triad on a project we are working on. I have been told that this is mainly for testing and reduction of "noise" to the PLC.

I am the designer on the project so I have submitted a drawing with the 3 driven rods with a test well at each rod, one going back to the PLC and one going to the main ground loop.

My question is: Is only one test well okay, and does the tie-in to the main loop need to be removed?

I have drawn it this way, because it's how I have seen it every time, in the 6 years I have been doing this. Only thing is, this is the first time I am with a company that doesn't have any other I&E Designers as they are just starting a design team here, and the engineer keeps pushing back and wants me to remove the other 2 test wells, and the tie-in to the main loop. I have no other information to back up my reasons and would like some insight as to what is necessary so that i can talk to him about it, and we can show it correctly.

any help is much appreciated,

thanks,
 
PLC inputs are very immune to "noise". Use of STP and bonding the shield at one end is a best practice. AB has a document on grounding and shielding for PLCS I don't have the link but it would be an excellent place to start. I have PLCs mounted inside a 480 motor starter no issues at all.
 
DIG_84 said:
That is what I am struggling with. The engineer's point is to remove the tie-in to the whole ground grid or it will not be "isolated". :?
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Under the NEC, you cannot have isolated grounding electrodes... i.e. isolated from other grounding electrodes serving the same building or structure.

250.50 Grounding Electrode System. All grounding electrodes
as described in 250.52(A)(1) through (A)(7) that are
present at each building or structure served shall be bonded
together to form the grounding electrode system. Where
none of these grounding electrodes exist, one or more of
the grounding electrodes specified in 250.52(A)(4) through
(A)(8) shall be installed and used.

Exception: Concrete-encased electrodes of existing buildings
or structures shall not be required to be part of the
grounding electrode system where the steel reinforcing bars
or rods are not accessible for use without disturbing the
concrete.
Click to expand...
 
DIG_84 said:
So it should be tied to the main grid/loop then. Thank you
Click to expand...

Correct but there may be more to it.

What exactly is this isolated triad intended to be connected to on the other end?

Is it the customers intention to use this triad in place of a proper EGC for line voltage equipment?
 
Re:

Re:

We are adding a new/upgraded PLC and SIS to an existing compressor station and the client only wants the new equipment connected to this new triad that I had originally planned to install with a connection to the main grid until the engineer on the project said to remove my ground that was running to the main ground grid
 
DIG_84 said:
We are adding a new/upgraded PLC and SIS to an existing compressor station and the client only wants the new equipment connected to this new triad that I had originally planned to install with a connection to the main grid until the engineer on the project said to remove my ground that was running to the main ground grid
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Though most providers aim to please, what the client wants isn't always what the client gets.

Is this an electrical engineer? Is he from the USA? Is he aware that NEC requirements (assuming the NEC is adopted into law wherever this project is located) supersede the authority granted him by the state and his client?
 
RE:

RE:

yes this is an electrical engineer from the USA. I am sure he is aware of that, but don't think he is aware of that being in the code book. I plan to discuss it with him and hopefully he will not fight me on it. Some guys I have worked with tend to have big egos.

thanks for all of the help.
 
FYI, I just installed a triad ground on an oilfield well pad this week. The triad was connected to the rest of the grounding on the pad and we had a test well in only one corner so the isolated ground could be removed and resistance tested on the grid. The isolated ground was for a PLC. For what is worth that'd how we were told to do it.

Sent from my SAMSUNG-SGH-I257 using Tapatalk
 
Bad juju

Bad juju

Such bad juju...why oh why do some engineers know nothing of the code? Even the IEEE references the code and states "isolated" grounds which are not bonded are a bad idea.

It just doesn't make sense.
 
The isolated ground I installed was bonded to the rest of the grounding grid (in the test well). I wasn't very clear on that.

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hbeery10 said:
The isolated ground I installed was bonded to the rest of the grounding grid (in the test well). I wasn't very clear on that.
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But ther is no such thing as a compliant isolated grounding electrode. Under Code, you can only have an an isolated ground circuit, but it still connects to the grounding electrode system though the EGC bus at the service disconnecting means. You cannot make an isolated jaunt from a feeder or branch circuit to any system electrode. The only electrode you can do that to is an auxiliary electrode... but the GEC of that auxiliary electrode must connect to the EGC of the power circuit.
 
I have struggled with this topic in my line of work also. I have been told by my peers that the NEC requires the instrument triad to be bonded to the main plant ground grid per 250.50. My confusion is with section 250.54, it says the aux grounding electrode doesn't need to comply with 250.50. What am I missing in the code to require the connection?

Thanks
 
MichEngin08 said:
I have struggled with this topic in my line of work also. I have been told by my peers that the NEC requires the instrument triad to be bonded to the main plant ground grid per 250.50. My confusion is with section 250.54, it says the aux grounding electrode doesn't need to comply with 250.50. What am I missing in the code to require the connection?

Thanks
Click to expand...

First off let's work this the other way.

If you did run a truly isolated ground to a remote electrode and the associated branch circuit faulted to that IG what would be the current path that would cause the breaker to trip?
 
OK, if the triad was really isolated there would be no path back to the source to enable an OCPD to trip. I'm just not sure how to address the 250.54 auxiliary electrode. Why would the code allow someone not to tie it back into the rest of the ground system? :?
 
MichEngin08 said:
OK, if the triad was really isolated there would be no path back to the source to enable an OCPD to trip. I'm just not sure how to address the 250.54 auxiliary electrode. Why would the code allow someone not to tie it back into the rest of the ground system? :?
Click to expand...
Code does not allow it. An auxiliary electrode is tied to the Grounding Electrode System through the Equipment Grounding Conductor.
 
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