PLC grounding

[joshtrevino]

I am working on a project in which we are upgrading a pipeline metering skid. The scope includes the replacement of several instruments on the skid, addition of supports for piping and cable tray, and the replacement of the control enclosure (new PLC, flow computer, I/O, etc).

We have installed a perimeter grounding grid (ground ring with rods) that we are bonding all supports to as well as the perimeter fencing. The meter skid is isolated from all cathodically protected undergound piping, so there is no need for decoupling on the skid.

My original design was to leave the PLC grounding grid isolated to eliminate any harmonics being transferred to the PLC from the site electrical ground grid. However, now the client is requesting that the PLC ground be tied into the site ground grid simply by bonding to the perimeter ground grid with a #2 bonding jumper.

Does the code address PLC grounding?

Should we look at installing an instrument triad between the PLC ground and the site ground grid? If yes, is this code driven or industry practice?

Any other thoughts/references?

 

[joshtrevino]

I forgot to mention that the PLC cabinet is inside of a small RTU building on the skid site.

 

[petersonra]

I am not sure what your intent is with such an elaborate grounding system.

I think you are better off having a single grounding system rather than trying to isolate some things from ground, whatever you may mean by that, or think you mean.

The reality is that the structure of the building probably has unintentional connections to your ground grid that will connect to the EGC of the PLC anyway.

 

[Smart $]

How many grounding grids are there???

A lot of industry practices are based on myth... to an extent. In many cases it leads to redundant grounding. In some cases it leads to potential for ground voltage gradients during power surges and lightning strikes, which in turn could cause equipment damage (not to mention personnel safety issues). In some cases it leads to totally inappropriate and non-compliant grounding.

PLC grounding should be that of the electrical supply for the structure... i.e. the equipment grounding system, same as the cabinet and all other non-current carrying metal. The structure equipment grounding system should be connected to the grounding electrode system for the structure at one point, and one point only.

 

[joshtrevino]

@Smart $; @petersonra
So there is no issue (operational or code) that you see in bonding the PLC ground to the site ground grid?

 

[petersonra]

@Smart $; @petersonra
So there is no issue (operational or code) that you see in bonding the PLC ground to the site ground grid?

The code clearly requires it. I do not see how you can get around that requirement.

 

[Smart $]

@Smart $; @petersonra
So there is no issue (operational or code) that you see in bonding the PLC ground to the site ground grid?

The code clearly requires it. I do not see how you can get around that requirement.

The first statement is too vague to say.

Assuming your skid is supplied by site power, it should contain an EGC. The skid should have a structure (skid) disconnecting means for that power supply on the exterior or inside nearest the point of entrance. The grounding electrode system for the structure should be bonded to both the supply and load EGCs (but not any grounded conductor) at the disconnecting means. This in itself will bond it to the site grounding electrode system. No additional bonding is required.