Running Multiple Ground Conductors to an Enclosure and Plant Grid

[saghalib]

We have a Metal Enclosure in a chemical plant that is installed outside in open. The Enclosure has some AC to DC power supplies and some DC equipment installed on a metallic back plate. In order to feed the power supply there is a 3 wire 220 VAC power entering the FEC. The green equipment ground conductor with this cable goes to a Grounding Bus Bar (which has insulated footing so it does not touch the enclosure or its back plate). From this bus bar the ground cables go to the AC to DC power supplies.

Apparently there is an another Grounding cable connected to the enclosure body from outside of enclosure. This is basically is a single conductor connecting to all metal poles, enclosures and structures around and also connecting to electrodes driven into EARTH after regular intervals. What I am not sure about is that if this Main ground cable is going back to the power source or not, but some of the inspectors are calling it Plant Grid.

Now what I want to do is to connect the equipment grounding conductor that is entering along with 220 VAC hot and neutral wire into the enclosure , to the enclosure body as well. My understanding is that the Grounding bus bar that has connection to this grounding cable as well as to power supply ground terminals, also needs to have bonding jumper connected to the enclosure body. But the inspectors are saying that you should not connect this equipment ground conductor to the enclosure body since Enclosure body is already grounded using that main grounding system (the plant grid).

Should I still connect to the enclosure and have multiple grounding conductors going back to the source? Only one conductor is enough? if one then which one is better option?

What exactly is this Plant Grid? if these are the ground conductors connecting to metal enclosures and then going back to the source then why do I need to even have 3rd equipment ground cable with main power cable then?

 

[Smart $]

Under the NEC, parts grounded via the "Plant Grid" are what is known as bonded... but bonding does not satisfy the requirement for an equipment grounding connection and the conductor which performs that function being run with the circuit conductors.

The grounding conductor in the supply cable would otherwise be considered an isolated grounding conductor (IGC). However, such a grounding conductor does not alleviate the requirement for a "standard" equipment grounding conductor (EGC).

To be NEC compliant, you are 1) required to bond the existing EGC to the enclosure [defeating the benefit of isolated grounding, if any], or 2) pull another cable with both an EGC and an IGC

 

[saghalib]

Under the NEC, parts grounded via the "Plant Grid" are what is known as bonded... but bonding does not satisfy the requirement for an equipment grounding connection and the conductor which performs that function being run with the circuit conductors.

The grounding conductor in the supply cable would otherwise be considered an isolated grounding conductor (IGC). However, such a grounding conductor does not alleviate the requirement for a "standard" equipment grounding conductor (EGC).

To be NEC compliant, you are 1) required to bond the existing EGC to the enclosure [defeating the benefit of isolated grounding, if any], or 2) pull another cable with both an EGC and an IGC

Thanks for the prompt response, it helped a lot. Just to ensure that I fully understand the concept here:

So the plant grid is actually a ground for all metallic structures and enclosures connected by a single ground conductor going all the way back to the source. In our case it seems that this conductor is not running along the main feed cables and rather it is taking different path back to the source (Mostly underground). But then it is also connected to EARTH by electrodes at different locations.
What I understand is that this ground should provide fault current path back to the source in case enclosure body becomes hot. But I do not understand why they are connecting this ground cable to electrodes at various locations while this conductor is already connected to Ground at the main source?

In addition to above ground, we have a second ground conductor which is isolated or "clean" (not connecting to metal structures) running within the regular power cable. This ground connects to all the devices or loads except to the body of enclosure. So effectively we have separated the equipment ground for the enclosures, raceways, and other exposed metal structures from the clean / isolated equipment ground for the loads /devices/ AC to DC power supplies etc.

Hence at no point except the source these 2 grounds connect to each other.

Also I wanted to know if I have a DC equipment installed inside this enclosure running on 12 VDC, and there is a DC ground terminal on the equipment, to which ground i should connect this terminal? to the Plant Grid Ground which is connected to the body of the enclosure? or to the clean/isolated ground cumming along the main power cable?

Waiting for any comments / confirmation on above understanding.

Regards

Ali

 

[Smart $]

Let me explain this another way...

The plant grid is for grounding of conductive non-electrical equipment, though in industrial locations many also prefer to bond electrical equipment, especially large-ish such as skids and machines... but it cannot be used as a substitute for an electrical equipment grounding conductor.

If you use an isolated equipment grounding conductor, you must still provide a regular equipment grounding conductor for the non-current-carrying metal parts from source to load, inclusive.

If you have a grounded DC power supply, you'd likely want to connect the ground terminal to the "clean" ground, given a choice... but it really depends on the circuit usage.

 

[saghalib]

Let me explain this another way...

The plant grid is for grounding of conductive non-electrical equipment, though in industrial locations many also prefer to bond electrical equipment, especially large-ish such as skids and machines... but it cannot be used as a substitute for an electrical equipment grounding conductor.

If you use an isolated equipment grounding conductor, you must still provide a regular equipment grounding conductor for the non-current-carrying metal parts from source to load, inclusive.

If you have a grounded DC power supply, you'd likely want to connect the ground terminal to the "clean" ground, given a choice... but it really depends on the circuit usage.

Very nicely explained. So it is clear that I need to attach that equipment ground cable coming along main power cable to the enclosure body, no matter if the enclosure is connected to this other plant grid ground.
But if I dont disconnect this plant grid ground cable, will I not create 2 different fault current paths?Possibly causing lower current in the circuit breaker in tern not resulting the breaker to work.

Also I was wondering why can't that plant grid ground cable serve as fault current return path for metallic enclosure ? I mean I understand that you are speaking in terms of NEC standards, but can you please explain the physics behind this? I mean won't we trip the circuit with this plant grid cable since the fault current path, back to the source is available from the enclosure? Only thing is that it is routed differently?

 

[GoldDigger]

Very nicely explained. So it is clear that I need to attach that equipment ground cable coming along main power cable to the enclosure body, no matter if the enclosure is connected to this other plant grid ground.
But if I dont disconnect this plant grid ground cable, will I not create 2 different fault current paths?Possibly causing lower current in the circuit breaker in tern not resulting the breaker to work.

Also I was wondering why can't that plant grid ground cable serve as fault current return path for metallic enclosure ? I mean I understand that you are speaking in terms of NEC standards, but can you please explain the physics behind this? I mean won't we trip the circuit with this plant grid cable since the fault current path, back to the source is available from the enclosure? Only thing is that it is routed differently?

1. The circuit breaker does not respond to fault current in the grounding conductor. It responds to fault current in the ungrounded conductor (ordinary circuit breaker) or additionally to the difference between the current in the grounded conductor and the ungrounded conductor (ground fault protection). In either case multiple ground return paths will not cause a problem.
2. The plant grid can physically serve as a fault current return path. But since its design is not subject to NEC regulation, the NEC will not blindly accept it for that purpose.

 

[Smart $]

In addition to what GD replied, some knowledgeable people here have reported that the inductive impedance for an AC fault increases with the distance between fault current conductors. As I have already surmised from your posts, you are aware that low-impedance is key to the CB operating quickly.