A contractor wants to have a "clean ground" at a location about 100' from the Service. The panel located 100' from the Service is being fed using a metal raceway as the EGC. Can the contractor just tie wrap an insulated conductor to the exterior of the conduit and use this conductor as his "clean ground"? I said No but need a code reference to back me up.

You can always say raceways are not to be used as "support" of other wiring methods but that is a pretty thin beef. Why isn't he just running it in the pipe? Even back in the olden days when IBM liked IG outlets we let them run with the circuit conductors. I suspect mythology has trumped reason with this guy.

George,
The clean ground is still an EGC and must be in the raceway with the other circuit conductors. 300.3(B). The rule cited by Greg also applies. Is he trying to install an illegal "isolated" ground?
Don

250.146(D)Isolated Receptacles.

The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor RUN WITH THE CIRCUIT CONDUCTORS.

Define "with".

That was my thought too Larry.

Use as it is in this location of the code it becomes a subjective word and in my mind it means in the same raceway, cable or trench.

There is a condition for running the grounding conductor separate from the other conductors as defined in 250.130(C) for receptacles and extentions and I would accept a single conductor for that type of installation. For example where it is a NM cable job. But after seeing what you guy/gals have posted I'm convinced that the proper installation would be to have the IG installed in the same wiring method as discribed above.

George this 'clean' ground still must be tired into the systems grounded conductor at some point.

What exactly is going on here?

I agree George. I am just afraid you will get tangled up in the mythology that got this guy's panties in a wad about the "clean ground" in the first place. You may hear the definition of "with" thrown around.
You can certainly toss back that the inside of the raceway is a "cleaner" environment than the outside but that is probably beyond the scope of your job.
Is this ground going to be protecting the case of metal equipment or is it just for his electronics in an insulated box. That could be the deciding factor about whether it requires the mechanical protection the raceway provides.

BTW one of the arguments I have heard against IG is that the "one way" penetrations through metal cabinets and raceways that are not bonded at both ends actually creates a choke that reduces the effectiveness of the IG. (similar to GECs)
IBM decided it was better to bond the hell out of everything. In the 60s we had single point grounding and we had to do a "baseplate ground" <continuity> check, lifting the single point ground and verifying that was the only path to ground.
After all the engineering changes were installed that "single point" was getting up to 20 or 30 additional grounding wires (added to fix problems) we had to remove to clear all paths to ground and sanity finaly won out. They stopped thinking star grounding was important. It was about that time that the IG dissapeared from the physical planning manual. In your typical PC you will have lots of DC to AC ground points once you start hooking up I/O.

The instllation consist of a subpanel fed with 100a conductors in a metal raceway and the contractor want's to have a "clean ground" for electronics. This ground would be connected to the Service panel and then to be used as a reference at the sub panel end. (I think) I agree that it must be tied down at the Service but it can be floating at the sub panel end as long as the raceway is being used as the EGC for the sub panel.
I think there is more to be gained by having this "clean ground" inside the raceway than having it outside the raceway aand subject to stray electrical noise in the plant.

George, how are they going to get this single conductor into and out of the electical enclosures? Did the contractor give a reason for not runing it inside of the conduit?

Beats me. Another reason for putting it in the raceway with the other conductors


[This message has been edited by George Little (edited 02-18-2006).]

If you have a single conductor in a ferromagnetic raceway, then you will get the choke effect similar to that experienced by GECs (and the reason that conduit surrounding GECs must be bonded at _both_ ends.

If the conductor is carrying fault current from the circuit conductors _in the same raceway_, then this 'choke effect' is neutralized; any current flowing in one conductor is balanced by current flowing in the other.

IMHO 'run with the circuit conductors' clearly means _inside_ of the corresponding raceway if the raceway is metallic, for the same reasons that phase conductors must not be run in separate metallic raceways.

Also, if the feeder conductors are run inside of the conduit, and the "quiet ground" run on the outside, then there will be considerable coupling between the "quiet ground" and any currents flowing on the conduit. This would seem to add noise to this quiet ground.

-Jon

The other function of the electronic ground is to shunt high frequency transients back to ground. That will usually be a one way trip from the load end. The "choke" slows down that shot.

George:

On the project I am on we have them both ways. Everything is a ground grid. For an Isolated ground in some instances they are installing a triad or Delta grid with insulated conductors connecting to a 1/4 in bus bar using insulated 4/0 copper and running all the isolated grounds to the bus. In other instances an Insulated 1/0 is run directly to the ground grid. EIther way all systems are connected to the grid (lightening, system, isolated grounds). WHat about ground conductors smaller than #6?

You say this is to be run from the main to the new panel? I thought feeders always needed a ground wire, that the conduit was not sufficient. Am I wrong here?

As far as branch circuits are concerned, it was oce exlained to me that the connections on conduit just might have little sparks, wherever a connection might not be perfect- and that this would create some "noise" that sensitive electronics (such as found in a recording studio) might pick up. If there is a wire run in addition to the conduit, this issue is prevented, as the wire is a much better conductor than the pipe.

True? I don't know. The guy who was so anal about having a ground wire also had some pretty basic errors in his wiring.

Reno the concept of the clean ground is based on the idea that a metalic raceway system may carry unwanted current from incidental things it touches along the way. An insulated wire that runs isolated from the raceway and intervening equipment/enclosures, all the way from the main grounding jumper to the equipment will not see this unwanted current (noise) but still provide a path for fault currrent.