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Posted By: dmattox Seperate conduit for IG ground - 07/30/05 02:09 AM
Working on a small project where we have a subpanel C2 thats fed from C1 thats fed from the switchgear. Data guys added some IG outlets so the EE changed the plans to add an IG run from C2 back to the switchgear. However he did this by requiring a 3/4" EMT run with a #6 ground seperate from the subpanel feeds. To me this seems to be a clear violation of 300.3(B). Thoughts?

[This message has been edited by dmattox (edited 07-29-2005).]
Posted By: iwire Re: Seperate conduit for IG ground - 07/30/05 10:07 AM
It is a clear violation of 300.3(B).

It is also a very poor design, do not install it like that.

Others with more theory knowledge can explain it better, but what I understand is that a EGC run away from the circuit conductors can have so much impedance during a short circuit that the OCP may not open quickly or at all.
Posted By: winnie Re: Seperate conduit for IG ground - 07/30/05 11:42 AM
If the circuit conductors are run in bonded EMT, and the 'quiet ground' conductor is run in EMT, then you have several problems.

1) Because the ground conductor is run separately from the supply conductors, you get 'loop area'. When you run current through a wire, you _always_ produce a magnetic field. The 'strength' of this magnetic field (flux density) is set by the current. But the total quantity of magnetic flux is the product of the flux density and the _area_. The bigger the area, the more total magnetic flux, and the more energy drawn from the circuit to create the magnetic flux. This is _inductance_, and acts to limit current flow. The larger the area, the greater the inductance.

2) Because you've run the conductors in a closed loop of EMT, you get a problem closely related to the issues of running different phases in separate pipes.

What you've done is create a transformer! You have two circuits (closed metallic paths capable of carrying current), closely magnetically coupled (any magnetic field created by one circuit must pass through the other circuit). When current flows in this ground conductor, you will get a corresponding current flowing in the opposite direction through the EMT. The steel of the EMT will also act to increase the inductance of your loop.

3) Finally, if there are any currents flowing in the EMT (the problem that an isolated ground is supposed to avoid), these currents will be transformer coupled to the ground conductor. Thus running this 'quiet ground' in a separate EMT will potentially inject exactly the noise that it is supposed to eliminate.

Posted By: George Little Re: Seperate conduit for IG ground - 07/30/05 04:56 PM
What Winnie is saying is only true when there is fault current involving the grounding conductor. Normally you wouldn't have any current flowing in the grounding conductor hence no magnetic field. It is important to have the grounded and the grounding conductor as well as the ungrounded conductors in the close proximity ie: in this case the same conduit to maximize conductivity and quick action of the overcurrent device. As someone has pointed out 300.3(B) needs to apply.
Posted By: dmattox Re: Seperate conduit for IG ground - 07/30/05 05:27 PM
Well, got the reply from the EE on my RFI suggesting their might be a code conflict. He said its allowed to run the IG separate from the other conductors. =\

So now my options are to do it the way he wants which is a code violation. Or run the ground wire back through the sub panel feeds.

Unfortunately the feeds for C1 are 2 2" conduits with 4 4/0s. Adding a #6 to each of these has an area of 368 mm^2, while 2" is allowed to only have 366 mm^2 (40%).

So my options are, do it the way shown on the plans. Add the ground to the parallel runs and slightly exceed code. Or eat the cost on replacing our 200' 2" runs with 2 1/2".

I think I'm going to have to do what the engineer says, I've done my job and pointed out what I think is not correct and he is standing by it.
Posted By: e57 Re: Seperate conduit for IG ground - 07/30/05 05:50 PM
If I read this right, if allowed, you can run the #6, to an isolated ground bar in each panel, and back to the main. Or individualy with the feeders back to the main. But not seperately.

250.146(D) Isolated Receptacles.
Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors. This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.20, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.
Posted By: e57 Re: Seperate conduit for IG ground - 07/30/05 06:08 PM
It's the EE that is wrong here, and following his original spec's is the conflict, if you can call it a change, charge accordingly, or tell the Data Guys to go for a long walk. (Hike)

However, for purposes of grounding racks, equipment and such, providing a seperately run for a ground bar is not un-common.

Then hand them this...

250.6 Objectionable Current over Grounding Conductors.
(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, and conductive non–current-carrying materials and equipment shall be installed and arranged in a manner that will prevent objectionable current over the grounding conductors or grounding paths.
(B) Alterations to Stop Objectionable Current. If the use of multiple grounding connections results in objectionable current, one or more of the following alterations shall be permitted to be made, provided that the requirements of 250.4(A)(5) or 250.4(B)(4) are met:
(1) Discontinue one or more but not all of such grounding connections.
(2) Change the locations of the grounding connections.
(3) Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections.
(4) Take other suitable remedial and approved action.
An increase in the use of electronic controls and computer equipment, which are sensitive to stray currents, has caused installation designers to look for ways to isolate electronic equipment from the effects of such stray circulating currents. Circulating currents on equipment grounding conductors, metal raceways, and building steel develop potential differences between ground and the neutral of electronic equipment.
A solution often recommended by inexperienced individuals is to isolate the electronic equipment from all other power equipment by disconnecting it from the power equipment ground. In this ill-conceived corrective action, the equipment grounding means is removed or nonmetallic spacers are installed in the metallic raceway system. The electronic equipment is then grounded to an earth ground isolated from the common power system ground. Isolating equipment in this manner creates a potential difference that is a shock hazard. The error is compounded because such isolation does not establish a low-impedance ground-fault return path to the power source, which is necessary to actuate the overcurrent protection device. Section 250.6(B) is not intended to allow disconnection of all power grounding connections to the electronic equipment. See also the commentary following 250.6(D).

(C) Temporary Currents Not Classified as Objectionable Currents. Temporary currents resulting from accidental conditions, such as ground-fault currents, that occur only while the grounding conductors are performing their intended protective functions shall not be classified as objectionable current for the purposes specified in 250.6(A) and (B).
(D) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment from being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section.
Section 250.6(D) indicates that currents that result in noise or data errors in electronic equipment are not considered to be the objectionable currents referred to in 250.6, which limits the alterations permitted by 250.6(C). See 250.96(B), which provides methods to minimize noise and data errors.
(E) Isolation of Objectionable Direct-Current Ground Currents. Where isolation of objectionable dc ground currents from cathodic protection systems is required, a listed ac coupling/dc isolating device shall be permitted in the equipment grounding path to provide an effective return path for ac ground-fault current while blocking dc current.
The dc ground current on grounding conductors as a result of a cathodic protection system may be considered objectionable. Because of the required grounding and bonding connections associated with metal piping systems, it is inevitable that where cathodic protection for the piping system is provided, dc current will be present on grounding and bonding conductors.
Section 250.6(E) allows the use of a listed ac coupling/dc isolating device. This device prevents the dc current on grounding and bonding conductors and allows the ground-fault return path to function properly. As part of the product testing, these devices are evaluated for proper performance under ground-fault conditions.
Posted By: renosteinke Re: Seperate conduit for IG ground - 07/30/05 11:35 PM
Just for ANOTHER source to cite, I am looking at IBM's "General Information Manual IM-PP," figure 5-1.
This section deals with power requirements of data processing equipment, and defines the "isolated ground" needs. The figure cited clearly shows the insulated ground wire being run in the same conduit as the power wires.
Posted By: iwire Re: Seperate conduit for IG ground - 07/31/05 11:02 AM

I don't think the EE has a leg to stand on if you show him 250.146(D) as e57 pointed out.

Posted By: luckyshadow Re: Seperate conduit for IG ground - 07/31/05 11:28 AM
Why would you do something you know is wrong! You are the one who will end up looking foolish. Isn't this what we teach our children? If you know it's wrong DON'T DO IT. Stand your ground and make the EE send out a correction which will result in a change order.
Posted By: walrus Re: Seperate conduit for IG ground - 07/31/05 11:43 AM
If the EE is a PE(professional Engineer) can't he sign off on the change and supercede code which would get your butt off the chopping block if something bad happens??
Posted By: iwire Re: Seperate conduit for IG ground - 07/31/05 12:37 PM
If the EE is a PE(professional Engineer) can't he sign off on the change and supercede code

No, an engineer can not change the adopted code.

What you may be thinking of is this.

310.15 Ampacities for Conductors Rated 0–2000 Volts.
(A) General.

(1) Tables or Engineering Supervision.

Ampacities for conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.15(B) and (C).

Under engineering supervision ampacities may be different than the NEC tables.
Posted By: walrus Re: Seperate conduit for IG ground - 07/31/05 02:36 PM
I thought an Engineer could sign off a change in an industrial or commercial setting??
Posted By: dmattox Re: Seperate conduit for IG ground - 07/31/05 03:14 PM
I understand what you are saying luckyshadow, but I have already appealed it to the engineer and he has said do it his way. At the end of the job I will have to sign the plans stating I did it per the plans. So I either need to keep confronting the EE, or lie about what I did on the job.

I am thinking about doing the 3/4" run + pulling a #6 ground in each 2". I'll be pushing the wire fill by 2 mm^2, but at least I'll know if there is a ground fault the breaker will trip. And I don't think its wrong for me to exceed what the plans call out.

My other option is to walk the inspector right by and have him issue a correction [Linked Image]
Posted By: renosteinke Re: Seperate conduit for IG ground - 07/31/05 03:35 PM
Engineers have assumed an armload of "powers" over the years......but the last I checked, codes are laws, and the only 'party' that can 'supercede' a law is a jury.

Not that anything will slow down an engineer in his assumptions that he, and he alone, is all-knowing and all-wise. I once put my job on the line, when an engineer insisted it was OK to stand on the bumper of a crane, to provide additional counter-weight!
Posted By: winnie Re: Seperate conduit for IG ground - 07/31/05 06:00 PM
Quoth George Little:

What Winnie is saying is only true when there is fault current involving the grounding conductor. Normally you wouldn't have any current flowing in the grounding conductor hence no magnetic field.

For my points 1 and 2 I agree with George above. The issues that I was describing would only occur when a fault current is flowing, and would act to change the impedance of the fault current path, and would also act to dissipate power in the conduit system. I should have stated this more clearly.

But for point 3, I am specifically pointing out that there is _always_ some noise current flowing in the conduit system, even without an electrical fault. This may be caused by things like ground coupled TVSS devices, ground referenced signaling devices, magnetic coupling to the conduit system, etc. The whole reason for an 'isolated' ground is to provide a _single_ path right back to the building ground reference, with no loops. By running the 'isolated' ground via a different path from the circuit conductors, you permit currents in the conduit system to transformer couple and induce voltages in the isolated ground, IMHO eliminating the benefit of the isolated ground in the first place.

I should add that this is conjecture on my part. I do not know the magnitude of the noise voltages on conduit in industrial settings, and I don't know how much current is actually flowing, so I don't know the magnitude of the transformer induced voltages that I'm describing. But running the isolated ground in a way that would pick up electrical noise seems as 'wrong' to me as spending extra money for an IG receptacle in a home wired with NM-B.

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