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).]
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.
Bob Badger Construction & Maintenance Electrician Massachusetts
Re: Seperate conduit for IG ground#94361 07/30/0507:42 AM07/30/0507:42 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.
Re: Seperate conduit for IG ground#94362 07/30/0512:56 PM07/30/0512:56 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.
Re: Seperate conduit for IG ground#94363 07/30/0501:27 PM07/30/0501:27 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.
Mark Heller "Well - I oughta....." -Jackie Gleason
Re: Seperate conduit for IG ground#94365 07/30/0502:08 PM07/30/0502:08 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.
Mark Heller "Well - I oughta....." -Jackie Gleason
Re: Seperate conduit for IG ground#94366 07/30/0507:35 PM07/30/0507:35 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.
Re: Seperate conduit for IG ground#94367 07/31/0507:02 AM07/31/0507:02 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.