Ground grid as equipment grounding conductor

[radug]

Hello,

I know that NEC 2011 explicitly forbids using the ground grid as an equipment grounding conductor while in NESC 2007 it is not explicitly covered.

Let's say you have a large power plant where you have buried a mesh type grounding grid so that you have only one earth for both HV, MV and LV (all voltage levels connected to a common ground). So you have MV/LV Dyn transformers with LV neutral connected to the ground grid.

Imagine you have a large LV load quite far from its corresponding supply switchgear. Then you would have to include a big cross section equipment grounding conductor together with the phases and neutral conductors.

If instead of doing that, you locally connect that load to the grounding grid, you are drastically reducing the length of the equipment grounding conductor.

From a technical point of view, which would be the problems associated with using the ground grid as equipment grounding conductor? I think the phase-to-ground loop is the same.

Any report about this topic?


Thanks.

 

[resqcapt19]

The NEC has never permitted a grounding grid to be used in place of an EGC. 300.3(B) requires that the EGC be run in the same raceway or cable as the ungrounded conductors. This rule has been a part of the NEC for many code cycles.

The change in the 2011 code was to prohibit the use of a single conductor as both a grounding electrode conductor and as an EGC. One common application of this before the code change was to use an oversized EGC run with the feeder conductors to a transformer as the EGC for the feeder and as the grounding electrode conductor for the grounded secondary.

If the fault current is high, your proposed fault clearing path will have a high impedance as a result of the supply and return paths being physically seperated. This increase in impedance can be enough so that the OCPD does not quickly clear the fault.

 

[jghrist]

As resqcapt19 said, the problem is high impedance for a ground fault making fault clearing slow. Even if the ground grid path to the LV ground source is shorter, the impedance will probably be higher because of the separation of the conductor from the ground return path. Inductance is proportional to log(D/r') where D is the distance between conductors and r' is the geometric mean radius of the conductor. The ground grid and EGC will be parallel paths for ground return current and the combination will have a lower impedance than either path.

 

[dpc]

There is a lot of history on this. Old IEEE papers by R. Kauffman and others showed that the impedance of grounding conductors outside of the raceway containing the phase conductors was so high as to render the grounding conductor ineffective. This is especially true for a steel conduit. As noted by others, this clearly not allowed by NEC and for good technical reasons.



David Castor

http://www.cvoes.com

 

[waross]

I was aware of some tests done on single conductor cable in conduit a lot of years ago. The tests showed that 10 feet of steel conduit would choke the current to about 100 amps. A further effect of this would be a large voltage drop across the portion of the cable contained in the conduit. A second test was done with the conductor bonded to the conduit at each end. It was found that the current through the conductor was still limited to about 100 amps in the section of the cable contained in the conduit. The balance of the current was conducted through the steel conduit.
The tests were done by an Electrical Contractors Association to illustrate to the member contractors that grounding in accordance with the electrical code was (and is) important and not arbitrary.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[radug]

dpc, thanks for your references to Kaufmann papers, I did a quick search at IEEE and as soon as I am able to access them I will read the papers carefully.

Waross, I had no luck finding Electrical Contractors Association tests. Could you provide some link?

I think that many times we lose perspective about the why of the rules. It is well to know the code, well it is a must have, but some rules would be better learnt if some explanation was included in an annex or handbook.
I am from IEC world rather than IEEE, but the question I made is not explained or addressed in IEC standards.

 

[jghrist]

The NEC handbook does include a short explanation of why all conductors of the same circuit need to be in the same raceway.

 

[waross]

Sorry, that was almost 40 years ago. It was not a national ECA. It was the provincial ECA of British Columbia, Canada. I doubt that the test results will be on line. I can't find any old information on the ECABC website.

http://www.eca.bc.ca/index.ph

Nothing was mentioned at the time but it is possible that the tests were done by one of the National ECAs and reported by the ECABC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[resqcapt19]

I think some of that information was published in the older Soares Grounding books. They were published by the International Association of Electrical Inspectors.

 

[radug]

Hello,

I will look for the info in Soares book. I can get a copy of the 2008 edition.

Also I was able to obtain some papers by Kaufmann.

I see that the main problem is the inductance of the circuit when the ground grid is used as a return for the ground fault. Speaking about inductance, do phase-current cables run in the same cable tray influence the inductance of other circuits? I have not seen any calculations made in this regard. When voltage drops are calculated I have always seen calculations done using the cable manufacturer catalogue inductance value (trefoil or flat).

 

[jghrist]

There will be mutual coupling among circuits, but since the fault current doesn't flow in the other circuits, it doesn't make any difference.