Ground grid as equipment grounding conductor
Ground grid as equipment grounding conductor
(OP)
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.
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.






RE: Ground grid as equipment grounding conductor
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.
RE: Ground grid as equipment grounding conductor
RE: Ground grid as equipment grounding conductor
David Castor
www.cvoes.com
RE: Ground grid as equipment grounding conductor
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
RE: Ground grid as equipment grounding conductor
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.
RE: Ground grid as equipment grounding conductor
RE: Ground grid as equipment grounding conductor
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
RE: Ground grid as equipment grounding conductor
RE: Ground grid as equipment grounding conductor
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).
RE: Ground grid as equipment grounding conductor