IEEE Standards for Connecting Ground Grids

According to NEC (NFPA-70, and NFPA-780) you SHOULD connect ALL the grounding grids in order to avoid induced dangerous potentials in separated grids.

The connection can be made between grounding rods from each grid.

The best IEEE reference for this is IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems. It is also known as the "Green Book". If you are studying power engineering, I strongly recommend you get very familiar with all of the IEEE Color Books. They are a great resource for beginners and experienced professionals.

The specific question you ask may not be answered directly. When two ground grids are connected together, you essentially have one larger grid. You must then review the design of the entire grid system as a single unit. Some areas which must be analyzed:

• Do the connections between the two grids have sufficient conductivity to allow fault current to flow to all grounding electrodes? Connecting the grids at one or two points may actually reduce the effectiveness of both grids.
• Does one grid have a higher earth resistance than the other? If so, fault current will tend to flow to the electrodes with the lower earth resistance.
• Are the grids of the same design (similar type & spacing of electrodes & conductors)?. Dissimilar materials may lead to galvanic corrosion of the grid, while unequal spacing may create localized differentials in earth resistance.

These are a few of the questions to resolve. If I were to analyze this, I would do a basic design for a single, large grid, then compare my design to the proposed combination of 2 existing grids. I would then perform in-depth analysis of any parts of the proposed grid combination which were different from my single-grid design, to ensure the proposed grid provided an adequate alternative.

Thank for the answers you really helped me.

Hi...I have some inquiries that would fall under this discussion.

I understand the part of calculation what you describe and thanks very much for that. My concern would be after the calculation, if everything is ok, how do you perform the task? In other words, when it comes to connect a bare copper between two grids? It is impractical to de-energize the whole substation during this task, while performing the task when the substation is energized..we have to consdier the we are working with a live system..any fault that happen during that time would go through the workers and a serious safety concern..in addition..as during the work we have to dig..step and touch potential could be violated...

i just dont know what would be the mitigation or how we can proceed..of-course my back ground is not utility scale grounding but that's an area i am looking into right now...any advise would be highly appreciated..

Thanks

Arif

IEEE_Std_142-1991_Grounding_of_Industrial_and_Commercial_Power_Systems

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