Substation Grounding: Ground Rods and Water Tables


We were having a discussion at my work today about how deep grounding rods must go for a substation in relation to the presence of ground water. Several of my colleagues were suggesting that the rods must be deep enough to penetrate that water table, and not much further than that. However, I and another coworker were of the mind that a water table will present itself in a soil survey, and thus we should be "ignorant" of the depth of the water table and design a grid that creates suitable potentials according to the provided soil model (and therefore, we may still need 24 foot rods even if the water table is 6 feet below grade).

The question seemed to boil down to trusting the models versus trusting that the groundwater would dissipate a fault. I was just curious of the opinions of others in this regard, would you stop the rods at the water table, or drill them as far as your "ignorant" model proposes?

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Section 250.53(A) of the NEC says, "Where practicable, rod, pipe, and plate electrodes shall be embedded below permanent moisture level."

Alternately, according to 250.53(G), you could dig a trench 30" deep and lay a rod in it..

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I would say ignore the water table. Have you ever seen a reference to the water table when reading IEEE substation grounding standards. Also, either way, keep in mind that water tables change.

Reply to
operator jay

Forget about the water for a ground based substation. Substation grounding can be extensive. I saw one that consisted of over twenty each 20 foot long ground rods in bored holes. Water does enter into the argument for salt water harbors and oil terminal berths. The grounding electrode for a harbor transformer on a floating dock is sometimes no more than a ground wire in the salt water. Grounding electrode bare copper conductors for a oil terminal berths also are laid in the water. I have seen it and been there.

Reply to
Gerald Newton

NEC, thankfully, does not apply to this situation.

Ignore the water table.

Charles Perry P.E.

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Charles Perry

Interestingly I was looking at taking this course when I saw your question.

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Peace dawg

Reply to
Wecan do it

Why aren't the utilities using Ufer grounds? Wouldn't they be much better?


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Reply to
Dean Hoffman

--------- The substation ground grid should be such as to ensure that the whole area of the ground (and equipment external chassis, within the area of the substation, including a "touch" distance outside the perimeter fence, is at the same or within safe limits of the same, potential under all conditions. Whether or not this is "true ground' with respect to some remote point, is not of importance with regard to safety. What is important is the relative potential between somebody within the sub or reach of the perimeter fencing is at a low potential difference between his/her self and anything that he/she may contact.The key for safety is "grid" rather than potential to some remote point. It doesn't matter if you, and anything you may contact, float up and down together. Ideally, one could cover the whole area with a sheet of copper (to be stolen overnight) but a proper grid does the same thing. Lowering the total ground resistance by multiple, long, ground rods is beneficial systemwise but doesn't actually deal with the me- equipment potentials (read safety). There is a lot of information in IEEE PAS that deals with this.

Don Kelly remove the X to answer

Reply to
Don Kelly

"Seems to me" that it would be a good idea to go a little further.

It only takes 100 volte and 100 ma to kill someone. Even if your "ground" island is works, you don't want potential voltage gradients to "charge" things like pipes and wire fences.

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Reply to
John Gilmer

Such "ground islands" are pretty common practice for substations (at least for large ones and should be for all) so I can't claim any credit for the concept and the practice. IEEE PAS would have a number of papers with respect to this- some of which I have read (and I am sorry that I can't pass on a direct reference). The proper practice is to tie fences etc to the grid (which would extend beyond the fence line by something more than the touch distance). A reasonable current limit would be well below 100ma-say 5ma (0.5% probability of a child being unable to let go and well below fibrillation levels) and calculations would be be based on zero body resistance (EPRI EHV book). Alternatively the fence could be well outside the grid - again by more than the distance one could reach. In either case, any above ground (and below ground in the case of pipes) metal must be bonded to the grid and the grid spacing is such that any step or touch potentials are safe -otherwise there wouldn't be much point to the expense.

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Don Kelly

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