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GPR in Concrete

GPR in Concrete

GPR in Concrete

Dear experts,

I have not come across sufficient information to assist me in designing grounding system for a switchyard control building using IEEEStd80:2013.
I am seeking your expertise to advise me if my thoughts are incorrect?

1. GPR in concrete is different from GPR in soil.
2. a. What is the formula to calculate grid resistance of horizontal grid conductor (either separate copper ring or rebar) embedded in concrete? Eqns 63, 64 and 65 refer to ground rods encased in concrete, however if I am to use copper or the rebars embedded in concrete horizontally only. (some sections of the rebar might be vertical but not penetrating the soil.
2. b. Failing to find this equation in 2.a.(above) I used equation 57, I substituted the soil resistivity with the concrete resistivity to obtain the grid resistance, then multiplied this with the max grid current to obtain the GPR.

If my view of solving this problem is wrong, I would appreciate your expert views in pointing me the right way.

Thank you.

RE: GPR in Concrete

The IEEE 80 standard presents the concrete influence in 3 different parts.
At first, when you calculate the permissible touch and step potentials. Then you use 27,32,33.
Second, when you calculate Ig [as Rf] and use 63
Third, when you calculate GPR=Ig*Rg then use 63
For Em and Es calculation only soil resistivity is considered.

RE: GPR in Concrete

Dear 7anoter4,

Appreciate sincerely your feedback.

I have covered item 1, item 2 and 3 are proving tricky as they refer to resistance calculation of vertical electrodes embedded in concrete.
For my case, am intending to use the rebars or run parallel copper conductor embedded horizontally in the concrete foundation (no rods penetrating soil) of the substation control building. I hope this clarifies why I am hesitant to use use eqn 63 or 64. I hope this has clarified my query more.

Thank you and appreciate you and other experts feedback.


RE: GPR in Concrete

The control building GPR will be the same as the substation ground grid GPR if the rebar is connected to the grid either deliberately or incidentally. Equation 63 is for a vertical rod and could be used for drilled pier foundations. I would ignore the building foundation in determining the substation grid resistance. Calculating the resistance will be of no use by itself, because you would then have to determine the mutual resistance between the ground grid (with rods) and the building. The GPR is a function of the resistance of the whole grounding electrode system. If the foundation is a slab with a vapor barrier, it would not be an effective grounding electrode anyway. Only the footers, if any, would be in contact with the soil.

You could consider the reinforced concrete foundation as an equipotential surface. Touch and step voltages within the building will be negligible. Touch voltage to the outside of the building will be the same as to any grounded structure.

If you really want to consider the voltage gradients in the foundation, you could model the rebar or embedded copper wire within a volume of concrete in a program such as SES CDEGS. I know of no way to do this by hand calculation.

RE: GPR in Concrete

Dear jghrist,

I appreciate your input here. I did not consider the the requirement to calculate the grid resistance, I now see how much of a pain that can be.

Thank you for the commentary on this.



RE: GPR in Concrete

As jghrist commented, utility scale grounding studies are typically done with software. The IEEE Std 80 equations help to show how the grounding system conceptually works through the equations, but those calculations have assumptions (assumes a superconductors and no voltage drop in the grid, unable to calculate transferred voltages, limit soil resistivity layers). Additionally many of the simple software tool have these same assumptions, so can give artificially good or artificially bad results.

EasyPower Grounding Resource page and E&S Grounding blog have a lot of good information as well as free workshop on grounding studies.

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