Ground resistance of horizontal conductor encased in concrete 3

[Kani77]

Hi!,

I am looking for calculation formula for ground resistance of horizontal conductor encased in concrete. There is a formula in Ieee80-2000 for the vertical ground rod. Appreciate if some one help me.

Thanks,

Kani

 

[waross]

You may find this interesting.

http://www.ecmag.com/section/codes-standards/what-ufer-ground[/URL]]Herbert G. Ufer was a vice president and engineer at Underwriters Laboratories who assisted the U.S. military with ground-resistance problems at installations in Arizona. Ufer’s findings in the 1940s proved the effectiveness of concrete-­encased grounding electrodes. The military required low-resistance (5 ohms or less) ground connections for lightning protection systems installed at its ammunition and pyrotechnic storage sites at the Navajo Ordnance Depot in Flagstaff and Davis-Monthan Air Force Base in Tucson. Ufer developed the initial design for a concrete-encased grounding electrode that consisted of ½-inch, 20-foot-long reinforcing bars placed within and near the bottom of 2-foot-deep concrete footings for the ammunition storage buildings. Test readings over a 20-year period revealed steady resistance values of 2 to 5 ohms, which satisfied the specifications of the U.S. government at that time. This work eventually resulted in what we know today as the concrete-encased electrode in the NEC. More details about Ufer’s research are provided in his October 1964 IEEE paper CP-978, “Investigation and Testing of Footing-Type Grounding Electrodes for Electrical Installations.”

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

 

[Kani77]

Thanks Waross. It is a good article. I really want to find the calculation formula for the horizontal conductors encased in cement. Hope someone help me.

 

[waross]

Read the article again or maybe I saw this in another article. The concrete is the grounding electrode. The horizontal conductors are the connection to the grounding electrode.
Too many variables including the moisture content of the soil and the amount of doping of the soil by the concrete.
If you do find a formula, please share it with us.
Thanks in advance.

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

 

[7anoter4]

Let’s take ,at first, the formula of horizontal electrode directly buried in ground.
Re=ro/pi()/l*(ln(2*l/b)-1) from:
file:///C:/Users/user/Downloads/158_HVDC%20Ground%20Electrode%20Design.pdf
In my opinion, you may at first consider concrete res.=earth resistance.
Second you may consider the resistance of the concrete 0 and increase the conductor diameter up to diameter of concrete.
1)Let's say ro=100 ohm.m; h=2.5 ft[0.762m]; d=1/2"=0.0127m; b=sqrt(d*h)=0.0984m;length=20’=6.1m
[roconc=ro=100 ohm.m]
R1[without concrete]=100/3.1415/6.1*(ln(2*6.1/0.0984)=2.27 ohm
2) Let's say ro=100 ohm.m; h=2.5 ft[0.762m]; dc=1/2"+2*2”=4.5”=0.0635m ; bc=sqrt(d*h)=0.22m
length=6.1 m;[roconc=0]
R2=100/3.1415/6.1*ln(2*6.1/0.22)=1.99 ohm
2.27/1.99=1.14
That means only 14% error maximum and in conditions on which the resistivity measurements are it is a sufficient accuracy.

 

[waross]

This effect will tend to lower the resistance:

https://en.wikipedia.org/wiki/Ufer_ground[/URL]]Concrete is naturally basic (has high pH). Ufer observed this meant that it had a ready supply of ions and so provides a better electrical ground than almost any type of soil. Ufer also found that the soil around the concrete became "doped", and its subsequent rise in pH caused the overall impedance of the soil itself to be reduced.[3] The concrete enclosure also increases the surface area of the connection between the grounding conductor and the surrounding soil, which also helps to reduce the overall impedance of the connection.

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

 

[cuky2000]

Enclosed is an approximated formula and graph for the resistance of horizontal grounding electrode encased in concrete.
This could be useful for medium and high resistivity soils
I hope this help.

 

[7anoter4]

First of all the numbers shown in my above post are for 300 ft. horizontal length. For 20 ft. it is 19.95/15.74=1.267[27% -I am sorry!].
Second, the formula in BS7430 CH.10.3[or Annex A] Horizontal strip or round conductor electrodes is the close to EPRI EL-2020 ELECTRODE TYPES AND CONFIGURATIONS.
As per cooky2000 attachment :
R=ro/2/pi()/L*(ln(2*L^2/w/h)-1) 13.6
BS7430 ch.10.3 or Annex A:
R=ro/P/pi()/L*(ln(2*L^2/w/h)+Q)
where for linear round wire P=2 and Q=-1.3 [Table 5]
By-the-way Q=-1 it is only for strip and not for round.
EPRI EL-2020 [equation 7-9]:
Re=ro/pi()/L*(ln(2*L/sqrt(w/h)-1)
[The difference is from 0.11% for 20 ft to 0.06% for 300 ft.]

 

[Kani77]

Thank you cuky2000 and 7anoter4. I think we are in good shape with your replies.
7anoter4, your reference to Table 5 for P and Q. I checked BS7430, but i couldn't find the values. Please let me know more detail of the reference table.

Thanks,

Kani

 

[7anoter4]

You are right. This formula is shown on:
BS7430/1998 [© BSI 05-1999] page 15/16 and Annex A connected with Table 5.
In new BS7430/2011 Ch. 9.5.5 Strip or round conductor electrodes
it is shown an other formula:
Rat=ro/(2*pi()*L)*ln(L^2/1.85/h/d) [in excel format].
but it still exist in new Annex B -page 87.
Never-the-less no indication of P and Q in Table 6 as indicated here.