GROUNDING CALCULATION 2

[bobziva]

Owner is preparing to make new Communication Tower inside Power Plant, too far from any Grounding Grid.
Tower must be grounded with 4 ground rods /square layout/. That what must be provided is Ground Resistance R = 5 Ohms minimum. Soil is good - no sand, no stone. If Ground Resistance is R > 5 ohms, I have to provide extra ground wires and extra ground rods.
I am trying to calculate this, but I do not have here any Grounding Software or any book with a calculation.
I welcome any article or softyware from Grounding Calculation.
THANKS

 

[jbartos]

Suggestion: Try the pencil and paper approach or the spreadsheet approach based on IEEE Std 80-2000 IEEE Standard for Safety in AC Substation Grounding, Annex D

 

[mvcjr]

From Furse's Consultant's Handbook:

R = (p/275L)* Log(400L/d)

R = apparent resistance of a single earth electrode (circular cross section), ohms
p = soil resistivity in ohm-cm
L = length (depth) of electrode in meters
d = diameter of electrode in centimeters

This should give you an approximate idea of the resistance of one electrode. For 4 electrodes you should get a lower value, though not exactly one-fourth of R.

Just make sure your electrode spacing should not be less than the length of one electrode.

God bless!

 

[jbartos]

Suggestion: There may be a safety concern regarding safe touch and step potentials

 

[Powersystems123]

Search for SAE grounding site

 

[sbergman]

To bobziva:

The equation that I use starts out like the one outlined by mvcjr but continues the calculation to give you the net resistance of the ground grid system.

Note: Equation does not take into acount the amount of copper conductor used.

p= soil Resistance in ohm-meters
l= length of ground rod in feet
d= diameter of ground rod in inches
R1= resistance of one ground rod
n= total rods in layout
k= combining factor
Rn= Total net resistance of grounding system
R= measured earth to ground resistance using IEEE 4-point method

p=38.30*R (for 10' ground rods 19.15*R or 38.30*R for 20' ground rods)
k=(.378 *log)+.89
R1=(p/(1.915*l))*(log(96*(l/d))-1)
Rn=(R1*k)/n

I have used this equation to design ground grids for Cell Sites across the Mid-Atlantic in the U.S. and most of the time the numbers were fairly accurate. Earlier this year I had the oppurtunity to perform a soil resistivity test for a new railroad substation and in order to due the test corectly I used the IEEE 80-2000. I compared the end results from the IEEE standard and the above calculation and both were within 1 Ohm of each other.

Also if you are using only four rods and better conductivitiy is needed you might want to look into using either chemical ground rods (Lyncole or Harger) or a soil additive in order to hit the 5 Ohm specification. You will also want to connect to any fence or cabletray sytem into your grounding system to protect against any touch or step potential as was mentioned earlier.

If you are looking for some literature

http://www.aemc.com,

under technical information might have some useful documentation.

Sorry about the long post.

Shawn

 

[DanDel]

bobziva, the actual soil resistance should probably be tested, by a fall-of-potential four-point ground test, to determine how many ground rods are needed and how deep they should be.
Check out Biddle(now called Megger) for some info on the tests and calculations.

http://www.megger.com/us/Index.asp

 

[jbartos]

Suggestion: IEEE 80-2000 page 64 states the simplest (approximate and estimated) formula for grounding system resistance in uniform soil as:
Rg = (rho/4) x sqrt(pi/A).............(10)
where
Rg is the substation ground resistance in Ohms
rho is resistivity of the soil in Ohm-m
A is the area occupied by the ground grid in square meters
pi=3.14

 

[jbartos]

Comment: sbergman posting refers to IEEE Std 80-2000; however, the step and touch potentials still may be posing a safety concern.

 

[WattsNew]

You might want to use standard softwares like ETAP that comply with IEEE 80-2000 to save you a lot of iteration time. Knowledge of 3Io LG fault current and X/R ratio is required.