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Hi Pmin,

Thank you for your question regarding the calculation of earth resistance for copper rods and tapes, it is our pleasure to help.

The simplified calculation for predicting the resistance-to-ground (earth) of a given length grounding rod is:

R=(p/(2*pi*Length))*(ln*(4L/a)-1))

Where:

• R= Resistance
• p= soil resistivity
• L = Length of the rod
• a= radius of the rod
• ln= natural log

The simplified calculation for a buried horizontal strip (tape) is actually really quite complex:

R=(p/(4*pi*L)*(ln*(4L/a)+((a squared)-pi*a*b)/(2*(a+b)squared))+(ln(4L/s)-1)+(s/2L)-(s squared/16L squared)+(s 4th power/512L 4th power)…)

Where:

• R= Resistance
• p= soil resistivity
• L = Length of strip
• a= section of strip
• b= section of strip
• ln= natural log
• s= buried depth

Keep in mind that both of these formulas are for uniform soil conditions, which simply doesn’t exist on the planet.  Also, these are for direct-current (DC) resistances and do not take into account the effects of alternating-current (AC) impedance issues.  As a result, it is really best to use a computer to model your earthing system.

That said, lowering the resistance-to-earth at your telecom tower can be a very tricky thing. If your soil resistivity lowers with depth, your best choice will be to install deep-earth electrodes.  If your soil resistivity increases with depth, your best choice will be to install long buried conductors running horizontally.  Keep in mind, these guidelines are simply for lowering the resistance-to-earth of your grounding system.  Blindly lowering the earth resistance may not be beneficial for Human Safety issues such as Step & Touch Voltage Hazards, electrical noise reduction (ground differentials), lightning protection issues, etc.  If protecting your telecommunications gear is your goal, you should be far more concerned with engineering factors like the Ground Potential Difference (GPD).  The earth resistance should be a secondary factor.

The problem will be in trying to calculate the earth resistance of a ground rod or tape in multi-layer soil resistivities.  You will need a computer to do this accurately.  E&S Grounding solutions is of course more than happy to help you with this process, should you need the assistance.  Please feel free to call us at 310-318-7151 California time if you should need these services.  But if not us, please get someone with the proper knowledge and engineering software to give you valid work plans, so that you are not waiting your time and money installing copper into the earth without getting the benefits you need.

Best regards,

The Engineering Team at E&S Grounding Solutions

Photo credit:  http://www.flickr.com/photos/franganillo/3734200307/sizes/l/in/photostream/

### 2 Responses

1. Naeemuddin Amin

Can we use ground water pump suction pipe as grounding the electrical system, is there any limitation?

2. EDDY MURRAY

I live in Barbados, why do I have to use an 8” earth rod when in some areas the soil depth is only about 5th, Why do we have to go pass the soil into hard rock, could you post the reply to my face book page ,, eddy murray .. or e-m me a eddy.47@live.com