Thank you for your question regarding how to calculate the resistance of an earthing pit for a particular area. It is our pleasure to help.
You first need to start by taking soil resistivity measurements in the area you are installing the earthing pits. This is the most important part of the task when calculating resistance-to-ground. We recommend the Wenner 4-Pin Method (or Wenner 4-Point) with spacing’s at least 2x the length of the proposed earth pit. Once you collect this data you will then to develop a soil model in order to accurately understand the nature of the earth and where the layers reside at you site. Developing this soil model may take computer assistance to get the job done correctly.
Once you have an understanding of the soil resistivity, you can begin your calculations. The formula for a single ground rod in uniform soil is: R=(p/2(pi)L)[1n(4L/a)-1] where p=soil resistivity in ohm-cm, L= length of the rod, and a=the radius of the rod. A simplified version of this formula that is within 15% accuracy is R=p/335 where p= resistivity in ohm-cm.
Now, keep in mind that the above formula’s are for uniform or single-layer soils, which simply don’t exist in the real world. Calculating earth pits or ground rods in multi-layer soils is far more complex. Additionally, these formulas are for Direct Current (DC) resistances, and not for Alternating Current (AC) impedances, and of course they do not take into consideration of current loading on the electrode. Additionally, if you are trying to calculate multiple earth pits, the distance the pits are away from each other will dramatically complicate the formulas. We recommend that your reference Section 14 of the “Handbook of Electric Power Calculations” by H. Wayne Beaty, published by McGraw-Hill. This book has all the hand formulas you will need for uniform soil conditions.
Our experience tells us that hand-calculations are rarely reliable as there are simply too many factors to take into account regarding soil conditions and the make-up of the electrodes themselves. We recommend the use of a computer modeling programs such as the engineering simulation software called CDEGS. This software program can accurately calculate multiple earth pits, comprised of differing materials and backfills, with as many soil layers as is needed.
Here is another blog on this subject that you may find of interest:
E&S Grounding Solutions is of course more than happy to help you in designing your grounding system. But if not us, please get someone with the proper measurement tools, engineering software and experience to get the job done right. Please feel free to call us at 310-318-7151 and we will be glad to speak with you about your project free of charge.
The Engineering Team at E&S Grounding Solutions