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#1

# earthing design

how to design an earthing system for 8x8metres communication room to keep the resistance below 1ohm where tha average soil restivity is 220 ohms-metre
09-01-2014 10:10 AM
Top #2
Assuming the requirement of 1 ohm is with respect to remote earth ground and is for lightning performance, a design approach would be to use a ground grid modeling software like SES Autogrid, develop a multi layered soil model and then layout out a copper grid consisting of a closed rectangular shape with internal x-axis and y-axis crossing conductors. Depending on the type of soil, the resistance of the grid to remote earth can be lowered using a combination of increasing the grid area and installing ground rods to penetrate into the deeper soil layers. Connecting the grid to a building foundation re-bar or structural steel system and other metallic structures such as incoming water lines and the power system equipment grounding conductors can also help in lowering the remote grid resistance provided that all local building and electrical codes are followed in the design. If your 1 ohm requirement is not for lightning performance but for signal noise reduction in communications circuits, the grid may need to be isolated from connections to these other building systems.
09-01-2014 12:32 PM
Top #3
thanx Jim :)
but using ieee std 80 for grid calculation it requires a very large area to bring down resistance to one ohm
09-01-2014 02:50 PM
Top #4
You are correct. IEEE Std. 80 uses equations that are based on a single layer soil model so a very large grid would be the expected result given those soil conditions. If you wish to refine your calculations, then you might consider using a multi-layer soil model. Of course, you will need additional soil resistivity measurements at increasingly wider probe spacings to measure the resistivity of the deeper soil conditions. If your soil is very dry and sandy, increasing the area of the grid will likely prove to be expensive. In that case you might consider using deep grounding wells that penetrate the high resistivity layer and go into lower resistivity regions if they exist. To determine the resistivity of the deeper soil, additional soil testing at wider probe spacings would be necessary.
09-01-2014 05:23 PM
Top #5
thnx man but why larger separation between probes?
09-01-2014 08:07 PM
Top #6
what about rods in line or hollow square (ring) with use of some conditioning agent
09-01-2014 11:05 PM
Top #7
plus any good free design software ?
09-02-2014 02:01 AM
Top #8
@M.Shoaib: "why larger separation between probes?"

The depth of the measurement is determined by the probe spacing when using the Wenner 4-point method. For more information, checkout the following link:

http://en.wikipedia.org/wiki/Soil_resistivity