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# Soil resistivity

Discuss Soil resistivity in the Electrical Forum area at ElectriciansForums.net

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#### laxmi

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi,
How can I change earth resistance (R) value into soil resistivity (Rho)? Is this formula correct, ⍴ (Rho) = 2πAR (A = Distance between pegs, R = Earth resistance).

Regards,

L.

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#### Wilko

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Hi - this is the simplified eqn for use with the Wenner Method, which to be accurate would need R to be found from using the Wenner layout of probes for current and voltage. But I'm not an expert ...

#### Lucien Nunes

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Yes agreed, you cannot simply use a 2-wire resistance measurement because that will include the contact resistance of rods / probes. The 4-wire measurement to obtain Rw eliminates this and only measures the resistance resulting from the bulk resistivity of the soil.

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#### laxmi

Thanks for your information, it was very usefull for me.

Can we not use earth resistance value (R) that is measured from earth electrode test method, to obtain soil resistivity. I have measursed existing earth electrode and value is 80 Ohm.

Does it not give us soil resistivity using formula ⍴ = 2πAR

Lax

#### Wilko

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Hi - if I understand correctly you have a measured value for electrode resistance. This will be as accurate as the particularly method allows and different methods will likely lead to different results as some are more accurate than others.

Using that eqn with your electrode resistance result you will get a number, but it is unlikely to accurately represent resistivity as per LN post#3 reasons.

Can I ask why you need resistivity?

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#### laxmi

Hi,

I am calculating substation earthing system. Existing earthing system was intalled approx 30 years ago. I can go for one for one replacement with similar earthing type but factirs might have changed by this time. And it is also good to know all information before hand.

Please see the formula, in which soil resistivity value is required.

Regards

#### Wilko

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Got it - GN8 if I'm not mistaken. For your purpose I would recommend you seek out the proper resistivity measurement kit and perhaps read up on their reference BS7430.

Just for fun as its -5C here this morning, I'm imagining I'm on a desert island with this challenge and no kit. I would estimate rho as you propose, design an earthing system using it, instal it, measure R actual, adjust rho in light of this result, design another system with say twice as much surface area, instal it, measure it and confirm R actual was as per design from rho estimate.

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