Ground Systems
Determining soil resistivity with 4-rods

The 4-Rod (Venner) Fundamental Method of Determining Soil Resistivity

Author: R.J.Edwards G4FGQ © 15th February 1998

The four rods need not be of same diameter but should all be forced into the soil to approximately the same depth and tamped in at the soil surface.

The rods should be in a straight line, uniformly spaced apart. Spacing should be at least three, preferably four times their depth. But very wide spacing needs a large test current and a fairly high supply voltage for accuracy.

A current is driven between the two outermost rods and measured. The resulting voltage between the inner pair of rods is measured. Then calculate as follows:

Soil resistivity = 2 * Pi * Spacing-in-metres * Volts / Amps ohm-metres

The voltmeter must be a high resistance type, at least 20 times the resistance of the soil between the pair of electrodes. To avoid errors due to extraneous earth currents or electro-chemical potentials, take the mean of measurememnts with rapid current reversals, or use an alternating 50 Hz supply current.

Example: Four rods driven to a depth of 500 mm, spaced two metres apart, current = 142 milliamps. Measured volts = 1.14 Therefore resistivity = 101 ohm-metres. Note: in this case, a 50 volt supply is needed to drive 142 mA into the rods.

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