GROUND RESISTANCE MEASUREMENT

[appunni]

Sir,

Can we measure the ground reistance of a test object by means of a dc currnet injection kit in the following way ?
Connect one lead of dc injection kit to test object.( In earthmegger, C1 and P1 are connected to test object.) Connect the other lead to a spike placed at about 30 meters apart.(In measurements using earthmegger this is C2.) Inject dc current. Measure voltage between a spike connected at 15 meters (P2 in earthmegger measurements) and test object connected lead. Measure current also. The ratio of voltage to current should give ground resistance of test object. The position of voltage spike may be altered for getting constant readings.
Is the above procedure correct? If yes, will current of the order of 1 to 3 amperes give good readings? I request for your valuable comments.

With regards,
appunni

 

[Skogsgurra]

appunni,

The principle is OK. It is the same that is being used in commercial ground resistance meters. There is one little thing, however. You might get some electrolysis if there is water in the soil and that will disturb the measured emf. To avoid that, simply change polarity and use (u- + u+)/2 and (i- + i+)/2 when calculating the resistance.

On the other hand, ground resistance varies a lot, so precision measurement is seldom of any use.

 

[alehman]

That's the basic concept. You need to make sure your distant electrode is sufficiently far away that it is outside of the "sphere of influence" of the electrode system under test. This is usually a several times the depth of the system under test.

Then make measurements with the middle electrode at periodic spacings (5 or 6) along the path from the system under test toward the distant electrode. If you plot these resistance values, you should see resistance increase as you move away from the system under test. It will plateau for a few intermediate positions and then increase again as you approach the distant electrode. If there is no plateau on the plot, your distant electrode is probably too close and the result will not be valid.

You should moisten the soil around the distant and intermediate electrodes. Using DC can subject the measurements to error due to stray earth currents. Your meter should be capable of rejecting AC if there is any chance of AC return current being present in the earth.

See

http://www.aemc.com/techinfo/techworkbooks/ground_resistance_testers/950-WKBK-GROUND.pdf

 

[jghrist]

Usually, ac is used with some frequency higher than 50 or 60 Hz (not an integer harmonic) so that stray currents can be rejected.

 

[jbartos]

Suggestion: Reference:
1. Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 14th Edition, McGraw-Hill, 2000,
includes methods for measuring the ground resistance in Section "Measuring Ground Resistance" on page 17-40.