Skip to content

Earth Resistance Testing – Techniques Part II

  • Uncategorized

Fall-of-Potential Method Continued …

Effects of Different Reference Probe Locations

Now, if you read “Earth Resistance Testing – Techniques Part I,” you may ask: if the right location for probe P is always 62 percent of the distance between the ground rod under test and probe C, why bother with all the tests at other locations for probe P? Why not just drive P in at the 62 percent distance and assume that the measured resistance is the correct earth resistance? The following should help answer these questions.

Minimum Distance for Probe C

Consider Illustration 1, which shows earth shells around the ground rod under test and reference probe C. Probe C is so close to the ground rod under test that the earth shells seriously overlap one another.  When these field or shells overlap you don’t get the leveling off of measured resistance as P is moved away from the ground rod under test; the shells of add to the shells of the ground rod under test so the resistance keeps increasing.

Note:  To obtain an ideal earth resistance curve at least 10 readings should be taken at equally spaced intervals.  True resistance will be obtained where the curve flattens out (typically 62% distance).  Taking readings over greater distances or in different directions from rod 1 will help to confirm results.  If the curve does not flatten the measurements should be retaken using more distance for the C probe placement.

In Illustration 2, C is placed farther away.  Then the measured resistance levels off enough and at the 62 percent distance it is very close to the actual earth resistance. The reason for having Cfarther away is to get assurance that the 62 percent value is “in line” with other values on the curve. The value could only be wrong (assuming there are no measuring mistakes) if the soil conditions at the 62 percent point vary from conditions at other points, causing changes in earth resistivity. Graded soil around construction sites or buried objects such as pipes can cause such localized deviations. Therefore, you want to get some degree of flatness or leveling off of your curve to make such a variation easily noticeable. At the same time, remember that the resistance will rise again in the electrical field of the current probe, so measurements in this area are to be avoided.

If you need to conduct earth resistance tests, consider renting equipment from Protec Equipment Resources. We carry power quality test sets from Megger and AEMC.  Our experienced staff can set you up with the right tools. Do not hesitate to call us at 1-866-352-5550 for assistance.