Method for Testing Ground Resistance Using a Multimeter
Normally, to test earth resistance, it is necessary to bury earth electrodes and lead out earth terminals for reliable earthing of instruments and equipment. To ensure the earth resistance meets specifications, a dedicated earth resistance tester such as the Kyoritsu 4105A / Kyoritsu 4102A earth resistance tester, or a relatively expensive clamp-on earth resistance tester is generally required for measurement.
However, dedicated earth resistance testers are costly and not easy to acquire. Is it possible to measure earth resistance with a multimeter? The author conducted earth resistance tests in soils of different types using a multimeter, and compared the multimeter readings with those obtained by a dedicated earth resistance tester. The two sets of results were very close. The specific measurement method is as follows:
Prepare two round steel bars of 8 mm diameter and 1 m length. Sharpen one end of each bar to serve as auxiliary test rods. Drive them into the soil 5 m away on each side of the earth electrode A under test, to a depth of more than 0.6 m, and keep the three points in a straight line.
Here, A is the earth electrode under test, and B and C are the auxiliary test rods.
Use a multimeter (R×1 range) to measure the resistance between A and B, and between A and C. Record the readings as RAB and RAC respectively, and also measure RBC. The earth resistance of electrode A can then be calculated.
Earth resistance refers to the contact resistance between the earth electrode and the soil. Let the earth resistances of A, B and C be RA, RB and RC respectively. Let RX denote the soil resistance between A and B. Since the distances AC and AB are equal, the soil resistance between A and C is also RX. Since BC = 2AB, the soil resistance between B and C is approximately 2RX. Then: RAB = RA + RB + RX ……① RAC = RA + RC + RX ……② RBC = RB + RC + 2RX ……③
Add equations ① and ②, then subtract equation ③: RA = (RAB + RAC − RBC) / 2 ……④
Equation ④ is the calculation formula for earth resistance.
Practical measurement example: The measured data of an earth electrode are as follows: RAB = 8.4 Ω, RAC = 9.3 Ω, RBC = 10.5 Ω.
RA = (8.4 + 9.3 − 10.5) / 2 = 3.6 (Ω)
Therefore, the earth resistance of the tested earth electrode A is 3.6 Ω.
Important note: Before measurement, polish the three electrodes A, B and C with sandpaper to a bright finish to minimize the contact resistance between test leads and electrodes and reduce measurement errors.
