How to Use Multimeter to Measure Ground Resistance

In many cases, it is necessary to bury the grounding body and lead out the grounding level so that the equipment can be reliably grounded. To ensure that the grounding resistance meets the requirements, a special grounding resistance tester (such as Japan Kyoritsu 4105A) is usually required for measurement.

However, in actual work, the special grounding resistance tester is expensive and difficult to find. Can a multimeter be used to measure the grounding resistance? The author used a multimeter to experiment with the grounding resistance in soils of different soils, and compared the data measured by the multimeter with the data measured by the special grounding resistance tester. The two are very close. The specific measurement method is as follows:

find two 8mm and 1m long round steels, sharpen one end of them as auxiliary test rods, and insert them into the ground 5m away from both sides of the grounding body A to be tested. The depth should be more than 0.6m, and keep the three in a straight line.

Here, A is the grounding body to be tested, and B and C are auxiliary test rods.

Then use a multimeter (R*1 gear) to measure the resistance between A and B; A and C, which are recorded as RAB, RAC, and RBC respectively. Then the grounding resistance value of the grounding body A can be calculated.

Since the grounding resistance refers to the contact resistance between the grounding body and the soil. Let the grounding resistances of A, B, and C be RA, RB, and RC respectively. Let the soil resistance between A and B be RX. Since the distances between AC and AB are equal, the soil resistance between A and C can be RX. Since BC=2AB, the soil resistance between B and C is approximately 2RX, then:

RAB=RA+RB+RX. . . . . .

①RAC=RA+RC+RX. . . . .

②RBC=RB+RC+2RX. . .

. . . ③ Substituting ①+②—③, we get: RA=（RAB+RAC—RBC）/2. . . . . ④

Formula ④ is the calculation formula for grounding resistance.

Actual measurement example: The data of a grounding body measured today are as follows: RAB=8.4∩, RAC=9.3∩, RBC=10.5∩. Then:

RA=（8.4+9.3-10.5）/2=3.6（∩）

So, the grounding resistance value of the grounding body A is 3.6∩.

It is worth noting that before measurement, the three grounding bodies A, B, and C need to be polished with sandpaper to make them shiny, and the contact resistance between the test lead and the grounding body should be minimized to reduce the error.