How to Test Grounding Resistance with a Multimeter
Usually, to test the grounding resistance, it is necessary to bury the grounding body and lead out the grounding level in order to reliably ground the instrument and equipment. In order to ensure that the grounding resistance meets the requirements, a special grounding resistance tester such as Japan’s Kyoritsu 4105A grounding resistance tester/Kyoritsu 4102A is usually required for measurement. Or use a more expensive clamp-type ground resistance tester.
But in use, the special grounding resistance tester is expensive and inconvenient to buy. Can a multimeter be used to measure the grounding resistance? The author used a multimeter to test the grounding resistance in different soil types, and compared the data measured by the multimeter with the data measured by a special grounding resistance tester, and the two are very close. The specific measurement method is as follows:
Find two 8mm, 1m long round steel, sharpen one end as an auxiliary test rod, 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, B and C are auxiliary test rods
Then use a multimeter (R*1 block) to measure the resistance value between A and B; A and C, which are recorded as RAB, RAC, RBC respectively, and then the grounding resistance value of grounding body A can be obtained by calculation.
Because 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 resistance of the soil between A and B be RX, because the distance between AC and AB is equal, the soil resistance between A and C can also be RX; and because 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 Combine ①+②—③ to get: RA=(RAB+RAC—RBC)/2. . . . . . ④
The formula ④ is the calculation formula of grounding resistance.
Actual measurement example: The measured data of a certain grounding body is as follows: RAB=8.4∩, RAC=9.3∩, RBC=10.5∩. but:
RA=(8.4+9.3—10.5)/2=3.6(∩)
Therefore, the grounding resistance value of the measured 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 minimize the contact resistance between the test lead and the grounding body to reduce errors.
