To test the resistance of the ground using a multimeter
Normally, when testing ground resistance, it is necessary to bury the ground body and lead out the ground level so that the instrument and equipment can be reliably grounded. In order to ensure that the ground resistance meets the requirements, a special ground resistance tester such as Japan Kyoritsu 4105A ground resistance tester/Kyoritsu 4102A is usually required for measurement. Or use a relatively expensive clamp-type ground resistance tester.
However, in use, a dedicated ground resistance tester is expensive and inconvenient to purchase. Can I use a multimeter to measure ground resistance? The author used a multimeter to conduct experiments on ground resistance in soils of different soil types, and compared the data measured by the multimeter with the data measured by a special ground resistance tester. The two were very close. The specific measurement methods are as follows:
Find two 8mm, 1m long round steels, sharpen one end as an auxiliary test rod, 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, B and C are auxiliary test rods
Then use a multimeter (R*1 gear) to measure the resistance values between A and B; record them as RAB, RAC, and RBC between A and C, and then calculate the grounding resistance value of grounding body A.
Because ground resistance refers to the contact resistance between the ground body and the soil. Let the grounding resistances of A, B, and C be RA, RB, and RC respectively. Suppose the resistance of the soil between A and B is 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. . . . . .
③Put ①+②-③ to get: RA=(RAB+RAC-RBC)/2. . . . . . ④
Equation ④ is the calculation formula for ground resistance.
Actual measurement example: The measured data of a certain grounding body today are as follows: RAB=8.4∩, RAC=9.3∩, RBC=10.5∩. but:
RA=(8.4+9.3-10.5)/2=3.6(∩)
Therefore, the ground resistance value of the measured ground body A is 3.6∩.
It is worth noting that before measurement, the three grounding bodies A, B, and C need to be polished and shiny with sandpaper to minimize the contact resistance between the test leads and the grounding body to reduce errors.
