Measuring Grounding Resistance with Multimeter Skillfully
In many cases, 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 is usually required for measurement.
But 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 uses a multimeter to test the grounding resistance in different soil types, and compares 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. . . . . .
③Compare 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.
