Using a multimeter skillfully to measure grounding resistance
In many cases, it is necessary to bury grounding electrodes and lead out grounding stages in order to reliably ground the instrument equipment. To ensure that the grounding resistance meets the requirements, a dedicated grounding resistance tester is usually required for measurement.
But in practical work, specialized grounding resistance testers are expensive and difficult to find. Can a multimeter be used to measure grounding resistance? The author conducted experiments on the grounding resistance of different soil types using a multimeter, and compared the data measured by the multimeter with the data measured by a dedicated grounding resistance tester. The two were 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 at a distance of 5m on both sides of the grounding body A to be tested, with a depth of more than 0.6m, and keep the three in a straight line.
Here, A is the grounding electrode to be tested, and B and C are auxiliary test rods. Then use a multimeter (R * 1 gear) to measure A and B; The resistance values between A and C are recorded as RAB, RAC, and RBC, respectively. After calculation, the grounding resistance value of grounding body A can be calculated.
Due to the fact that 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, because the distance between AC and AB is equal, the soil resistance between A and C can also be RX; Because 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 Replace ①+②? ③ Namely: RA=(RAB+RAC? RBC)/2... ④
④ The formula is the calculation formula for the grounding resistance.
Actual measurement example: The data of a certain grounding electrode measured today are as follows: RAB=8.4 Å, RAC=9.3 Å, RBC=10.5 Å. Then:
RA=(8.4+9.3? 10.5)/2=3.6 (Å)
So, the grounding resistance value of the tested grounding body A is 3.6 Å.
It is worth noting that before measuring, the grounding bodies A, B, and C need to be polished and polished with sandpaper to minimize the contact resistance between the probe and the grounding body, in order to reduce errors.
