Cleverly using a multimeter to measure ground resistance
In many cases, it is necessary to bury grounding electrodes and lead out grounding stages in order to reliably ground instruments and 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 as an auxiliary testing rod, and insert them into the ground at a distance of 5m on both sides of the grounding body A to be tested. The depth should be at least 0.6m, and the three should be kept 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 A and B; The resistance values between A and C, denoted as RAB, RAC, and RBC, can be calculated to determine the grounding resistance value of grounding body A.
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 Add ①+②? ③ That is to say: RA=(RAB+RAC? RBC)/2... ④
④ The formula is for calculating the grounding resistance.
Actual measurement example: The data of a certain grounding body is 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 measurement, the grounding bodies A, B, and C need to be sanded and polished with sandpaper to minimize the contact resistance between the probe and the grounding body, in order to reduce errors.
