Introduction to Clever Use of Multimeter for Ground Resistance Measurement
In many cases, it is necessary to bury grounding bodies and lead out grounding levels in order to reliably ground instruments and equipment. To ensure that the grounding resistance meets the requirements, a dedicated grounding resistance tester (such as Japan's Kyoritsu 4105A) is usually needed 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 that measured by a dedicated grounding resistance tester. The two were found to be very close. The specific measurement method is as follows:
Find two 8mm and 1m long round steel bars, sharpen one end as an auxiliary testing rod, and insert them into the ground 5m away on both sides of the grounding body A to be tested, with a depth of at least 0.6m, and keep the three 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 the resistance values between A and B; A and C, denoted as RAB, RAC, RBC respectively, and calculate the grounding resistance value of grounding body A.
Since 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. Assuming the soil resistance between A and B is RX, since the distance between AC and AB is equal, the soil resistance between A and C can also be RX; and since 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 Combining ①+② - ③ yields: RA=(RAB+RAC - RBC)/2...... ④
④ The formula is the calculation formula for grounding resistance.
Actual test example: The data obtained for a certain grounding body today are as follows: RAB=8.4 ∩, RAC=9.3 ∩, RBC=10.5 ∩. Then:
RA=(8.4+9.3-10.5)/2=3.6(∩)
So, the grounding resistance value of the tested 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 probe and the grounding body and reduce errors.
