How to measure ground resistance with multimeter
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 A and B; The resistance values between A and C are denoted as RAB, RAC, and RBC, respectively. After calculation, the grounding resistance value of grounding body A can be determined.
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. Let the soil resistance between A and B be RX again, because the distance between AC and AB is equal, the soil resistance between A and C can also be RX; 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.
