Methods for Measuring Ground Resistance with Multimeter
The method of using a multimeter to test grounding resistance is difficult to purchase due to the high cost of specialized grounding resistance testers. 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 round steel bars that are 8mm and 1m long, sharpen one end of them as an auxiliary testing rod, and insert them into the underground 5m away from 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, B and C are auxiliary test rods. Then, use a multimeter (R * 1 gear) to measure the resistance between 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 obtained. 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, 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...... ① RAC=RA+RC+RX...... ② RBC=RB+RC+2RX...... ③ Combining ①+② - ③ yields: RA=(RAB+RAC - RBC)/2...... ④ Formula ④ 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 ∩. So: RA=(8.4+9.3-10.5)/2=3.6 (∩), therefore, 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.
