How to measure ground resistance with a multimeter
The grounding resistance was tested on soils with different soil qualities with a multimeter, and the data measured by the multimeter was compared with the data measured by a special grounding resistance tester. The two are very close. The specific measurement methods are as follows:
Find two 8mm and 1m long round bars, sharpen one end of them as auxiliary test rods, and insert them into the ground 5m away from both sides of the grounding body A to be tested, the depth should be more than 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 recorded as RAB, RAC, RBC respectively, and then the grounding resistance value of grounding body A can be obtained by calculation.
Because the 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. Then let the resistance of the soil 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. . . . . .
①RAC=RA+RC+RX. . . . . .
②RBC=RB+RC+2RX. . . . . .
③Compose ①+②-③ to get: RA=(RAB+RAC-RBC)/2. . . . . . ④
④Formula is the calculation formula of grounding resistance.
Measured example: The measured data of a grounding body are as follows: RAB=8.4∩, RAC=9.3∩, RBC=10.5∩. but:
RA=(8.4+9.3—10.5)/2=3.6(∩)
Therefore, the grounding resistance value of the grounding body A under test is 3.6∩.
Why is the grounding resistance the smaller the better?
The smaller the resistance, the smaller the resistance of the current passing through, so if there is leakage, all the electricity will be uploaded from the grounded ground wire to the ground.
There are many types of grounding, such as lightning protection grounding (that is, the common lightning rod): the smaller the grounding resistance, the faster the lightning discharges to the ground through the grounding wire once it is struck by lightning, that is, the safer it is; electrical equipment is safely grounded (such as The washing machine casing is grounded): The smaller the grounding resistance, when the casing leaks, the leakage will be transmitted to the ground through the grounding wire. If the grounding resistance is too large (if it is greater than the human body resistance), when a person touches the casing, the human body becomes The ground wire, the current flows from the human body into the earth, that is, electric shock; and the working grounding inside the electronic equipment, etc., the smaller the better.
