How to test earth resistance using a multimeter
Normally, to test the earth resistance, it is necessary to bury the earth body and lead to the earth level in order to ground the instruments and equipment reliably. To ensure that the grounding resistance meets the requirements, a specialised earth resistance tester such as the Kyoritsu 4105A Earth Resistance Tester / Kyoritsu 4102A is usually required. Alternatively, a more expensive clamp-on earth resistance tester can be used.
But in use, the special earth resistance tester price is high, inconvenient to buy, can use a multimeter to measure grounding resistance? The authors used a multimeter in different soil quality of the soil on the grounding resistance of the experiment, and will be measured by the multimeter and dedicated grounding resistance tester measured data were compared, the two are very close. Specific measurement methods are as follows:
Find two 8mm, 1m long round steel, one end of the grinding tip as an auxiliary test rod, were inserted into the grounding body to be tested on both sides of the A 5m away from the ground, the depth should be more than 0.6m, and make the three keep a straight line.
Here, A for the grounding body to be tested, B, C for the auxiliary test rod
Then use a multimeter (R * 1 stop) to measure the resistance between A and B; A and C, respectively, recorded as RAB, RAC, RBC, and then calculated to find the grounding resistance value of grounding body A.
As the grounding resistance refers to the contact resistance between the grounding body and the soil. Let the grounding resistance of A, B, C are RA, RB, RC. and then set the resistance of the soil between A and B is RX, because AC, AB distance is equal, it can be A and C between the soil resistance is also RX; and because BC = 2AB, so B and C between the soil resistance approximated as 2RX, then:
RAB=RA+RB+RX......
①RAC=RA+RC+RX......
②RBC=RB+RC+2RX......
③ ① + ② - ③ that is: RA = (RAB + RAC - RBC)/2...... ④
④ type is the formula for grounding resistance.
Example of measurement: today measured the following data of a grounding body: RAB = 8.4 ∩, RAC = 9.3 ∩, RBC = 10.5 ∩. Then:
RA=(8.4+9.3-10.5)/2=3.6(∩)
Therefore, the value of ground resistance of the measured grounding body A is 3.6∩.
It is worth noting that: before the measurement, you need to polish the three grounding bodies A, B and C with sandpaper shiny to minimise the contact resistance between the meter pen and the grounding body to reduce the error.
