By hvhipot 
Let’s take the commonly used clamp ground resistance tester as an example. The meter is equipped with two jaws, a voltage clamp and a current clamp. The voltage clamp excites an induced potential E in the circuit under test and generates a current I in the circuit under test. The instrument can measure the value of I through the current clamp. By measuring E and I, the value of R can be obtained by Ohm’s law: R=E/I.
The specific test methods on site are as follows:
(1) When measuring the grounding resistance of the two terminals of E-E, short-circuit it with a chrome-plated copper plate and connect it to the 5m-long pure copper wire that comes with the meter. The other end of the wire is connected to the test point of the grounding body to be tested. When measuring the resistance of the shield, loosen the chrome-plated copper plate, connect one E terminal to the grounding body, and connect the other E terminal to the shield.
(2) The P-pillar is connected to the 20m pure copper wire that comes with the instrument, and the other end of the wire is connected to the pin.
(3) C-pillar is connected to the 40m pure copper wire that comes with the instrument, and the other end of the wire is connected to pin 2.
Technical requirements of grounding resistance tester test site:
(1) The grounding resistance tester should be placed 1 to 3m away from the test point, and it should be stable and easy to operate.
(2) The terminal of each terminal must be in good contact and the connection must be firm.
(3) The two grounding electrode pins should be set at 20m and 40m to the left and right of the grounding body to be tested; if the two pins are connected by a straight line, the grounding body to be tested should be basically on this straight line.
(4) Do not use other wires to replace the 5m, 20m, and 40m pure copper wires that come with the instrument configuration.
(5) If the grounding resistance tester is the center of the circle, the angle between the two pins and the tester should not be less than 120°, and they should not be set in the same direction.
(6) The soil for the two-pin installation must be solid, and cannot be installed in mud, backfill soil, beside tree roots, grass, etc.
(7) The grounding resistance test can only be carried out after 7 consecutive sunny days after rain.
(8) The grounding body to be tested should be derusted first to ensure reliable electrical connection.