By hvhipot 
Clamp-on resistance meter measurement principleThe ground resistance tester is a novel measuring tool. It is convenient and fast, and its appearance is exactly like a clamp-on ammeter. It does not need auxiliary test piles during testing, and only needs to go to the ground wire to be tested. The measurement results can be obtained in a few seconds after the last clip, which greatly facilitates the ground resistance measurement work. Another great advantage of the clamp-on ground resistance meter is that it can measure the ground resistance of the equipment in use online without cutting off the power supply of the equipment or disconnecting the ground wire.
The grounding resistance value measured with a clamp-type grounding resistance meter is the comprehensive resistance of the grounding branch. It includes the contact resistance of the branch to the common ground wire, lead resistance, and grounding body resistance. The clamp ground resistance tester uses the traditional voltage and current method under the condition of tripping, and the measured value is only the resistance of the grounding body. Obviously, the measured value of the former is larger than that of the latter. The magnitude of the difference reflects the magnitude of the contact resistance between this branch and the common ground wire. It should be noted that the grounding resistance specified in the national standard includes the grounding lead resistance.
Due to the special structure of the clamp-on ground resistance meter, it can be easily used as an ammeter. Many of these meters also have the function of a clamp-on ammeter. On the other hand, although the clamp-on ground resistance tester uses signals of a certain frequency to eliminate interference, when there is a large current on the cable under test, the measurement will also be interfered, resulting in inaccurate results. Therefore, according to the requirements, the current on the cable should be measured first when in use, and the ground resistance can be further measured only when the current is not very large. Some meters automatically perform noise interference detection when measuring ground resistance, and will give a prompt when the interference is too large to make the measurement impossible.