By hvhipot 
Ground resistance testers usually have the following measurement methods: two-wire method, three-wire method, fHV Hipot Electric Co., Ltd.-wire method, single clamp method and double clamp method. Each has its own characteristics. In actual measurement, try to choose the correct method to make the measurement. The results are accurate.
1. Two-line method
Condition: There must be a known good ground. Such as PEN and so on. The measured result is the sum of the resistance of the measured ground and the known ground. If the known ground is much smaller than the resistance of the measured ground, the measurement result can be the result of the measured ground.
Applicable to: buildings and concrete floors, etc. Seal areas where piles cannot be driven.
Wiring: e+es receives the measured ground. h+s receive known ground.
2. Three-line method
Conditions: There must be two ground rods: an auxiliary ground and a detection electrode, and the distance between each ground electrode is not less than 20 meters.
The principle is to add a current between the auxiliary ground and the measured ground. Measure the voltage drop measurement result between the measured ground and the probe electrode. This includes measuring the resistance of the cable itself.
Applicable to: ground grounding, construction site grounding and lightning ball lightning rod, QPZ grounding.
Wiring: s connect to the detection electrode. h then auxiliary ground. After e and es are connected, the measured ground is connected.
3. FHV Hipot Electric Co., Ltd.-wire method
It is basically the same three-wire method, which eliminates the influence of the measurement cable resistance on the low ground resistance measurement result when it replaces the three-wire method. When measuring, e and es must be directly connected to the measured ground, which is very accurate in all grounding resistance measurement methods.
4. Single clamp measurement
Measure the ground resistance of each position in the multi-point grounding, and do not disconnect the ground connection to prevent danger.
Applicable to: multi-point grounding and cannot be disconnected. Measure the resistance of each connection point.
Wiring: Use a current clamp to monitor. The current at the location being tested.
5. Double clamp method
Conditions: Multi-point grounding, no auxiliary grounding stakes. Measure the ground.
Wiring: Use the current clamp specified by the grounding resistance test instrument manufacturer to connect to the corresponding socket. Clamp the two clamps on the grounding conductor, and the distance between the two clamps should be greater than 0.25 meters.