Ground resistance measurement principle and measurement method

Principle and method of ground resistance measurement
There are usually several methods for measuring ground resistance: two-wire method, three-wire method, four-wire method, single clamp method and double clamp method. Each has its own characteristics. When measuring, try to choose the correct method to make the measurement result correct. 
　　(Take the 8986 and 9066 ground testers of BEHA in Germany as an example)
　　1.  Two-wire method
　　Condition: There must be a known well-grounded ground, such as PEN, etc. 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 used as the result of the measured ground.
　　Applicable to: areas with dense buildings or sealed cement ground where ground piles cannot be driven.
　　Wiring: E+ES is connected to the measured ground, and H+S is connected to the known ground.
　　2.  Three-wire method
　　Condition: There must be two ground rods: an auxiliary ground and a detection electrode.  The distance between each grounding electrode is not less than 20 meters. 
　　The principle is to add current between the auxiliary ground and the measured ground, measure the voltage drop between the measured ground and the detection electrode, and the measurement result includes the resistance of the measurement cable itself.
　　Applicable to: foundation grounding, construction site grounding and lightning protection grounding.
　　Wiring: S is connected to the detection electrode, H is connected to the auxiliary ground, and E and ES are connected to the measured ground. .
　　3.  Four-wire method
　　Basically the same as the three-wire method, when measuring low ground resistance and eliminating the influence of the measurement cable resistance on the measurement result, E and ES must be directly connected to the measured ground separately. This method is the most accurate of all ground resistance measurement methods.

　　4.  Single clamp measurement
　　Measure the ground resistance of each grounding point in multi-point grounding, and the ground connection cannot be disconnected to prevent danger.
　　Applicable to: multi-point grounding, the connection cannot be disconnected, and the resistance of each grounding point is measured.
　　Wiring: Use a current clamp to monitor the current at the measured grounding point.
　　5.  Double clamp method
　　Conditions: multi-point grounding, no auxiliary ground piles, and measure a single ground.
　　Wiring: Use the current clamp specified by the 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