Ground resistance measurement methods usually include the following: two-wire method, three-wire method, four-wire method, single-clamp method, and double-clamp method. Each method has its own characteristics. In actual measurement, the correct method should be selected to ensure accurate results.
1. Two-wire method
Conditions: There must be a known good ground, such as PEN, etc. The measured result is the sum of the resistance of the tested ground and the known ground. If the known ground is much smaller than the tested ground resistance, the measurement result can be used as the result of the tested ground.
Applicable to: places where grounding rods cannot be driven due to dense buildings or sealed concrete floors.
Wiring: Connect E+ES to the tested ground, and H+S to the known ground.
2. Three-wire method
Conditions: Two grounding rods are required: an auxiliary ground and a detection electrode. The distance between each grounding electrode should not be less than 20 meters.
The principle is to apply current between the auxiliary ground and the tested ground, measure the voltage drop between the tested ground and the detection electrode, and the measurement result includes the resistance of the measuring cable itself.
Applicable to: foundation grounding, construction site grounding, and lightning protection grounding.
Wiring: Connect S to the detection electrode, H to the auxiliary ground, and connect E and ES together to the tested ground.
3. Four-wire method
Basically the same as the three-wire method, but it replaces the three-wire method in low ground resistance measurements and when eliminating the influence of measuring cable resistance on the measurement results. During measurement, E and ES must be directly connected to the tested ground separately. This method has the highest accuracy among all ground resistance measurement methods.
4. Single-clamp measurement
Measures the ground resistance of each grounding point in a multi-point grounding system, and cannot disconnect the grounding connection to prevent danger.
Applicable to: multi-point grounding, where connections cannot be disconnected, measuring the resistance of each grounding point.
Wiring: Use a current clamp to monitor the current on the tested grounding point.
5. Double-clamp method
Conditions: Multi-point grounding, without driving auxiliary ground rods, measuring a single ground.
Wiring: Use the current clamp specified by the manufacturer and connect it 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.
