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 has its own characteristics. In actual measurement, try to choose the correct method to make the measurement result accurate.
1. Two-line method
Condition: There must be a known well-grounded ground, such as PEN, etc. The measured result is the resistance sum of the measured ground and the known ground. If it is known that the resistance of the ground is much smaller than the measured ground, the measurement can be used as the result of the measured ground.
Scope of application: The areas where the building is dense or the cement floor is sealed and cannot be piled.
Wiring: E+ES is connected to the measured ground, H+S is connected to the known ground.
2. Three-line method
Condition: There must be two ground rods: an auxiliary ground and a probe electrode. The distance between each ground electrode is not less than 20 meters.
Principle: 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.
Scope of application: 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-line method
Basically the same as the three-wire method, it replaces the three-wire method when measuring low ground resistance and eliminating the influence of the measurement cable resistance on the measurement results. E and ES must be directly connected to the measured ground separately during measurement. This method is the most accurate of all earth resistance measurement methods.
4. Single clamp method
Measure the grounding resistance of each grounding point in the multipoint grounding, and do not disconnect the grounding connection to prevent a hazard.
Scope of application: multi-point grounding, can not be disconnected, measure the resistance of each grounding point.
Wiring: Monitor the current on the ground point under test with a current clamp.
5. Double clamp method
Conditions: Multi-point grounding, no auxiliary ground piles, and single grounding is measured.
Wiring: Use the current clamp specified by the manufacturer to connect to the corresponding socket, clamp the two clamps on the ground conductor, and the distance between the two clamps should be greater than 0.25 meters.
2. Common factors affecting the detection of grounding resistance
1. The soil resistivity is too large or has a sudden change. When testing in sandy soil sites with high soil resistivity and poor water absorption, the measured grounding resistance is often too large due to poor contact between the auxiliary test electrode and the soil. If the soil resistivity between the ground grid of the grounding device and the auxiliary ground pole changes abruptly, it will cause the auxiliary current or voltage loop to be open or nearly open, resulting in a very large measured resistance value, usually dozens or hundreds of times the normal value. gigantic.
2. The resistance of the test line itself is too large. Due to frequent bending or mechanical extrusion, part of the copper wire of the test wire is dislocated and broken, resulting in the high resistance of the test copper wire itself, and due to the existence of the protective sleeve, it is difficult to find, resulting in a large grounding resistance test value or unmeasurable .
3. Corrosion phenomenon. Due to the corrosion on the surface of the test point of the lightning protection device or the long use of the detection rod and the vise clamp, there is oxidation and corrosion, which will also affect the measured value.
4. Leakage current interference. With the widespread use of electrical and electronic equipment, such as the grounding of transformers in factories and comprehensive buildings, and the grounding of various electrical and electronic equipment, more and more stray currents flow into the ground. If the auxiliary test electrode is placed around it, a potential difference will be generated around the auxiliary ground electrode, which will affect the measurement accuracy.
5. The auxiliary ground electrode is located in the ground grid. For a single vertical grounding body or a combined grounding body with a small footprint, the distance between the current electrode and the grounding body under test can be 40m, and the distance between the voltage electrode and the grounding body under test can be 20m. For the network grounding body with a large area, the distance between the current electrode and the grounding body under test can be 2-3 times of the diagonal of the grounding grid. The density of buildings in modern cities is getting bigger and bigger, and the location of auxiliary ground poles is very limited. In the measurement of grounding resistance, it is sometimes difficult to meet the spacing requirements, and even the auxiliary poles are arranged in the ground grid. The resistance value is too small, or even a negative value.
6. Mutual inductance between test leads. When measuring the grounding resistance of a large-scale grounding grid, the voltage and current test lines are very long. If the distance is very close, the mutual inductance between the test lines will be large, which will cause a large measurement error.