1. What is grounding resistance?
The grounding resistance is the resistance encountered by the current flowing into the ground from the grounding device and then flowing to another grounding body or spreading to a distance through the ground. It includes the resistance of the grounding wire and the grounding body itself, and the contact resistance between the grounding body and the ground. And the resistance of the ground between the two grounding bodies or the grounding resistance of the grounding body to infinity. The size of the grounding resistance directly reflects the good degree of contact between the electrical device and the “ground”, and also reflects the scale of the grounding grid. Under the conditions of single-point grounding system and strong interference, the measurement method of auxiliary grounding can be used for measurement. Ground resistance is mainly divided into the following three types.
Protective grounding: The metal casing of electrical equipment, concrete, poles, etc., may be charged due to insulation damage. The grounding is set up to prevent this situation from endangering personal safety.
Anti-static grounding: grounding flammable fuel oil, natural gas storage tanks and pipelines, electronic equipment, etc. to prevent the impact of static electricity hazards.
Lightning protection grounding: In order to introduce lightning into the ground, the grounding end of lightning protection equipment (lightning rods, etc.) is connected to the ground to eliminate the damage of lightning overvoltage to electrical equipment, personal property and grounding, also known as overvoltage protection grounding.
2. Several methods of grounding resistance measurement
Two-wire method
Condition: There must be a known good 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 result can be used as the result of the measured ground.
Applicable to: dense buildings or cement grounds and other areas where the seal cannot be piled.
Wiring: E+ES is connected to the measured ground, H+S is connected to the known ground.
Three-line method
Condition: There must be two ground rods: an auxiliary ground and a probe electrode. The interval between each ground 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.
Suitable for: 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.
Four-wire method
Basically the same as the three-wire method, replace 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 individually connected directly to the measured ground during measurement. This method is the most accurate among all grounding resistance measurement methods.
Single clamp measurement
Measure the grounding resistance of each grounding point in the multipoint grounding, and do not disconnect the grounding connection to prevent a hazard.
Suitable for: 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.
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 interval between the two clamps should be greater than 0.25 meters.
3. How to use the grounding resistance tester
Use the grounding resistance tester to prepare for work
①Familiarize yourself with the instruction manual of the grounding resistance measuring instrument, and fully understand the structure, performance and usage of the instrument.
② Prepare the necessary tools and all instrument accessories for measurement, and wipe the instrument and grounding probe clean, especially the grounding probe, be sure to clean the surface of the dirt and rust that affect the conductivity.
③ Disconnect the connection point between the grounding trunk line and the grounding body or the connection points of all grounding branch lines on the grounding trunk line, so that the grounding body is separated from any connection and becomes an independent body.
Measurement steps using a grounding resistance tester
(1) Insert the two grounding probes into the ground 20m and 40m away from the grounding body along the radiation direction of the grounding body, and the insertion depth is 400 mm, as shown in the figure below.
Grounding resistance test usage diagram: a) actual operation b) equivalent principle
(2) Place the grounding resistance measuring instrument flat near the grounding body and wire it up. The wiring method is as follows:
①Use the shortest special wire to connect the grounding body to the terminal “E1” of the grounding measuring instrument (the measuring instrument of the three-terminal button) or the common terminal (the measuring instrument of the four-terminal button) after short-circuiting with C2 and “.
②Connect the measuring probe (current probe) 40m away from the grounding body to the connecting button “C1” of the measuring instrument with the longest dedicated wire.
③ Connect the measuring probe (potential probe) ⒛m away from the grounding body to the terminal “P1” of the measuring instrument with the remaining special wire with the center length.
(3) After placing the measuring instrument horizontally, check whether the pointer of the galvanometer points to the center line, otherwise adjust the “zero adjuster” to make the pointer of the measuring instrument point to the center line.
(4) Set the “multiplier scale” (or coarse adjustment knob) to the maximum multiplier, and slowly turn the generator handle (the pointer begins to shift), while rotating the “measurement dial” (or fine Adjust the knob) so that the galvanometer needle points to the centerline.
(5) When the pointer of the galvanometer is close to the balance (the pointer is close to the center line), speed up the shaking of the handle to make the speed reach more than 120r/min, and adjust the “measurement dial” at the same time, so that the pointer points to the center line.
(6) If the reading of the “measurement dial” is too small (less than 1) and difficult to read accurately, it means that the magnification scale is too large. At this time, the “magnification scale” should be set to a smaller multiple, and the “measurement dial” should be re-adjusted so that the pointer points to the center line and an accurate reading is read.
(7) Calculate the measurement result, that is, R = “magnification scale” reading × “measurement dial” reading.
