What is ground resistance? Several methods for measuring ground resistance

1. What is ground resistance?

Ground resistance is the resistance encountered when current flows from a grounding device to the earth and then flows through the earth to another grounding body or spreads to a distance. It includes the resistance of the grounding wire and the grounding body itself, and the contact resistance between the resistance of the grounding body and the earth. And the resistance of the earth between two ground bodies or the resistance of the earth from the ground body to infinity. The size of the ground resistance directly reflects the good degree of contact between the electrical device and the "ground", and also reflects the size of the ground network. In single-point grounding systems and conditions with strong interference, the measurement method of auxiliary ground pole can be used for measurement. There are mainly three types of ground resistance.

1. Protective grounding: The metal shell of electrical equipment, concrete, poles, etc. may be charged due to damage to the insulation. Grounding is provided to prevent this situation from endangering personal safety.

2. Anti-static grounding: Ground flammable oil and natural gas storage tanks and pipelines, electronic equipment, etc. to prevent the dangerous effects of static electricity.

3. Lightning protection grounding: In order to introduce lightning into the ground, the grounding end of the lightning protection equipment (lightning rod, etc.) is connected to the earth to eliminate the harm of lightning overvoltage to electrical equipment, personal property, and grounding. It is also called overvoltage protection grounding.

2. Several methods for measuring ground resistance

1. Two-line method

Conditions: There must be a known well-grounded ground, such as PEN, etc. The measured result is the sum of the resistances of the measured ground and the known ground. If the resistance of the known ground is much smaller than the ground being measured, the measurement result can be taken as the result of the ground being measured.

Suitable for: areas with dense buildings or cement floors that are sealed and unable to drive ground piles.

Wiring: E+ES is connected to the measured ground, H+S is connected to the known ground. 

2. Three-line method

Conditions: There must be two ground rods: an auxiliary ground and a detection electrode. The distance between each grounding electrode shall not be less than 20 meters. The principle is to add current between the auxiliary ground and the measured ground, and measure the voltage drop between the measured ground and the detection electrode. The measurement results include measuring the resistance of the 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, E and ES are connected to the measured ground.

3. Four-wire 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. During measurement, E and ES must be individually and directly connected to the ground being measured. 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 a multi-point grounding, and do not disconnect the ground connection to prevent danger.

Suitable for: multi-point grounding, the connection cannot be disconnected, measuring the resistance of each grounding point.

Wiring: Use a current clamp to monitor the current at the ground point under test.

5. Double clamp method

Conditions: Multi-point grounding, no auxiliary ground stakes, single grounding measurement.

Wiring: Use the current clamp specified by the manufacturer to connect to the corresponding socket, and clamp the two clamps on the ground conductor. The distance between the two clamps should be greater than 0.25 meters.

3. How to use the ground resistance tester

1. Preparation for using ground resistance tester     

① Familiarly read the instruction manual of the ground 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 dirt and rust stains on its surface that affect the conductivity.

③ Disconnect the connection point between the grounding main line and the grounding body or the connection points of all grounding branches on the grounding main line, so that the grounding body becomes independent from any connection relationship.

2. Measurement steps using a ground resistance tester

(1) Insert two grounding probes into the ground 20m and 40m away from the grounding body along the radiation direction of the grounding body, with an insertion depth of 400mm, as shown in the figure below.

▲ Illustration of ground resistance test: a) actual operation b) equivalent principle

(2) Place the ground resistance measuring instrument flatly near the ground body and perform wiring. The wiring method is as follows:

① Use the shortest dedicated wire to connect the grounding body to the terminal "E1" of the grounding measuring instrument (a three-terminal measuring instrument) or to the common terminal after short-circuiting C2 and "(a four-terminal measuring instrument).

② Use the longest special wire to connect the measurement probe (current probe) 40m away from the ground body to the wiring button "C1" of the measuring instrument.

③ Use the remaining length of the special wire in the middle to connect the measuring probe (potential probe) ⒛m away from the ground body to the terminal "P1" of the measuring instrument.

(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 "magnification scale" (or coarse adjustment knob) to the maximum multiple, and slowly turn the generator handle (the pointer begins to shift), and at the same time rotate the "measuring dial" (or fine adjustment knob). Adjust the knob) so that the galvanometer pointer points to the center line.

(5) When the pointer of the galvanometer is close to balance (the pointer is close to the center line), shake the handle faster to make the rotation speed reach more than 120r/min. At the same time, adjust the "measuring dial" so that the pointer points to the center line.

(6) If the reading of the "measuring dial" is too small (less than 1) and difficult to read accurately, it means that the magnification scale multiple is too large. At this time, the "magnification scale" should be set to a smaller multiple, readjust the "measuring dial" so that the pointer points to the center line and read the accurate reading.

(7) Calculate the measurement results, that is, R = "magnification scale" reading × "measurement dial" reading.