Grounding resistance test - electrical performance test

Date：2021-10-29 17:05:00Views：1391

Introduction to grounding resistance test

Grounding resistance is an important parameter used to measure whether the grounding state is good. It is the resistance encountered by the current flowing from the grounding device to the earth and then flowing through the earth to another grounding body or spreading to a distance. It includes the resistance of the grounding wire and the grounding body itself, and the contact resistance between the grounding body and the earth, And the earth resistance between the two grounding bodies or the earth resistance from the grounding body to infinity. The grounding resistance directly reflects the good contact between the electrical device and the "ground", and also reflects the scale of the grounding grid. The concept of grounding resistance is only applicable to small grounding grid; With the increase of the floor area of grounding grid and the decrease of soil resistivity, the role of inductive component in grounding impedance is becoming larger and larger. Large grounding grid should adopt grounding impedance design.

What are the influencing factors and results of grounding resistance measurement

1. Geological Influence

Different geology around the grounding grid will have a certain impact on the test results of grounding resistance, because the soil composition, tightness, dry and wet in geology are different. If it is dispersed, its current will also be stray, which will have a great impact on the test results. In this case, if you want to measure well, you can measure it in different places, Then calculate the average value.

2. Test line influence

If there is an error in the direction and distance of the grounding wire, it will also affect the test results. Therefore, it is necessary to find the correct test point.

3. Influence of grounding resistance

If the resistance of the auxiliary grounding electrode is large, it will have some impact on the measurement results. It is necessary to pour water and use resistance reducing agent to reduce the resistance.

4. Interference effect

When testing the grounding resistance, it is easy to be disturbed. At this time, anti-interference measures need to be taken. For example, the setting out direction can be adjusted to less runout of the instrument reading.

Grounding resistance test requirements:

a. For AC working grounding, the grounding resistance shall not be greater than 4 Ω;

b. For safe working grounding, the grounding resistance shall not be greater than 42;

C. For DC working grounding, the grounding resistance shall be determined according to the specific requirements of the computer system;

d. The grounding resistance of lightning protection ground shall not be greater than 10 Ω;

e. For shielding system, if combined grounding is adopted, the grounding resistance shall not be greater than 1 Ω.

Grounding resistance test method

The measurement methods of grounding resistance can be divided into voltage ammeter method, ratio meter method and bridge method. According to the specific measuring instruments and the number of poles, it can be divided into: hand-operated ground resistance meter method, clamp type ground resistance meter method, voltmeter and ammeter method, three pole method and four pole method.

When measuring the grounding resistance, some factors cause the inaccurate grounding resistance:

(1) The soil composition around the grounding grid is inconsistent, the geology is different, the compactness and the degree of dryness and wetness are different, and it is dispersive. The stray current on the ground surface, especially the overhead ground wire, underground water pipe, cable sheath, etc., has a great impact on the test. Solution: take different points for measurement and take the average value.

(2) The direction of the test line is wrong and the distance is not long enough. Solution: find the test direction and distance.

(3) The resistance of auxiliary grounding electrode is too high. Solution: pour water on the ground pile or use resistance reducing agent to reduce the grounding resistance of current electrode.

(4) The contact resistance between the test clip and the grounding measuring point is too large. Solution: polish the contact point with a file or sandpaper, and fully clamp the polished contact with a test wire clamp.

(5) Interference effects. Solution: adjust the setting out direction and try to avoid the direction with large interference to reduce the runout of the instrument reading.

(6) Instrument use problems. The battery is insufficient. The solution: replace the battery.

(7) Instrument accuracy decreases. Solution: recalibrate to zero.

The accuracy of the test value of grounding resistance is one of the important factors to judge whether the grounding is good. Once the test value is inaccurate, it will either waste human and material resources (the measured value is too large) or bring potential safety hazards to the grounding equipment (the measured value is too small).