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How to measure ground resistance

time:2020/7/9   source:华天电力  reading:948 time

There are two important parameters in the grounding device: 1. Grounding resistance value; 2. Grounding network structure. It now appears that although the structure of the grounding grid and the equipotentiality of the system are important, the low-impedance grounding device is the basis for the normal and safe operation of the equipment, especially in the lightning protection grounding, to discharge dozens of KA lightning current in an instant When flowing to the ground, the smaller the grounding resistance is, the faster the stray current is. The high-potential retention time after a lightning strike is short, and the danger is small. In short, the smaller the grounding resistance, the better the effect and the safer the protected object.

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The accuracy of the grounding resistance measurement is an important factor in judging whether the grounding device is qualified. In our daily work, whether it is the engineering party or the testing organization and the contractor, there are objections to the measurement method of the grounding resistance, especially in different ways. The measurement values appearing during the measurement vary greatly, and I don’t know how to judge. It is worth mentioning that in some of our inspection agencies, there are even many inspection personnel who do not understand the measurement principle, which makes the measurement values inaccurate and there is a detection When the engineer of the organization used a 4102 electronic meter to test a ground network, he first poured two waters on the cement ground to put the voltage pole and current pole upward; then he used a 100M long 6 square multi-strand copper wire to connect to the ground electrode ( The wires are coiled together). The measurement result is 5 ohms. At my request, the 100M wire is unfolded, and the voltage and current are unchanged. The measured result is 2 ohms. Then insert the voltage and current poles in the evenly distributed soil Here, the measured result is 1.2 ohms, and then the 100M line is changed to 5M line, the measured result is 0.4 ohms. From the analysis of the above example, we can summarize the following conclusions:

1. The lengthening of the measuring line, especially when it is rolled together, due to the large inductance, the measured value is too high. Therefore, when we measure the ground value of high-rise buildings, the measurement of the ground value of the elongated wire is inaccurate. This is because there is a certain resistance between the ground lead of the high-rise building and the ground (R ground wire). The test line leading from the measuring point above the high-rise building to the ground instrument has a line inductance in the air, (WL) Therefore, the resistance value measured at the ground point of the high-rise building is R=R ground line+WL+R ground, ground measurement ground Resistance R=R ground.

The measurement data is more bounced than the ground measurement. This is because the length of the test line in the air is extended, just like an antenna directs some radio, electromagnetic clutter and other signals in the air to the instrument through the test line, which causes serious interference and makes the measurement data jump. The solution is to use a coaxial wire as the test lead, connect the coaxial wire and the core wire together, and connect it to the test point, and connect the shield wire of the other end of the coaxial wire to the C2 end of the instrument (Namely the current pole), connect the core wire of the coaxial line to the P2 terminal of the instrument (namely the voltage pole), which can better solve the interference effect caused by the too long lead when measuring the high-level grounding resistance.

2. When the 102 electronic meter measures the grounding body and penetrates the contact voltage and current pole with water, the contact resistance due to poor contact will affect the measurement true value. This method is only used when there is no place to insert the voltage and current pole, but The water must penetrate into the soil, and the voltage and current must be in good contact with the water to minimize contact resistance and errors.

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