Several commonly used methods to reduce grounding resistance According to technical design and engineering construction management experience, the following methods are currently used:
1 When the resistivity of soil or water deep in a deep buried ground electrode is low, a deep buried ground electrode can be adopted to reduce the ground resistance. This method is most effective on sandy soils. According to relevant data, the soil resistivity is 100% at 3m depth, 75% at 4m depth, 60% at 5m depth, 60% at 6m depth, 50% at 6.5m depth, and 9m at 9m depth. 20%, this method can not consider the increased resistivity of soil freezing and drying, but the construction is difficult, the volume of earthwork is large, the cost is high, and the difficulty is greater in rocky areas.
2 Soil replacement This method is to replace the soil with higher resistivity with lower resistivity soil (such as clay, black soil and sandy clay, etc.). The replacement range is within 0.5m around the grounding body and 1 / 3 places. However, this method of soil replacement requires much labor and man-hours.
3 Artificial soil treatment (chemical treatment of soil)
Add chemicals such as salt, charcoal, furnace ash, nitrogen fertilizer slag, calcium carbide slag, and lime to the soil around the grounding body to improve the conductivity of the soil around the grounding body. The use of table salt has different effects on different soils. For example, after sandy clay is treated with table salt, the soil resistivity can be reduced by 1/3 to 1/2, and the sand resistivity can be reduced by 3/5 to 3/4. The resistivity of sand is reduced by 7/9 ～ 7/8; for rocky soil, the conductivity can be increased by 70% after immersed in 1% common salt solution. Although this method has a low engineering cost and obvious effect, after the soil is artificially treated, it will reduce the thermal stability of the ground, accelerate the corrosion of the ground, and reduce the life of the ground. Therefore, in general, it is recommended to use it only under conditions that must be avoided.
4 The use of a ground resistance reducing agent to lay down the resistance reducing agent around the ground electrode can increase the overall size of the ground electrode and reduce the contact resistance with the surrounding earth medium, thereby reducing the ground to a certain extent. Ground resistance of the electrode. When the resistance reducing agent is used in a small area of concentrated grounding and a small grounding grid, its resistance reducing effect is more significant.
Drag reducer is a chemical drag reducer formulated from several substances. It is a strong electrolyte and moisture with good electrical conductivity. These strong electrolytes and water are surrounded by reticular colloids, and the spaces of the reticular colloids are filled with partially hydrolyzed colloids, so that it will not be lost with groundwater and rain, so it can maintain good electrical conductivity for a long time. This is a newer and aggressive approach to popularization.
5 Multiple external lead-through grounding devices can be used if there are rivers and lakes with good conductivity and not freezing near the grounding device. However, in the design and installation, the influence of the resistance of the grounding electrode main line must be taken into consideration. Therefore, the length of the external lead-in grounding electrode should not exceed 100m.
6 Use of water and water-reinforced concrete as a dispersing medium Make full use of hydraulic structures (water wells, pools, etc.) and other metal bodies in concrete in contact with water as natural grounding bodies, which can be used in underwater reinforced concrete structures Among the many reinforced meshes that have been bundled, some vertical and horizontal intersections are selected for welding and connected to the ground mesh.
When using a hydraulic structure as a natural grounding body still cannot meet the requirements, or it is difficult to use a hydraulic structure as a natural grounding body, it should be preferred to lay an external (artificial) grounding device in the nearest water (river water, pool water, etc.). (Underwater grounding grid), the grounding device should be laid in the place where the water flow rate is not high or in still water, and some large stones should be backfilled to fix it.
7 An extended horizontal grounding body is combined with the actual application of the project. After analysis, the results show that when the length of the horizontal grounding body increases, the influence of the inductance increases, which increases the impact coefficient. When the grounding body reaches a certain length, If the length is increased, the impact ground resistance will no longer decrease. Generally speaking, the effective length of the horizontal grounding body should not be greater than. The effective length of the grounding body is determined according to soil resistivity as shown in Table 1.
Table 1 Effective lengths of horizontal grounding bodies under different soil resistivities
Soil resistivity (Ωm) 500 1000 2000
Effective length of horizontal grounding body (m) 30 ～ 40 45 ～ 55 60 ～ 80
8 Deep grounding can be used when conditions permit. Use a drilling rig to drill holes (also can use exploration drilling), drill the steel pipe ground electrode into the well hole, and pour mud into the steel pipe and well.
When determining the specific measures to reduce the ground resistance in areas with high soil resistivity, a comprehensive and comprehensive analysis should be carried out based on the original local operating experience, climatic conditions, topographic features, and the level of soil resistivity, and determined through technical and economic comparison. Choose a reasonable method according to local conditions. In this way, not only the normal operation of the lines and equipment can be guaranteed, but excessive investment in grounding equipment engineering can be avoided.
9 Take sewage introduction In order to reduce the resistivity of the soil around the grounding body, the sewage can be led to the buried grounding body. The grounding body is a steel pipe, and a small hole with a diameter of 5mm is drilled in the steel pipe every 20cm to make water penetrate into the soil.