In sandy and stony soils with high ground resistivity, in order to meet the requirements of low grounding resistance, a grounding grid composed of multiple grounding bodies in parallel is often used. But sometimes a lot of steel is required, and the grounding area is very large. It is often difficult to achieve the required ground resistance. At this time, the earth resistivity of the soil near the grounding body can be reduced, and the purpose of reducing the grounding resistance can also be achieved. How to use a ground resistance tester in harsh environments?
1. Use soil with low resistance coefficient (ie soil replacement method)
Clay, peat, black soil and sandy clay can be used to replace soil with high original resistance coefficient, and coke and charcoal can also be used if necessary. The replacement range is within 1 to 2m around the grounding body and within 1/3 of the length of the grounding electrode near the ground. After this treatment, the grounding resistance can be reduced to about 3/5 of the original.
2. Use artificial treatment methods such as adding salt
Add salt, cinder, carbon dust, furnace ash, coke ash, etc. to the soil around the grounding body to improve the conductivity of the soil. Because table salt has a good effect on improving soil resistance coefficient, it is less affected by seasonal changes, and the price is low.
3. Use external introduction to ground
Especially in hilly areas, when the grounding resistance value is small and difficult to reach on the spot, if there is a water source or soil with low resistivity nearby, it can be used to make grounding electrodes or lay underwater grounding nets. Then use a ground wire (such as flat steel strip) as the external ground. However, the grounding resistance tester needs to pay attention that the external grounding device should avoid pedestrian passages to prevent electric shock from the step voltage; when crossing the road, the buried depth of the outer lead should be greater than or equal to 0.8m.
4. Conductive concrete
The carbon fiber is mixed into the cement and used as a ground electrode. For example, about 100 kg of carbon fiber is mixed into 1 cubic meter of cement to make a hemispherical (1 meter diameter) ground electrode. After measurement, its power frequency grounding resistance (compared with ordinary concrete) can usually be reduced by about 30%. This method is often used in lightning protection and grounding devices. In order to further reduce the impact grounding resistance value, the needle-shaped grounding electrode can also be embedded in the conductive concrete at the same time, so that the discharge corona continuously fluctuates from the needle tip and carbon fiber, which has a significant effect on grounding and reduces the impact grounding resistance value.
5. Chemical treatment of resistance reducing agent
The drag-reducing agent with carbon powder and quicklime as the main raw materials does not contain dielectrics and will not be lost due to groundwater, and can be used in the soil for a long time. Therefore, a long-term pollution-free and stable low grounding resistance (about 1/2 lower than before using the resistance reducing agent to treat the soil) can be obtained. For hard rock slabs, the method of adding drag reducer to buried wires is very effective. Compared with only buried wires, the grounding resistance of the grounding resistance tester can be reduced by about 40%. This method can achieve good results as long as the powder drag reducer or long-acting drag reducer is sprinkled in the trenches excavated and laid with ground wires, and then backfilled with old soil.