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The composition and working principle of lightning protection system

2022-08-12

2022-01-06

Principle of lightning protection and grounding:

In the grounding system, grounding is the most important part of lightning protection technology, whether it is direct lightning, induced lightning, or other forms of lightning, it will ultimately send lightning current into the earth. Therefore, it is impossible to reliably avoid lightning without a reasonable and good grounding device. The smaller the grounding resistance, the faster the dissipation, the shorter the holding time of the high potential of the object struck by lightning, and the less dangerous.

The lightning protection grounding device includes the following parts:
1) Lightning receiving devices: metal poles (air-termination devices) that directly or indirectly receive lightning, such as lightning rods, lightning protection belts (nets), overhead ground wires and lightning arresters, etc.
2) Grounding wire (down-lead): the metal conductor used to connect the lightning receiving device and the grounding device.
3) Grounding device: the sum of the grounding wire and the grounding body. Grounding body refers to resistance reducing agent, ion grounding electrode, flat steel, etc.

The composition of the lightning protection system:
The lightning protection system mainly consists of lightning rods (lightning rods, lightning strips, lightning wires and lightning nets), down conductors and grounding devices. The lightning resistance level of a building refers to the current (Unit: Ka) when the building's lightning protection system can withstand the maximum lightning current impact without being damaged.

Lightning rod: suitable for protecting buildings or structures with a high ratio of thinness, open-air transformer and distribution equipment, power lines, etc. It can be made of Ф25 galvanized steel or SC40 steel pipe. The upper end of the needle is flattened and tinned to facilitate the tip discharge. Independent lightning rods are suitable for protecting low-rise warehouses and factories, especially for those occasions where lightning protection conductors are required to be isolated from various metals and pipelines in buildings. Sea urchin-shaped multi-needle lightning rods can also be used.

Lightning protection belt and lightning protection net: Lightning protection belt refers to the wires laid along the roof ridge, gable, ventilation duct, and the edge of the flat roof that are most likely to be struck by lightning. When the roof area is large, use lightning protection nets. It is to protect the surface of the building from being damaged. The lightning protection net and lightning protection belt should be galvanized round steel or flat steel. Round steel should be preferred. Its diameter should not be less than 8mm, and the width of flat steel should not be less than 12mm. Should be less than 4mm. The lightning protection cable is suitable for lightning protection of long-distance high-voltage power supply lines. Overhead lightning protection wires and lightning protection nets should adopt galvanized steel strands with a cross-sectional area greater than 35mm2.

Down Conductor: Divided into two forms of concealed installation and exposed installation. Concealed down conductors usually use structural column steel bars as down conductors, but the diameter of the steel bars cannot be less than 12mm. And when the main reinforcement in the column is used as the down conductor, the IEC specification states that "it is usually not necessary to install a special ring conductor to connect the down conductors, because the steel bars connected in the reinforced concrete horizontal beam can achieve this function." When special flat steel is used as down conductors in high-rise buildings, it is difficult to lay on the one hand, and on the other hand, the number of down conductors is small, and the current flowing through is relatively large, which is likely to cause counterattack accidents due to high potential. Therefore, it is not a good practice for high-rise buildings.

Grounding device: The grounding electrode in the grounding device generally uses Ф19 or Ф25 round steel or L40x4 or L50x5 angle steel. For steel pipe, it is G50. The buried depth of the grounding electrode is not less than 0.6m; the length of the vertical grounding body is not less than 2.5m, and the spacing is not less than 5m. Between the two grounding poles, the grounding bus bar, that is, flat steel welding, is used. In order to prevent damage to the human body by the step voltage, the distance between the grounding body and the outer wall shall be no less than 3m, and the sidewalk shall be no less than 1.5m.

The grounding electrode can also be smashed into a circle of vertical grounding body around the building, that is, the surrounding grounding method. At this time, it is not necessary to leave the outer wall 3m, and it is reasonable to set the outer edge of the trench close to the foundation of the building. Because it is close to the base steel bar, it can achieve the effect of equalizing the potential. However, if the main reinforcement of the building can be used as the grounding body, the effect will be better, not only saving steel, but also lower grounding resistance.