Why can lightning rod prevent lightning strike?

In thunderstorm weather, when charged clouds appear over high-rise buildings, lightning rods and the top of high-rise buildings will induce a lot of charges. Because the needle of the lightning rod is sharp, the tip of the conductor always accumulates the most charge during electrostatic induction.

In this way, the lightning rod collects most of the charge. The lightning rod and these charged clouds form a capacitor. Because it is pointed, that is, the relative area of the two plates of this capacitor is very small, and the capacitance is also very small, that is to say, it can hold very little charge.

Moreover, it collects most of the charge, so when there is more charge on the cloud, the air between the lightning rod and the cloud is easily broken down and becomes a conductor.

In this way, the charged cloud and the lightning rod form a path, and the lightning rod is grounded, so that the lightning rod can lead the charge on the cloud to the earth, thus not causing danger to the high-rise building and ensuring its safety.

According to China's statistics, the maximum lightning current amplitude is about 300 kA, and the corresponding lightning height is 408 m. With the lightning point height of 400 m, the surface radii of lightning rod protection zones and scattered lightning strike zones with different heights can be obtained. See table 1.

The height of old houses in China is generally below 10 m, the height of lightning protection belt and lightning protection network is the same as that of houses, and the lightning protection height of short needles is 1 ~ 2 m, with no obvious scattering phenomenon; High-rise buildings and overhead lightning rods lead to an increase in lightning strike rate and disperse the existence of lightning strike areas.

China lightning protection scholars have never advocated the use of overhead lightning rods to protect buildings. They advocate using short roof nails and lightning protection belts to protect buildings, considering that this can not only attract lightning, but also avoid increasing scattered minefields.

Extended data:

I. Functions

Conventional lightning protection can be divided into direct lightning protection, inductive lightning protection and comprehensive lightning protection. The lightning protection device against direct lightning generally consists of three parts: lightning receptor, down conductor and grounding body; Lightning arrester is divided into lightning rod, lightning wire, lightning belt and lightning protection network.

The lightning protection principle with lightning rod as lightning receptor is that the lightning rod is connected with the ground through wires, forming an equal potential difference with the ground, and using its own height, the Thundurus electric field whose electric field intensity increases to the limit value is distorted, and it begins to ionize and discharge downward.

The lightning rod produces tip discharge under the action of strong electric field, forming upward pilot discharge; The two meet to form a lightning path, which is then poured into the earth to achieve lightning protection effect.

The lightning rod is actually a lightning needle, which can lead the surrounding lightning and discharge it in advance, and conduct the lightning current to the ground through its own grounding conductor, so as to prevent the protected object from being directly struck by lightning.

Second, discrimination.

The lightning rod is made of iron, copper (or silver), with its top facing the sky, connected to the lightning protection network and buried underground. In thunderstorm season, lightning enters the lightning protection network from the sky until it is buried, which is a facility to eliminate lightning strikes and protect buildings or instruments.

Mostly used in buildings, transformers, wires, computer rooms, transmitters, etc. Lightning rods are divided into two types: circular section and flat section. The base metal of cladding metal is divided into lead-clad steel, lead-clad copper, copper-clad steel and lead-clad steel lightning rod.

Lightning rods are used in high-rise buildings, chimneys or oil tanks. The down conductor can be connected by lightning rod. The lightning rod consists of a needle body and an installation structure. The needle is made of stainless steel; The needle body must be made of copper-clad steel round bars or steel pipes.

Lightning protection zone refers to the wires laid along the ridge, gable, ventilation pipe and flat roof edge, which are most vulnerable to lightning strike. When the roof area is large, lightning protection network is used.

This is to protect the surface of the building from damage. Galvanized round steel or flat steel should be adopted for lightning protection network and lightning protection belt, and round steel is preferred, with a diameter of not less than 8mm, a width of not less than 12mm and a thickness of not less than 4 mm. ..

References:

Baidu encyclopedia-lightning rod