What are the preventive measures against direct lightning?

Thundercloud's voltage on the ground is several million volts, high is tens of millions of volts, or even higher. The peak current of a lightning flash discharge on the earth is more than 30 kA. Its instantaneous power is very high. The power is large, so its destructive power is quite large.

Until now, direct lightning protection has used lightning rods, belts, cables, and nets as lightning receptors to draw down the lightning current, and then quickly and safely introduce it to the ground through a good grounding device.

Commonly used air-termination devices, such as lightning rods, belts, cables, and nets, are made of metal and installed at building points, such as roof ridges or tail corners, where they are most vulnerable to lightning strikes. The lightning protection net is a grid made of metal wires and bands, which is placed in the top space of the building or connected to the grid by the building roof ribs, and then reliably connected to the ground.

When a thunder cloud appears at high altitude, due to the electrostatic induction effect on the ground, it will inevitably carry the opposite charge to the thunder cloud, but the lightning-receiving equipment (lightning rod, lightning belt, lightning line, lightning net, etc.) are located at the buildings on the ground. The distance to Thunder Cloud is the closest, and it has a good electrical connection with the earth, so it has the same potential as the earth, so that the electric field strength in the space near the air-termination equipment is relatively large. It is easier to attract the power-consuming pilot, so that the main discharge is concentrated to the ground Therefore, the probability of being struck by lightning, especially near objects lower than it, is greatly reduced. The chance of lightning strike by the lightning receptor is greatly increased, so as far as the lightning receptor itself is concerned, it not only cannot avoid lightning strikes. On the contrary, it attracts more lightning strikes, and it is more subject to lightning strikes to protect the surroundings from lightning strikes.

Because the air-termination device has a good electrical connection with the earth, the charge energy accumulated on the earth is quickly neutralized with the charge of the thundercloud. In this way, the time of overvoltage caused by lightning strikes is greatly shortened. The danger of lightning strikes is greatly reduced.

During a lightning strike, the process by which Thundercloud discharges to the earth through the lightning receptor can be approximated by the RC discharge process. Because the ground and thundercloud are equivalent to a charged capacitor, as shown in Figure 1.5. The capacitance between the thundercloud and the ground is represented by the capacitor C. The resistance inside the thundercloud and the lightning current channel is represented by R1, and the resistance between the lightning receptor and the connection between it and the ground (including the resistance of the connection line and the ground Drift resistance) is represented by R2.

From the equivalent circuit diagram, it can be seen that the relationship between the current i and R and the high voltage on the air-termination device is suitable during the lightning strike.

RC discharge equation:

iR-Uc = 0

R = R1 + R2

In the formula: R1- resistance of thundercloud and thunder current channel;

R2-the air-termination system and the connection resistance between it and the ground.

The resistance of the lightning current source includes the resistance of the main discharge channel, which is about several thousand ohms. If a charged thundercloud is used as a power source, the lightning receptor to the ground is regarded as a load. Then, when discharging, it is equivalent to a power supply with several thousand ohms of internal resistance, connected to a load with only a few ohms of ground resistance and a little lead resistance (as shown in Figure 1.5). This power supply is generally several million volts. And tens of millions of volts, or even higher. The voltage of the lightning receptor to the ground during a lightning strike is the voltage of the thundercloud. The smaller the internal resistance (including the channel resistance) and the ground resistance (including the lead resistance) of the thundercloud, the smaller the ground resistance. The smaller the partial voltage value, the relative The safer it is. Therefore, theoretically, the smaller the grounding resistance of the lightning protection device is, the better, but if the grounding resistance is required to be small, the cost will be high. In engineering, it is often only required to achieve a sufficiently safe range. The above description indicates that the lightning protection device must have a grounding device that is sufficiently reliable and sufficiently small in grounding resistance, otherwise it will not only serve the function of lightning protection, but will increase the risk of lightning strikes.

It should be pointed out that the atmospheric changes are large-scale, and the occurrence of thunderclouds is also large-scale, and the movement of thunderclouds is governed by many variables. Many conditions are random. Therefore, the idea of ​​foolproof devices is foolproof. it's wrong. Lightning protection can only greatly reduce the possibility of being struck by lightning.