The earth's magnetic field changes, and the atmosphere always contains a large number of positive and negative charged particles in gaseous form. According to the lightning detection editor, the movement of the air flow makes the atmosphere conductive. The generation movement of these charged particles and the separation and aggregation of different charged particles cause the atmosphere to generate an electric field, which causes lightning. When a charged thundercloud is close to a protrusion on the ground or a cloud layer with a different charge, a violent discharge will occur between them. During the discharge process, the arc in the lightning channel generates high temperature, which can cause the surrounding air to expand at a rapid rate, and then cool down quickly after the heat disappears, causing the air to contract at a rapid rate, thereby causing violent acoustic vibration. The strong flashes and explosions that appear at the discharge site are the lightning flashes that people see and hear.



The discharge current of lightning is more than 20,000 amps, the discharge voltage is about l to 10 billion volts, the discharge time is about one thousandth of a second, and the local range will produce high temperatures above 6000 to 10,000 degrees, and the average power of a single lightning strike reaches 200 Above 100 million watts, its energy is equivalent to the output power of a small nuclear power plant. One can imagine the degree of damage caused by such huge energy.



There are many forms of lightning strikes, but those directly endangering the security of the air defense command and control center and the air defense sirens mainly have the following four forms:



Direct thunder. The rapid discharge of the charged cloud layer to a certain point on the ground will directly cause serious damage to the objects in the discharge path (such as alarms, transceiver antennas, buildings themselves, power distribution facilities and related personnel), and cause fires.



Lightning wave intrusion. Lightning is not directly discharged on the alarm or control equipment itself, but on the cables that are placed outside the building. The lightning wave or overvoltage on the cable spreads along the cable path almost at the speed of light and invades the indoor electronic equipment. The indoor electronic equipment is often damaged before people hear the thunder. Because electronic equipment composed of large integrated components, such as civil air defense alarm control systems and computer network systems, generally have the shortcomings of transient overvoltage and overcurrent tolerance.



Induced overpressure. Lightning occurs near the civil air defense warning facilities. Even if the ground is not directly discharged, the discharge phenomenon only occurs between clouds, and a huge magnetic storm will also be generated. A strong transient magnetic field is formed in the lightning area. We call it electromagnetic pulse or electromagnetic radiation. Lightning electromagnetic pulses (LEMP) are less harmful, causing network equipment to be interfered, data loss, misoperation or temporary paralysis of the system. In severe cases, they break down electronic components and burn circuit boards.



Geoelectric counterattack. If lightning strikes a building or grounding grid with a lightning protection device directly, the voltage will be raised by tens of thousands or hundreds of thousands of volts within a few seconds. A highly destructive lightning current will flow from each device node to the power supply system or various Network signal system, and can penetrate the earth insulation and flow to other networks. Thereby, the electric counterattack will damage the electronic equipment, and at the same time, it will induce a high potential to the conductive circuit that does not implement the equipotential connection and generate a spark discharge phenomenon. It may also cause the conversion of electromagnetic energy inside the power system, resulting in internal overvoltage, and eventually the current will endanger personal safety and damage electronic equipment in a surge manner.