Thunder and lightning discharges may occur between clouds or inside clouds, or between clouds and ground; in addition, internal surges caused by the use of many large-capacity electrical equipment can affect the power supply system (China's low-voltage power supply system standard: AC 50Hz 220/380V) and The impact of electrical equipment and the maintenance of lightning protection and surge protection have become the focus of attention.　　
The lightning discharge between the cloud layer and the ground consists of one or several separate lightnings, each of which carries a number of currents with high amplitude and short duration. A typical lightning discharge will consist of two or three lightning strikes, with a time interval of approximately one-twentieth of a second between each lightning strike. Most lightning currents fall between 10,000 and 100,000 amperes, and their duration is generally less than 100 microseconds.　　 The use of large-capacity equipment and frequency conversion equipment within the power supply system has caused increasingly serious internal surge problems. We put it back as the effect of transient overvoltage (TVS). Any electrical equipment has a permitted scale of power supply voltage. Sometimes even a very narrow overvoltage surge can cause the power supply of the equipment or be completely damaged. This is the effect of transient overvoltage (TVS) damage. Especially for some sensitive microelectronic devices, sometimes a small surge may cause fatal damage.　　
The influence of power supply system surge 　　
The origin of power supply system surge is divided into external (causes of thunder and lightning) and internal (start and stop of electrical equipment, faults, etc.).　　
Lightning to ground lightning may be effective in low-voltage power supply systems in two ways: 　　
(1) Direct lightning strike: The lightning discharge directly hits the components of the power system, injecting a large pulse current. The probability of seizures is relatively low.　　
(2) Direct lightning strike: The lightning discharge hits the ground around the equipment and induces a moderate current and voltage on the power line.　　
The cause of the internal surge is related to the start and stop of the equipment inside the power supply system and the malfunction of the power supply network: 　　
The power supply system will cause internal surges due to the start and stop of high-power equipment, line faults, switching actions, and the operation of variable frequency equipment, which will bring adverse effects to electrical equipment. In particular, microelectronic devices such as computers and communications have brought fatal impacts. Even if there is no permanent equipment damage, the abnormal operation of the system and the stoppage will bring serious results. For example, nuclear power plants, medical systems, large-scale factory automation systems, securities trading systems, telecommunications bureau switches, network key, etc.　　
Direct lightning strikes are the most serious thing, especially if the lightning strikes close to the user entrance and crowd out the power line. When these things happen, the squeezing out power line voltage will rise to hundreds of thousands of volts, generally causing a flashover. The interval of lightning current transmission on the power line is one kilometer or more, and the peak current around the lightning strike point can reach 100kA or more. The current of the low-voltage line at the user inlet can reach 5kA to 10kA per phase. In areas with frequent thunder and lightning activities, power facilities may be directly struck by thunder and lightning several times a year, causing serious lightning currents. Regarding the choice of underground power cables for power supply or in areas with infrequent thunder and lightning activities, the above-mentioned things rarely happen.　　
The probability of direct lightning strikes and internal surges is high, and most of the electrical equipment damage is related to them. Therefore, the focus of power surge prevention is to absorb and suppress this part of the surge energy.　　
Regarding the low-voltage power supply system, transient over-voltage (TVS) maintenance caused by surges is best done in a graded maintenance method. Starting from the entrance of the power supply system (for example, the main power distribution room of the building), the surge energy is gradually absorbed, and the transient overvoltage is suppressed in stages.　　
[The first line of defense] It should be a large-capacity power supply anti-surge maintainer connected between the incoming phases of the user's power supply system and the earth. Generally, this level of power supply maintenance is required to have a maximum impact capacity of more than 100KA/phase, and the required constraint voltage should be less than 2.8kv. We call it CLASS I power surge preventive maintenance device (SPD for short). These power anti-surge maintainers are specially designed to withstand the high current and high energy surge energy absorption of lightning and induced lightning, and can shunt a large amount of surge current to the earth. They are only for the restrained voltage (when the inrush current flows through the SPD, the maximum voltage appearing on the line becomes the restrained voltage) for medium-level maintenance, because the CLASS I maintainers mainly absorb large surge currents. They alone cannot completely maintain the sensitive electrical equipment inside the power supply system.　　
[The second line of defense] should be the power surge protector installed at the branch distribution equipment that supplies power to important or sensitive electrical equipment. These SPDs have a more complete absorption of the remaining surge energy through the surge arrester imported from the user's power supply, and have an excellent suppression effect on the transient overvoltage. The power surge protector used here requires a maximum impact capacity of 40KA/phase or more, and the required restraint voltage should be less than 2.0kv. We call it a CLASS II power surge protector. The general user power supply system can meet the requirements of the operation of the electrical equipment after the second level of maintenance.
[The last line of defense] A built-in power surge protector can be used in the internal power supply of electrical equipment to achieve the purpose of completely eliminating small transient transient overvoltages. The power surge protector used here requires a maximum impact capacity of 20KA/phase or lower, and the required restraint voltage should be less than 1.8kv. Regarding some particularly important or particularly sensitive electronic equipment, it is necessary to have the third level of maintenance. At the same time, it can also maintain the electrical equipment from the transient overvoltage that occurs inside the system.