When lightning strikes, the strong current and the electromagnetic pulse generated by the device can be generated through the conduction, induction, coupling and other methods in the electronic equipment over-voltage, over-voltage along the power line or signal line transmission, the formation of lightning surge.
Usually, the lightning will exposed to high voltage power line induction in abroad, this voltage will not only directly to the equipment, the surge current in the power transmission line, electromagnetic induction will surge signal line is coupled to the surrounding. Similar to the surge of electronic products will cause great damage, so the product has a certain surge immunity.
Three types of lightning transient surge
1 direct lightning, lightning on external (outdoor) power grid, large current flows into the grounding resistance or the external circuit impedance and voltage, breaking force is very strong, there is no protection against direct lightning equipment.
2 conduction lightning strike: the transmission of the overhead line from the distance, because the equipment connected to the power network has different suppression ability to the over-voltage, so the conduction overvoltage energy decreases with the extension of the line.
3 induction lightning electromagnetic field in the clouds or between clouds or lightning strike in attachment objects caused by lightning, acting on the conductor, high voltage induction, the overvoltage is very steep and fast decay frontier.
Among them, there are two kinds of inductive lightning surge induction:
Static induction electric field of lightning surge with negative charges generated by lightning, induces a positive charge is bound on the wire electric field. When Leiyun discharge, charge moment in the clouds is greatly reduced or disappeared, the lines are bound to positive charge lost instantly bound, in the electric energy under the effect of surge on the line.
Electromagnetic induction: surge current of lightning magnetic field formed around the magnetic field changes with time, excitation induced electromotive force around the conductor, lightning occurs in the power supply line or near lightning, will have such a situation.
In addition, there is a relatively rare earth potential mechanism of lightning surge, the industry rebound, often called the "rebound thunder", when the equipment installation of surge protection devices, surge voltage of a cable equipment, surge protector will trigger surge energy to bypass the earth, at this time, due to the large current flowing through the equipment ground, ground potential of the device increases suddenly. At this time the other equipment connected to the device, the ground potential is still zero, between the two devices produced a high common mode voltage, the common mode voltage may damage the device interface.
Two, lightning induction factors
Lightning induction is the energy received on the guidance line, and its influence factors are as follows:
1 the amount of charge thundercloud directly affect the current size, size, and the influence of electromagnetic field intensity.
2 lightning current steepness in the wave tail under the same steepness, harmonic energy is more abundant, high voltage due to the higher.
The frequency and amplitude of the 3 lightning current decrease with the increase of the frequency.
4 there is a direct relationship between the length of the different wires and the overvoltage. With the increase of the length of the wire, the peak value of the overvoltage increases, but the increase is smaller when the length is longer.
5 wire from the cloud height and over voltage is directly related to height increase, induction voltage increases rapidly, this is because the high from the ground, and a wire loop area is large, can more electromagnetic energy coupling.
6 when the two ends of the wire are connected with different resistance, the magnitude of the induced over-voltage waveform is affected. When the starting point is the matching resistance, the terminal induced voltage increases with the increase of the terminal resistance.
7 the influence of the electromagnetic environment around the conductor, the better the shielding protection, the smaller the wire induced over-voltage.
Three, surge test standards
Surge test is to simulate the transient phenomena of the power supply line and the signal line induction when the test equipment is attached to the device. This kind of energy is usually very large, the surge test is based on the international standard is IEC61000-4-5, corresponding to the national standard is GB/T17626.2:200X electromagnetic compatibility test and measurement technology surge (impact) immunity test.
The standard is applicable to the electronic equipment in the normal working state, the induction lightning strike, conduction lightning surge voltage test. The standard does not test the insulation resistance to high pressure, so it does not consider the direct lightning.
Among them, the direct output voltage / current waveform of surge signal generator is described in detail, corresponding to the connection to the symmetrical communication line port, should use the 10/700 s combined wave generator. In other cases, in particular, to connect to the power line and the short distance signal line port, should use 1.2/50 s combination wave generator. Communication systems generally use other standards, such as the United States using Telcordia GR-1089 (the provisions of the open circuit voltage waveform 2/10 s, short circuit current waveform of 10/1000 s).

Four, test level and selection
The rank test range is shown in Table 1

Table 1 test level form from IEC61000-4-5".
The level of application depends on the environment, which is subject to the possibility of surge environment and installation conditions, the main categories are as follows:
Level 1: better protection of the environment, such as the control room of a factory or power station.
Level two: there is a certain protection of the environment, such as the factory without strong interference.
Level three: general electromagnetic harassment environment, the equipment does not require special installation requirements, such as the general environment of the cable, industrial premises.
Level Four: under strong interference environment, such as civil overhead lines, without the protection of high-voltage substation.
Level X: by user and manufacturer.
Five, surge generator
Surge generator circuit shown in Figure 1
Figure 1 combination wave generator "picture from IEC61000-4-5"
The internal resistance of combinationwave generator is 2 ohms. Different tests require different energy, which is regulated by external resistance. Specific for different location, differential mode power line between the injected energy maximum, external resistance is zero, the internal resistance is 2 ohms. The common mode surge voltage between the lines of the power supply is injected through a 10 external resistor. For the signal line test, the injected surge is injected through a series resistor of 40. Different resistance, the relation of voltage and current as shown in table 2.

The relationship between voltage and current resistance when table 2
Six, the two main surge test waveform
According to the test standards, the two main types of waveforms are 1.2/50 s (8/20 s) combined wave signal and 10/700 s (5/320 s) combined wave signal.
1 1.2/50 s (8/20 s) combined wave: 1.2/50 s voltage surge waveform (open circuit state) and 8/20 s current surge waveform (short circuit)

Figure 2 open circuit voltage waveform (1.2/50 s) image from IEC61000-4-5"

Figure 3 short-circuit current waveform (8/20 s) image from IEC61000-4-5"
2.10/700 s (5/320 s) combined wave: 10/700 s voltage surge waveform (open circuit state) and 5/320 s current surge waveform (short circuit)

Figure 4 open circuit voltage waveform (10/700 s) image from IEC61000-4-5"

Figure 5 short-circuit current waveform (5/320 s) image from IEC61000-4-5"
Two surge signal range:
1 pairs of communication networks and long-distance signal circuit port, recommended using 10/700 s shock wave. Common electronic devices such as RJ45, RS232, XDSL, RS485, security cameras, etc.
2 pairs of AC and DC power supply port and short distance signal circuit port, recommended using 1.2/50 s shock wave. Common electronic equipment such as LED lighting equipment, base stations, etc.
Seven, common equipment port surge resistance test
1 AC power supply voltage withstand level
Level 1: the differential mode applies the 2kV voltage positive and negative for each of the 5 times without damage; the common mode 4kV voltage is not damaged for the 5 time.
Test waveform: 1.2/50 s (8/20 s) combination wave
Test method: according to IEC61000-4-5.
2 DC power supply voltage withstand level
Level 1: the differential mode applies the 1kV voltage positive and negative for each of the 5 times without damage; the common mode 2kV voltage is not damaged for the 5 time.
Level 2: the differential mode applies the 0.5kV voltage positive and negative for each of the 5 times without damage; the common mode 1kV voltage is not damaged for the 5 time.
Test waveform: 1.2/50 s (8/20 s) combination wave
Test method: in accordance with IEC61000-4-5, level 2 for the basic requirements, if the device is not equipped with lightning protection, port tolerance level requirements should reach level 1.
3 signal voltage withstand level (signal line)
Level 1: the differential mode applies the 2kV voltage positive and negative for each of the 5 times without damage; the common mode 4kV voltage is not damaged for the 5 time.
Level 2: the differential mode applies the 1kV voltage positive and negative for each of the 5 times without damage; the common mode 2kV voltage is not damaged for the 5 time.
Level 3: the differential mode applies the 0.5kV voltage positive and negative for each of the 5 times without damage; the common mode 1kV voltage is not damaged for the 5 time.
Test waveform: 1.2/50 s (8/20 s) combination wave.
Test method: in accordance with IEC61000-4-5, level 3 for the basic requirements, if the line is more than 10m, no more than 30m, no lightning protection, port tolerance level requirements should reach level 1.
4 signal voltage withstand level (out of signal line).
Level 1: the differential mode applies the 4kV voltage positive and negative for each of the 5 times without damage; the common mode 4kV voltage is not damaged for the 5 time.
Level 2: the differential mode applies the 1kV voltage positive and negative for each of the 5 times without damage; the common mode 2kV voltage is not damaged for the 5 time.
Test waveform: 1.2/50 s (8/20 s) combination wave.
Test method: in accordance with the IEC61000-4-5, the construction of the line outside the signal cable, into the room should go through the first level of the distribution frame. The requirements of the test port are: to test the device port itself, should meet the level 2; in front of the signal port connection wiring frame (with a protection unit), in front of the test frame, need to meet the level 1.