The surge protector has an important parameter, 8/20us or 10/350us, which many people are unaware of. The main function of the surge protector is to respond to lightning strikes, including direct and induced lightning. The forms of direct lightning and inductive lightning have different energies, and their current waveforms are similar to 10/350us (direct lightning) and 8/20us (inductive lightning).

Therefore, artificially created currents of these two waveforms to simulate direct lightning and induced lightning, used to test the parameters of surge protective devices under this current waveform.

1、Lightning waveform
The two waveforms represent the curve of time and current, with 10/350us being the typical lightning current curve of lightning breaking through the earth and the lightning current curve of lightning directly attacking power lines and lightning rods. We generally refer to it as a direct lightning waveform.
Among them, 10 (microseconds) represents the time when the impulse pulse reaches 90% of the current peak, while 350 represents the time from the current peak to half peak, and 8/20 is also defined similarly.
8/20us is a typical electromagnetic pulse induced overvoltage caused by lightning breaking through the ground (lightning rod or adjacent lightning arrester), which is the current curve when the induced overvoltage breaks through and burns down the equipment. We generally refer to it as the induced lightning waveform.

2、The difference between direct lightning waveform and induction lightning waveform
Differences between standards and regulations: The IEC International Electrotechnical Commission standards IEC1024 "Tag_1 Lightning Protection Specification" and IEC1312 "Lightning Electromagnetic Pulse Protection" both implement 10/350uS and 8/20uS lightning waveforms.
The national standards GB11032-2000 "AC Lightning Arresters" and GB3482-3483-83 "Electronic Equipment Lightning Tests and Guidelines" all implement 8/20uS lightning waveform.
According to theoretical calculations, under the same lightning current action, the ratio of 10/350uS and 8/20uS lightning Joule energy is 17.5:1 (for example, if an 8/20uS lightning current of 10KA has a lightning Joule energy of 1000J, then a 10/350uS lightning current of 10KA has a lightning Joule energy of approximately 17500J), there is a fundamental difference between 10/350uS and 8/20uS lightning Joule energy.

According to the "new requirements" of IEC, when testing "surge protectors for direct lightning protection", 10/350 waveform impulse pulses should be used, while when testing "surge protectors for indirect lightning protection", 8/20 waveform should be used.

From this perspective, 10/350us and 8/20us each have their own characteristics. When choosing lightning protectors, we should choose the corresponding lightning protector based on the equipment to be protected, and the suitable one is the best.

3、Principles for selecting surge protective devices
1. The voltage protection level Up of SPD should always be less than the impulse withstand voltage Uchoc of the protected equipment, and greater than the maximum operating voltage Usmax of the power grid based on the grounding type, that is, Usmax considered at 80% of its value;

2. The leads at both ends of SPD and the protected equipment should be as short as possible, controlled within 0.5m;

3. If the Up of the incoming SPD, coupled with the induced voltage and reflected wave effect of the leads at both ends, is too high compared to the impulse withstand voltage of the protected equipment far away from it, a second level SPD should be installed at this equipment, and its nominal discharge current In should not be less than 8/20 μ S 3kA; When the distance between the incoming SPD and the protected equipment is not more than 10m, if the Up of the SPD and the induced voltage of its two end leads are less than 80% of the Uchoc of the equipment, SPD can generally not be installed at the equipment;

4. When there is a distribution board between SPDs installed according to the requirements of point 3 above, if the Up of the first level SPD and the induced voltage of its two end leads cannot protect the equipment inside the distribution board, a second level SPD should be installed inside the distribution board, and its nominal discharge current In should not be less than 8/20 μ S 5kA;

5. When installing SPDs in multiple locations on the line, the length of the line between voltage switch SPDs and voltage limiting SPDs should not be less than 10m, and the length of the line between voltage limiting SPDs should not be less than 5m. For example, if the distance between the protected equipment and the distribution center is relatively close, more wires can be intentionally wound on the line laying;

6. When the distance between the SPD at the incoming end and the protected equipment is greater than 30m, another SPD should be installed as close as possible to the protected equipment, with a flow capacity of 8kA;

7. When selecting SPD, attention should be paid to ensuring that it does not burn out due to power frequency overvoltage, as SPD is designed to prevent transient overvoltage（ μ S level), power frequency overvoltage is a transient overvoltage (ms level), and the energy of power frequency overvoltage is several hundred times that of transient overvoltage. Therefore, attention should be paid to selecting SPD with higher power frequency working voltage;

8. SPD protection: Each level of SPD should be equipped with protection, which can be protected by circuit breakers or fuses. The breaking capacity of the protector is greater than the maximum short-circuit current at that location;

9. In addition, when selecting SPD, it should also be noted that the response time should be as fast as possible; The length of service life, price factors, good maintainability, flow capacity, moisture resistance, and other aspects.