Classification of surge protective devices:

SPD is an indispensable device in the lightning protection of electronic equipment. Its function is to limit the instantaneous over-voltage of the power line and signal transmission line to the voltage range that the equipment or system can bear, or to drain the strong lightning current into the ground to protect the protected equipment or system from impact.

According to the working principle, surge protective devices are divided into:

According to its working principle, SPD can be divided into voltage switch type, voltage limiting type and combination type.

(1) voltage switch SPD. When there is no instantaneous over-voltage, it presents high impedance. Once it responds to lightning instantaneous over-voltage, its impedance will suddenly change to low impedance, allowing lightning current to pass through, also known as "short-circuit switch SPD".

(2) pressure limiting SPD. When there is no instantaneous over-voltage, it is high impedance, but with the increase of surge current and voltage, its impedance will continue to decrease, and its current voltage characteristic is strongly nonlinear, sometimes referred to as "clamp type SPD".

(3) combined SPD. It is composed of voltage switch type components and voltage limiting type components. It can be displayed as voltage switch type or voltage limiting type or both, which depends on the characteristics of applied voltage.

Surge protective devices are divided into:

1. Power line SPD:

Because the energy of lightning stroke is very huge, it needs to discharge the lightning energy to the earth step by step through the method of grading discharge. At the junction of lpz0a or lpz0b and lpz1, surge protective devices (SPD) or voltage limiting surge protective devices (SPD) which have passed the level I classification test are installed as the first level protection to discharge the direct lightning current, or when the transmission line of electric source is hit by direct lightning, the transmitted huge energy is discharged. The voltage limiting surge protective device (SPD) shall be installed at the junction of each partition (including lpz1) after the first protection zone as the second, third or higher level protection. The second level protector is the protection equipment for the residual voltage of the former level protector and the induced lightning stroke in the area. When the former level has a large lightning energy absorption, there is still a part of considerable energy for the equipment or the third level protector, which will be transmitted and need to be further absorbed by the second level protector. At the same time, the transmission line passing through the first level lightning arrester will also induce lightning electromagnetic pulse radiation. When the line is long enough, the energy of induced lightning becomes large enough, and the second level protector is needed to further discharge the lightning energy. The third level protector protects the residual lightning energy through the second level protector. According to the voltage withstand level of the protected equipment, if two levels of lightning protection can limit the voltage lower than the voltage withstand level of the equipment, only two levels of protection are needed; if the voltage withstand level of the equipment is low, four or more levels of protection may be needed.

To choose SPD, we need to know some parameters and their working principles.

(1) 10 / 350 μ S wave is the waveform of simulating direct lightning, with large energy; 8 / 20 μ S wave is the waveform of simulating lightning induction and lightning conduction.

(2) nominal discharge current in refers to the peak current flowing through SPD and 8 / 20 μ s current wave.

(3) the maximum discharge current Imax, also known as the maximum flow, refers to the maximum discharge current that SPD can bear when using 8 / 20 μ s current wave.

(4) maximum continuous withstand voltage Uc (RMS) refers to the effective value of maximum AC voltage or DC voltage that can be continuously applied on SPD.

(5) residual voltage ur refers to the residual voltage value under rated discharge current in.

(6) the protection voltage up represents the voltage characteristic parameter between SPD limit terminals, and its value can be selected from the list of preferred values, which should be greater than the maximum value of the limit voltage.

(7) the main discharge of voltage switch SPD is 10 / 350 μ s current wave, and the main discharge of voltage limiting SPD is 8 / 20 μ s current wave.

2. Signal line SPD:

Signal line SPD is actually signal arrester, which is installed in the signal transmission line, generally at the front end of the equipment, to protect the follow-up equipment and prevent lightning wave from entering the damaged equipment from the signal line.

1) Selection of voltage protection level (up):

The up value shall not exceed the impulse voltage rating of the protected equipment. Up requires that the insulation of SPD and the protected equipment shall have a good coordination.

In the low-voltage power supply and distribution system, the equipment shall have a certain capacity of withstanding surge, that is, withstanding impulse overvoltage. When it is not possible to obtain the withstand overvoltage value of various equipment of 220 / 380V three-phase system, it can be selected according to the given index of IEC 60664-1 and GB 50057-1994 (2000 version).

2) Selection of nominal discharge current in (impulse through current capacity):

Peak current flowing through SPD and 8 / 20 μ s current wave. It is used for level II Classification Test of SPD, and also for pretreatment of level I and level II Classification Test of SPD.

In fact, in is the maximum peak value of impulse current that SPD can pass through a specified number of times (generally 20 times) and a specified waveform (8 / 20 μ s) without substantial damage.

3) Selection of maximum discharge current Imax (limit impulse through current capacity):

The peak current flowing through SPD and 8 / 20 μ s current wave is used for class II classification test. IMAX and in have many similarities. They all use the peak current of 8 / 20 μ s current wave to carry out level II Classification Test on SPD. The difference is also obvious. IMAX only conducts one impact test on SPD, and there is no substantial damage to spd after the test; in can do 20 such tests, and there is no substantial damage to spd after the test. Therefore, IMAX is the limit value of impulse current, so the maximum discharge current is also called the limit impulse current carrying capacity. Obviously, IMAX > in.