Basic understanding of surge protector (SPD)

- Aug 10, 2020-

Thunder and lightning is a common natural phenomenon. In addition to endangering personal safety, it can also cause huge damage to electrical equipment, especially electronic equipment. From the point of view of the areas where lightning disasters occur, my country's densely populated and economically developed large and medium-sized cities are all located in lightning areas above the medium level. In these areas, computers and other electronic information products have been widely used, and the probability of encountering lightning damage is high. Therefore, we must continuously improve our defense against lightning disasters.

1. Surge protector (SPD) application common sense

SPD is mainly used to limit the transient overvoltage (ie, lightning surge) and most of the operating overvoltage caused by lightning in the power system. Lightning surges can invade equipment through power or signal lines. They can counteract the equipment due to the rise of ground potential during lightning strikes, or they can be induced in cables and loops due to pulsed electromagnetic fields generated by lightning strikes on the building itself (or nearby).

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Therefore, in addition to external lightning protection measures such as good lightning rods, down conductors, and grounding devices, SPDs need to be installed. Because they cannot prevent the conduction intrusion along the lightning induced surge and the secondary lightning strike. Especially if the information electronic equipment and/or power electronic equipment with high value and large impact in the building, its ability to withstand lightning surge is much lower than that of conventional electrical equipment, and SPD needs to be installed.

2. Composition and classification of surge protectors

  1. Surge protectors are generally composed of gas discharge tubes, discharge gaps, semiconductor discharge tubes, zinc oxide varistors, Zener diodes, filters, fuses and other components.

  2. Surge protectors can be divided into three categories in terms of usage: power surge protectors, signal lightning surge protectors, and antenna feeder surge protectors.

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  3. In terms of working principle and performance, surge protectors can be divided into three types: voltage switch type, voltage limit type, and combined type.

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The voltage switch type SPD is high impedance when there is no surge, and suddenly becomes low impedance when there is a voltage surge. It is also called "short-circuit switch type" SPD. Usually use discharge gap, gas-filled discharge tube, thyratron and triac as components. Its characteristic is strong discharge capacity, but high residual voltage, 2-4kV. Testing the device generally uses the 10/350μS analog lightning current waveform. Generally installed at the junction of the building LPZ0 and PZL1 area.

The voltage-limiting SPD has high impedance when there is no surge. With the increase of the surge current and voltage, the impedance continuously decreases. It is also called "clamping" SPD. Varistors and suppression diodes are usually used as components. Its characteristic is low residual pressure. Testing the device generally adopts 8/20μS analog lightning impulse current waveform. It is generally installed in buildings in lightning protection areas, and drains 8/20μS lightning impulse current.

The combined SPD is a combination of voltage switching components and voltage limiting components. In general lightning overvoltage protection, the voltage-limiting component is subjected to surge current, and its nominal discharge current can reach 10-20kA; in the event of a relatively large lightning current overvoltage, the first level is composed of voltage-limiting circuit insurance The tube can be automatically disconnected, and the lightning overvoltage protection is performed by the second-level voltage switch component. The combined SPD can withstand inrush current capacity generally greater than 100kA.

3. The main performance indicators of the surge protector

(1) The 10/350μs wave is a waveform that simulates direct lightning strikes, with large waveform energy; the 8/20μs wave is a waveform that simulates lightning induction and lightning conduction.

(2) The nominal discharge current In refers to the peak current flowing through the SPD, 8/20μs current wave.

(3) The maximum discharge current Imax is also called the maximum flow rate, which refers to the maximum discharge current that the SPD can withstand once using an 8/20μs current wave.

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

(5) Residual voltage Ur refers to the residual voltage value under the rated discharge current In.

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

(7) The voltage switch type SPD mainly discharges the 10/350μs current wave, and the voltage-limiting SPD mainly discharges the 8/20μs current wave.

Under normal circumstances: ①Voltage protection level. Generally, the lower the voltage protection level, the better the protection effect. ②Circulation capacity. Generally, the higher the circulation capacity, the better the safety under lightning. But the larger the circulation capacity, the higher the SPD price. ③Maximum continuous operating voltage. Generally, the higher the maximum continuous operating voltage, the better the long-term safety, but the higher the maximum continuous operating voltage, the higher the voltage protection level.

4. Installation principles of surge protectors

1. Surge protectors must be able to withstand the lightning current expected to pass through them. The maximum surge voltage at the entrance of the building and at the interface of other lightning protection zones should be consistent with the basic insulation level of the system and the maximum surge voltage allowed by the equipment. In order to make the maximum surge voltage low enough, the leads at both ends should be the shortest, not more than 0.5 meters

2. When the distance between the surge protector at the inlet end of the power supply and the protected equipment is far (more than 30m), or the voltage protection level of SPD Up plus the induced voltage of the leads at both ends and the reflected wave effect are insufficient to protect sensitive For equipment, the second-level SPD should be installed at the protected equipment, and its nominal discharge current should not be less than 8/20μS5kA.

3. When multiple SPDs are installed on the line, the line length between the voltage switch type SPD and the voltage limit type SPD should not be less than 5m.

4. In order to prevent the SPD from being damaged due to various reasons, each level of SPD must be protected by a fuse or circuit breaker.

5. When the line of the protection device is shielded, the SPD should be installed near the end of the shielded line.