Switching power supplies are classified by working principle
(1) Voltage switch type SPD. When there is no instantaneous overvoltage, it exhibits high impedance. Once it responds to lightning instantaneous overvoltage, its impedance suddenly changes to low impedance, allowing lightning current to pass through. It is also known as a "short-circuit switch type SPD".
(2) Voltage limiting SPD. When there is no instantaneous overvoltage, it has high impedance, but as the surge current and voltage increase, its impedance will continuously decrease, and its current voltage characteristics are strongly nonlinear, sometimes referred to as "clamp type SPD".
(3) Combination SPD. Composed of voltage switch type components and voltage limiting type components, it can display the characteristics of voltage switch type, voltage limiting type, or both, depending on the characteristics of the applied voltage.
Function
(1) Power line SPD
Due to the enormous energy of lightning strikes, it is necessary to gradually release the lightning energy to the ground through a graded discharge method. Install surge protectors or voltage limiting surge protectors that have passed the Class I classification test as the first level protection in the non protected zone (LPZ0A) or at the junction of the protected zone (LPZ0B) and the first protection zone (LPZ1), to discharge the direct lightning current, or to discharge the huge energy conducted when the power transmission line is directly struck by lightning. Install voltage limiting surge protectors at the junction of each zone (including LPZ1 zone) after the first protection zone, as second, third or higher level protection. The second level protector is a protective device against residual voltage of the previous level protector and induced lightning strikes in the area. When a large amount of lightning energy is absorbed in the previous level protector, there is still a considerable amount of energy that is transmitted to the equipment or the third level protector, which needs 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 electromagnetic pulse radiation from lightning strikes. When the line is long enough, the energy of induced lightning becomes large enough, and a second level protector is needed to further discharge the lightning strike energy. The third level protector protects against residual lightning energy passing through the second level protector. According to the withstand voltage level of the protected equipment, if two-level lightning protection can limit the voltage to be lower than the withstand voltage level of the equipment, only two-level protection is needed; If the voltage resistance level of the equipment is low, it may require four or even more levels of protection.
(2) Signal line SPD
With the widespread application of information systems, due to the large number of network lines and low voltage resistance of electronic devices, lightning strikes are becoming increasingly harmful to information systems. The harm of lightning to information systems is mainly caused by lightning electromagnetic pulses, including lightning overvoltage waves conducted along the line, high potential counterattacks generated by lightning currents on grounding wires, electrostatic induction and electromagnetic induction of lightning electromagnetic fields. The protective measures for electromagnetic pulses include interception, diversion, equipotential bonding, shielding, grounding, and reasonable wiring. Installing SPD on signal lines is an important measure for information systems to prevent electromagnetic pulses, which can simultaneously intercept, divert, and equipotential connect. The signal line SPD should be connected to the signal port of the protected equipment. Its output end is connected to the port of the protected device, and there are series and parallel connections, usually installed in series on the signal line.
