SPDs (Sustainable Power Distribution Devices) are indispensable devices for lightning protection of electronic equipment. Their function is to limit transient overvoltages entering power lines and signal transmission lines to within the voltage range that the equipment or system can withstand, or to divert powerful lightning currents to the ground, protecting the protected equipment or system from impact.
Based on their working principle, SPDs can be classified into voltage-switching type, voltage-limiting type, and combined type.
(1) Voltage-switching type SPD: Presents high impedance when there is no transient overvoltage. However, upon response to a lightning transient overvoltage, its impedance abruptly changes to low impedance, allowing lightning current to pass through. It is also known as a "short-circuit switch type."
(2) Voltage-limiting type SPD: Presents high impedance when there is no transient overvoltage, but its impedance continuously decreases as the surge current and voltage increase. Its current-voltage characteristics are strongly nonlinear, sometimes referred to as a "clamping type SPD."
(3) Combined type SPD: Composed of voltage-switching and voltage-limiting components, it can exhibit voltage-switching, voltage-limiting, or both characteristics, depending on the characteristics of the applied voltage.
