The main function of the SPD backup protector in the circuit is to quickly cut off the short circuit freewheeling flowing into the SPD to protect the SPD from fire; and it does not act when the lightning strikes, allowing the lightning current to smoothly discharge through the SPD, thereby protecting the electronic equipment from damage.
Due to the working characteristics of SPD, as a backup protection device of SPD, the following conditions need to be met:
1. It has the ability to respond quickly to small currents, and can quickly remove SPD from the line when a small power frequency current flows;
2. It can distinguish the difference between lightning current and power frequency current, and selectively allow lightning current or surge current to discharge to the ground through SPD;
3. Reliable high breaking capacity, able to withstand the expected short circuit current of the transmission line and break it;
4. Safe and fast, able to withstand the impact stress when breaking high short-circuit current without bursting;
5. Maintain a low residual voltage when discharging overvoltage or overcurrent energy.
Currently, there are two typical SPD backup protector designs on the market, one is bypass trip SCB, and the other is main circuit trip SCB.