Surge not only exists in the environment of the power supply, but is also ubiquitous in ordinary inductive circuits. The following figure can well explain why inductive circuits are prone to surge voltages.
Under normal working conditions, the voltage at the test point is maintained at a stable state of 500V, but if the switch Q is suddenly disconnected, since the inductor current cannot be abruptly changed and the back electromotive force effect occurs, a high voltage surge will be generated at the test point.
Surges can occur for many reasons, such as large equipment, short circuits, power switching, or large motors that can cause surges in a circuit. To sum up, the reasons for the surge in the power supply system are divided into two types: external and internal. The external cause is mainly lightning, which causes surge and overvoltage, and the internal cause is mainly the start-stop and failure of electrical equipment.