Surge protectors are generally installed on a primary system to prevent direct intrusion of lightning waves and to protect overhead lines and electrical equipment. SPD surge protectors are installed on secondary systems, which eliminate lightning waves in surge protectors. After direct intrusion, or the surge protector does not have additional measures to eliminate the lightning wave; therefore, the surge protector is installed at the incoming line; the SPD is installed at the end outlet or signal circuit.
The flow capacity is different: the main function of the surge protector is to prevent lightning overvoltage, so its relative flow capacity is relatively large; for electronic equipment, the insulation level is much smaller than that of electrical equipment in general sense, so SPD is required for lightning. Voltage and operating overvoltage are protected, but their current capacity is generally small. (SPD is generally at the end and will not be directly connected to the overhead line. After the current limit of the upper stage, the lightning current has been limited to a lower value, so that the SPD with a small flow capacity can fully protect the flow. The value is not important, the important thing is the residual pressure.)