Surge Protective Device(Surge Protector,Arrestor) It protects the electric network shocked by the thunder or over voltage . Surge Protective Device(Surge Protector, ... Arrestor)Usage and application scopeBY40 type surge protective device (short name "SPD") is suitable for AC 50/60 Hz ... electric power systems,such as TT, IT, TN-S, TN-C, TN-C-S. It protects the electric network shocked by the ...