Surge Protective Devices,surge protector,arrester,SPD Surge Protective Devices,surge protector,arrester,SPD BY3 type SPD is suiting to A. C 50/ ... zones. It protects the electric network shocked by the thunder or over voltage. This type belongs to Class 3 and the ... is that there must be installed the Class 1 or Class 2 SPD, or it will be damaged by the over impulse ...