http://www.chnze.com by3 type spd is suiting to a.c 50/60 hz , 220v electric power system and used at the joint of the lpz2 and lpz3 zones. it protects the electric network shocked by the thunder or surge over voltage. this type belongs to class 3 and the usage condition is that there must be installed the class 1 or class 2 spd, or it will be damaged by the over impulse current