These high-voltage dual bidirectional thyristor protectors are designed to protect ground backed ringing central office, access and customer premise equipment against overvoltages caused by lightning and a.c. power disturbances. Offered in five voltage variants to meet battery and protection requirements, they are guaranteed to suppress and withstand the listed international lightning surges in both polarities. Overvoltages are initially clipped by breakdown clamping until the voltage rises to the breakover level, which causes the device to switch. The high crowbar holding current prevents d.c. latchup as the current subsides.