These medium voltage dual symmetrical transient voltage suppressor devices are designed to protect ISDN and telecommunication applications with ground backed ringing against transients caused by lightning strikes and a.c. power lines. Offered in three voltage variants to meet battery and protection requirements they are guaranteed to suppress and withstand the listed international lightning surges in both polarities. Transients are initially clipped by breakdown clamping until the voltage rises to the breakover level, which causes the device to