These devices are designed to limit overvoltages on the telephone and data lines. Overvoltages are normally caused by a.c. power system or lightning flash disturbances which are induced or conducted on to the telephone line. A single device provides 2-point protection and is typically used for the protection of ISDN power supply feeds. Two devices, one for the Ring output and the other for the Tip output, will provide protection for single supply analogue SLICs. A combination of three devices will give a low capacitance protector network for the 3-point protection of ISDN lines.