Dedicated copper lines are susceptible to electromagnetic interference. The coupling of extraneous voltages can be inductive or capacitive, for example due to the effects of lightning. Direct conductive coupling is also possible due to bad insulation.

The LTOP overvoltage protection limits external voltage and overvoltage to a non-critical level. The floating transformer additionally provides electrical isolation, and the transfer of voltages to other cable sections are therefore prevented.

LTOP protects people and investments, and is therefore an essential safety element in private trunk line networks.

Note:
The LTOP modules are not suitable for protecting dial-up connections (telephone or ISDN network). Companies such as Phoenix and Dehn offer TAE6, S₀, RJ-12, and RJ-45 jacks with overvoltage protection.