Isolation transformer (dry)

A line isolation transformer for telephone use is a special type of impedance matching transformer that is used to couple telephone land lines to subscriber equipment to prevent dangerous common mode voltages from appearing on the line. It has a specific 600Ω input impedance which matches the telephone equipment.

Its primary winding may or may not have a direct current passing through it. It is also made with a gapped magnetic core that is not used in normal transformers.