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Line isolation transformer
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<p>A line isolation transformer for telephone use is a special type of <a href="/facts/Impedance_matching/0mdy9TAp">impedance matching</a> transformer that is used to couple <a href="/facts/Telephone/P7uEgWQX">telephone</a> land lines to subscriber equipment to prevent dangerous common mode voltages from appearing on the line. It has a specific 600Ω <a href="/facts/Input_impedance/AQCLaTj9">input impedance</a> which matches the telephone equipment. </p><p>A "wet" type will maintain its characteristics while passing the DC current needed to seize the telephone line (signal off-hook to the central office). It is also made with a gapped <a href="/facts/Magnetic_core/yojywmuo">magnetic core</a> that is not used in normal <a href="/facts/Transformer/EYYfXWrF">transformers</a>. A "dry" isolation transformer need not carry DC current and an alternative DC path would be provided. </p>