Transposition (transmission lines)

In telecommunications this applies to balanced pairs whilst in power transmission lines three conductors are periodically transposed.

Transposition also equalizes their impedance relative to the ground, thus avoiding one-sided loads in three-phase electric power systems.

Also, transpositions on pylons near power substations are used to get an optimal arrangement of the feeding system without crossing of conductors.

Modern power lines are normally not transposed on the go as the difference in the inductance of the phases are negligibly small due to asymmetrical spacing, However, intermediate switching stations, where the transposition takes place, are implemented whenever it is necessary.

The principal measure is the pitch or lay length,[3] the distance over which the pairs of a circuit are twisted.

In open wire lines used for long-distance (trunk or toll) telephone circuits, transposition was used for reducing cross-talk.

Overhead trunk lines were rare in Europe and the German term for transposition “Drehkreuzachse” remained a mystery at the Bletchley Park codebreaking centre until late in World War II: General Fellgiebel of the Army Signals Service and Heinz Guderian developed a landline system for newly occupied territories to carry multiple telephone and telegraph or teleprinter channels.

Overhead lines were common in larger and less densely populated countries like Australia, New Zealand, and the western United States.

For example, in star quad twisting the two conductors of a quadruple run parallel over the entire cable length.