Click here for Overhead Construction Menu

On very long lines (Trunk circuits), of multiple open wires, overhearing can occur.  This is because the wires can induce currents in nearby wires, causing speech to break through or noise from telegraph wires, producing "overhearing".

The can also occur in long lengths of multiple wire cable.  In this case the wires are laid or twisted to prevent overhearing.  Today, data break through can occur and all computer and telephone wire pairs are twisted to prevent this.

A standard scheme of crossing the pair of wires at intervals along the line was introduced - this was called Transposition.  Trunk lines were balanced at 8 miles so each pair had to cross the same amount of times as each other to maintain the balance.

Transposition cards were introduced and the poles were labelled to assist in location pairs of open wires.

This is an excerpt from the Engineering Instructions, LINES, OVERHEAD, F 3003, 1934.


1. General
Open telephone circuits are subject to inductive influence, which would cause interference with speech if preventive measures were not taken. Generally, this influence is both electrostatic and electro-magnetic but, usually, the electrostatic effect preponderates. There are three main sources of induction, namely:-

a) Other telephone circuits on the same pole line.
b) Telegraph circuits.
c) Adjacent power circuits.

2. The two methods adopted for neutralizing the inductive effects consist in transposing the two wires of a telephone circuit, with the object of securing that each wire is situated at the same average distance from all likely sources of disturbance.

3. The standard method of effecting this, both for trunk and local lines, is known as the Transposition System. In this system, crosses of the A and B wires of the circuits are inserted at poles along the line in accordance with a standard scheme.

4. The other method is known as the Twist System
In this system, units of four wires are rotated in a right-hand direction, and a revolution is completed every four spans. This method should only be employed on Trunk Lines having more than eight arms on which twisted trunk circuits already exist.

5. The effectiveness of both systems
As regards the prevention of disturbance from power circuits is subject to the proper design of the power circuits, which should be comparatively free from currents of audible frequency. If this is not the case, and interference with telephone circuits occurs, the appropriate remedy is the alteration of the power system.

6. Regulation and Insulation are of vital importance in securing quiet circuits, whether the transposition or the twist system is used. If one wire of a circuit is out of regulation with the other, the condition of average equidistance is upset, and noise or cross-talk results. It is important that the two wires should have equal insulation ; otherwise, although the condition of average equidistance is fulfilled, the currents circulating in the two wires will differ and noise will result.

7. Exactitude of Detail and Symmetry of Construction form very important elements in securing immunity from inductive disturbance. With this object in view, care should be taken to use arms and spindles which provide average equidistance, and, also, to bind-in all the wires on any pole on the same side of the insulators.

On this system, all wires are run straight, and "flat" pairs are used i.e. two adjacent wires on the same arm are used for each circuit. To prevent inductive interference, the two wires of each circuit are crossed at intervals along the line; to obviate cross-talk between circuits, the crosses in those circuits which are likely to interfere with one another are arranged systematically, being so placed that, over a given section, each circuit is balanced against the others. Further, the positions and spacings of the crosses in a group of neighbouring circuits - which, from their relative positions, are most likely to be sources of mutual interference - are selected to obtain the maximum number of changes of position of the wires of any circuit relative to those of any other circuit in the group. Theoretically, the effect of electrostatic induction can only be eliminated by the insertion of an infinite number of crosses but, in actual practice, it can be reduced to meet the practical requirements by the insertion of crosses in accordance with this system.

9. Standard Transposition Section
The line is balanced in eight-mile sections, the minimum distance between crosses in any pair of wires being a quarter of a mile and the maximum distance two miles. When surveying for a new line, care should be taken, therefore, to fix the positions of the transposition poles as accurately as possible at quarter-mile intervals; otherwise, balance will not be obtained. There is no necessity to arrange for the same number of spans in each quarter-mile section. It will almost invariably be found that an incomplete section will occur at the "down" end of the line, but the absence of balance in such a section should not be sufficient to cause trouble.

10. Lines less than Eight Miles in Length. In the application of the scheme to lines less than eight miles in length, the unit transposition section should be built up gradually as the line is extended.

11. Arrangement of Crosses
The arrangement of the crosses should be used for trunk, junction, and subscribers' lines. It will be seen that this arrangement is suitable for circuits on arms of all types, saddles and swan-neck spindles, the relevant pairs being indicated.

12. This arrangement allows 4-way arms to be replaced by 6-way or 8-way arms without necessitating any change of existing transpositions for example, in changing from 4-way to 6-way, it is only necessary to place the existing circuits in the outside positions on the new arms, and to cross the new wires. When 4-way arms are replaced by 8-way arms, the existing circuits should occupy the inside positions and the new circuits should be erected in the outer positions.

13. The arrangement of crosses for pairs on the top set of four arms, should be repeated for each additional set of four arms except that, on odd-numbered "S" poles - which occur at the end of the first 8 miles and each subsequent 16 miles - pairs on the second and fourth sets of arms should be crossed. With this arrangement, the distance between circuits similarly crossed on adjacent sets of four arms is sufficient to prevent overhearing.


The four wires occupying the four insulator positions forming the corners of a square (or rectangle) are considered as a unit, the diagonally opposite wires forming a telephone pair. Each wire is displaced one insulator position in a clockwise direction at each successive pole. Each of the four wires thus occupies, in turn, each of the four insulator positions, and is in a similar position at each end of any four consecutive spans.

30. Under normal conditions of insulation, circuits erected on the twist system are not subject to interference from outside sources, due to the fact that both wires of each circuit are at the same average distance from all sources of interference. For the same reason, overhearing is absent between the two circuits forming a twist unit.

31. Overhearing may take place, however, between circuits in different twist units where the condition of average equidistance does not exist, as in the case of circuits which are parallel. To prevent interference between such circuits, especially on long lines, it may be found necessary to cross, at certain points, the two wires of each circuit forming a twist unit or group. The relative position of each wire will thus be reversed and, consequently, the inductive interference from other circuits will be neutralized.

33. Overhearing on a circuit does not necessarily mean that crosses are required throughout its whole length, since the source of the interference may often be localized to a certain section.


BACK Home page BT/GPO Telephones Search the Site Overhead Construction Quick Find All Telephone Systems

Last revised: March 12, 2022