Transposition of Conductors

By transposition of conductors, we mean the exchanging of the position of the power conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor over an equal distance. If the spacing is unequal, even though the system operates under balanced condition, voltage drops of different magnitude will be there in the 3 conductors due to unequal inductance in the 3-phase. Also due to unsymmetrical spacing, the magnetic field external to the conductors is not zero, thereby causing induced voltages in adjacent electrical circuits particularly telephone lines,  that may result in interference. To reduce this effect, the conductors are transposed as shown below:

Transposition of Conductors

It is enough to transpose either the power lines or the communication lines under balanced operating conditions, the magnetic field linking an adjacent telephone line is shifted 120 degrees in the time phase with each relation of the conductor positions in the transposition cycle. Over the length of one complete transposition cycle of power lines, the net voltage induced in the telephone lines is zero as it is the sum of the 3 induced voltages which are displaced by 120 degrees in time phase under balanced conditions of course.