Interference in Telecommunication Circuit in Electric Traction:

In all traction systems track is used as a return path for the current. Some part of the current flows through the track and the remaining portion of the current flows through earth, as shown in Fig. 15.25. Current flowing through earth causes interference in telecommunication lines. Now to constrain the return current to flow through restricted path i.e., along the track some steps are required to be taken. In ac traction system use of booster transformer is made while in dc traction system negative booster is used to reduce the potential drop across the track.

Path of return current is also constrained by reducing the track resistance. The resistance per km length of continuous track rail is of order of 0.05 Ω but the resistance of fish-plate joints employed in jointing the commercial length rail mechanically is more than that of a rail. Hence in order to reduce the resistance of the track as a whole the joints are supplemented with a good conductor or “bone”. Modern bonds are of the flexible type and are designed to be as short as the conditions of locations permit. The bonds are usually fixed under the fish plates as they are protected from damage while carrying out the repairs to the paving.

In single phase ac traction system presence of harmonics in supply system also causes interference in telecommunication circuits.

Interference in Telecommunication Circuit and signalling lines is also caused due to electromagnetic induction. Current flowing through contact wire sets up a magnetic field around itself. This magnetic field links with nearby circuits and induces emf in circuits.

Electrostatic induction in signalling and telecommunication circuits also causes interference in these circuits. There occurs a voltage gradient between hv contact wire and earth. Now statically induced emf in communication and singnalling lines is produced because of its positioning voltage gradient.

The Interference in Telecommunication Circuit in Electric Traction owing to reasons mentioned above (last three paras) can be reduced by taking following steps.

• All telecommunication circuits are installed under-ground in cables instead of in overhead lines.
• Telecommunication or signalling lines are provided with aluminium sheath instead of lead.
• Use of PVC or rubber covering is made to prevent sheath corrosion.
• Telecommunication circuits may be removed to the distant place, to the extent practically possible, from the railway track.
• Long telecommunication circuits are broken by providing isolating transformers.
• Booster transformer employing return feeder is also used to avoid magnetic linking of telecommunication lines with power lines.