Page 531 - IRSEM_Main Book

Chapter 22: Requirements of Signalling

in 25 KV AC Electrified Area
Section 1: General

22.1.1 Objectives

(a) To ensure that signalling equipment continues to function normally in the
presence of traction current.

(b) To ensure that interference from the traction system cannot, under any
circumstances, cause the Signalling system to exhibit false indications, or
in any other way imperil the safe operation of trains. This objective must be
achieved even if the traction or Signalling system is in a faulty or
anomalous condition.

(c) To safeguard equipment and staff from electric shock.

22.1.2 Factors Affecting Signalling in AC Electrified Area

(a) Visibility of signals due to OHE structure.

(b) Electrical clearances of signals due to live OHE close by.

(c) Traction return current.

(d) Electrostatic induction and Electro-magnetic Induction.

22.1.3 Visibility of Signals shall be taken care of by the proper implantation of OHE
structures during the design stage of OHE. Where electrical clearances are not
possible to maintain, suitable protective shields for signal structures shall be
provided. Lay Out Plan for OHE shall be based on approved Signalling Plan.

Note: (i) Protective wire-mesh screen of approved design and size shall be provided
wherever clearance of more than 2 meter for any part of the signal from live OHE
conductor is not possible to get.
(ii) OHE bonds (Structural, longitudinal and cross bonds) to be provided by electrical
department in the yard and in the block section wherever required for proper
functioning of field S&T equipment. These bonds should be properly fastened with the
structure/mast/rails and must be insulated for portion passing under positive rail of
track circuit.

22.1.4 The electrostatic induction is practically eliminated by transferring a circuit into
underground cables protected by a metal sheath. The Electro-magnetic
Induction causes various currents and voltages to develop in conductors
parallel to the track. These include the rails, traction return conductors where
provided, cable sheath, any other conductors in the vicinity, and signalling and
telecommunication circuits. The voltages that occur in the conductors appear
as potential gradients. In addition, there are potential differences between
various points along the conductors and the adjacent Earths. The value of this
induced voltage depends on various factors, such as

Chapter 22: Requirements Of Signalling in 25 KV AC Electrified Area Page 474 of 535