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17.3.13 Rusty Rails: Where there are rusty rails in the track circuited areas zig-zag
                        welding using steel wire shall be done by P-Way staff on top of rail to ensure
                        shunting of track circuit by the vehicle. Such identified locations to be provided
                        with Axle Counters. Till such time axle counters are being provided, working
                        instructions  should  be  issued  that  points  are  operated  only  after  physical
                        verification of complete arrival of train by ASM, and facility of sectional route
                        release be restricted to be provided only where it is essential. (Axle Counters
                        shall not be provided in parallel to “Rusty Rail Track circuit”)

                        Note: Refer to Annexure: 17-MS2 for maintenance schedule of DC Track Circuits.

                                 Section 4: Audio Frequency Track Circuits (AFTC)


                17.4.1  Audio Frequency Track Circuits (AFTC) – General

                        (a) The use of audio frequency permits the physical limits of an individual track
                            circuit to be defined by tuned short circuits between the rail rather than the
                            insulation in the rails themselves. These types of track circuit have distinct
                            advantage  of  not  requiring  IRJs  (except  Point  Zones)  and  offers
                            considerable  saving  in  IRJs  and  impedance  bond,  especially  on  tracks
                            subjected to high speed, high axle load traffic or where there is an intensive
                            service.


                        (b) Main features of Audio Frequency Track Circuit are:

                            (i)  Built in time delay, therefore a slow to pick up QSPA1 Relay as TPR is
                                not required.

                            (ii)  Can be used in most AC, DC electrified and non-electrified areas.

                            (iii) Can be used in end or centre-fed configuration.

                        (c) Only approved type Audio Frequency track circuit shall be used.

                        (d) Track circuit shall not be configured in single rail mode operation.


                        (e) Audio Frequency Track Circuit may be operated as local fed or in a remote
                            fed mode.


                        (f)  Audio Frequency Track Circuit‟s components are

                            (i)  Tuning unit
                            (ii)  Transmitter

                            (iii) Receiver

                            (iv) Power Supply

                            (v)  Track Connections

                            (vi) Impedance Bond (as applicable)






                                        Chapter 17: Train Detection Track Circuits & Axle Counters   Page 340 of 535
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