ALLOCATION OF FREQUENCIES| RAILWAY AUDIO FREQUENCY TRACK CIRCUITS |PRINCIPLES OF THE OPERATION|QUADRUPLE LINES|TRANSMITTER -Tx|RECEIVER -Rx|TUNING UNITS|CENTER FED AFTC| TRACK CIRCUIT LENGTHS|TRACK CIRCUIT LENGTHS|PHYSICAL REQUIREMENTS|WIRING DIAGRAM| CONFIGURATIONS OF AFTC
Railway Audio Frequency Track Circuits
- It’s a type of AC track circuit in which audio frequency signals are used instead of pure AC or DC signals.
- Its generally a joint less track circuit, extremity of the tracks are defined by tuned units but can also be used with IRJs.
- it is least prune to interference and hence can be used in ac, dc, and non electrified areas.
- It has built in time delay and hence do not need slow to pick up track repeater.
- It is a double rail track circuit and must not be connected in single rail mode. Hence generally not suitable for S&C area.
PRINCIPLES OF THE OPERATION
ALLOCATION OF FREQUENCIES
- A Tuned capacitor/inductor circuit is used to provide correct termination defining the end of a TI.21 Track circuit. Tuning units are two types, ETU and TTU
- ETU – Use of ETU at an IRJ enables the train shunt to be effective up to the IRJ. In this case ETU can be used either with Tx or Rx.
- Max distance permitted between IRJ and ETU rail connection shall not exceed 1m.
- ETU also enables the Tx to feed two receiver units of the same frequency.
- TTU – Where two joint less track circuits abut a TTU from each track circuit is connected to the rails 20m apart to form a tuned zone.
- The shunt zone of each track circuit extends beyond mid point between TTUs.
- Each TTU provides specified load for the correct frequency and acts as a short circuit to other frequencies of the adjacent tracks
- Each TTU is sensitive to a specific frequency and blocks other permitted track circuit frequency
- Separate TTU are to be used with different frequency track circuit
- If the next section of the line is non track circuited then a tuned zone shall be followed by A IRJ at 18.5m.
CONFIGURATIONS OF AFTC
- AFTC comes in two configuration – End fed and center fed
Center Fed AFTC
TRACK CIRCUIT LENGTHS
- The limits of the Track circuits shall be defined by IRJs in both the rails or by tuned zones.
- As TI.21 is used in double rail configurations, impedance bonds shall be used for cross bonding and in plain line shall be implemented with series bonding on both rails.
- Min length of track circuit is based on two factors:
No Rx is to be positioned less than 200m from any normal power TX of same frequency
No Rx is to be positioned less than 50m from low power Tx of same frequency.
- The min length restricted in End fed is required to false feeding of a RX by a TX of the same frequency, which is being used by another track circuit on the same line.
- Low fed is to be used only in End Fed mode and it must be ensured that no normal TX is within 200m of Rx of the same frequency
- Length shall also be imposed in locations susceptible to dampness such as tunnels
- In LC gate area, only normal power AFTC is recommended
- There are lot of restrictions for use of TI.21 and other frequency based TCs such as reed/Aster and axle counter
In TI.21 area structure bonds cannot be connected to rail as rail can not be earthed.
Tail Cable – max loop resistance and length
- Tx -0.5 Ohms and max length with 2.5 sq mm cable is 30m
- Rx – 6.0 ohms and max length with 2.5 sq mm cable is 365m
- Track cable 70and 35 sq mm cables
- Power supply units – BR 929 24V Current rating 4.4A and can feed any number of Rx & Tx combination within max limit (Tx power – 2.2A and RX power is 0.5A)
- Track relay – 50V BR931 style relay and should be in same loc case as Rx