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2. Track Circuit
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Sri Lanka

Track circuit is one of the primary input for a signal interlocking plant. Opps ! hold on. What is an 'interlocking plant'? It is the control logic behind the signaling system. The signal cannot be 'green' while there is another train on track segment ahead. The system should able to detect the condition of the track segment: occupied or not. 

The tracks are segmented into 'blocks'. Each block  is track circuited separately. The figure below illustrates a track circuit.

The track circuit consists of a power supply on one end and a directional (polarized) relay on the other end. The power supply has a 6V battery kept charged by a 6V/6A rectifier. In case of power failure the battery will supply power to the circuit.

The track relay (TR), which has a resistance of 30 ohm and a pickup voltage of 1.4 volt, is normally held in picked-up state the circuit being completed via the rails. When a train enters the segment the axels of the train short circuit the supply to the relay and the relay drops. The contacts of the track relays appear in most of the safety circuits of the interlocking plant. The interlocking logic is arranged such that only one train can be permitted to enter a section. If you carefully observe, the track circuit is fail safe; if the circuit fails it will indicate occupancy.

The variable resister is introduced into the circuit such that it can be tuned to make the system works under all weather conditions.

The rails are insulated to separate the adjacent track circuits. The polarity of the adjacent track circuit is always reversed, so that the power supply of one circuit cannot operate the relay of the other circuit should the insulate between the circuits breakdown. Within one track circuit the rails are electrically connected by two wires (for safety).

The minimum length of track circuit is depends on the degree of control necessary and the maximum length is limited by the weather conditions. On the Northern line  from Loco Junction (Maradana) to Veyangoda,  the  segments have a maximum length of 2000 feet. On the Southern line from Loco Junction to Wadduwa, due to the saline atmosphere along the cost line, the track circuits are limited to 1500 feet max.

Now that we have looked upon the track relay we will get into discuss the types of Relays used in the railway signaling.


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1. Introduction ] [ 2. Track Circuit ] 3. Relays ] 4. Signals ] 5. Moter Points ] 6. Gate barriers ] 7. Automatic Blocking System ]

R. Jayanthan
Last updated on 01 January, 2002