DC TRACK CIRCUITS

DC track circuits

What is a Track circuit

• Track circuit is a train detection device consisting of a simple electrical circuit in which the running rails form a part of the circuit of a given portion of a track.

Why we need a Track circuit

• Every length of track in signalled area is split in to logical sections called Track sections or Blocks.

• The State of track sections (clear or occupied) to be used to prevent or allow signals and points to operate, the state must be detected electrically.

DC track circuits

• Failsafe track circuit was invented in 1872 by William Robinson.

Different types of track circuits

1. DC track circuit.

2. AC track circuit.

3. HVI track circuit.

4. Jointless track circuit.

5. Coded track circuit.

6. TI21 track circuit.

DC track circuits

Basic Operation• These work on the principle of relay voltage regulation when

vehicles shunt the track.

• In a track circuit, a portion of rail track is electrically isolated from adjoining rails and included in a circuit to energize a relay. The occupation or vacancy of the track portion is detected by the condition of track relay.

• In the basic form of DC track circuit, power is applied to the rails at one end of the section and is transmitted via the rails to a DC track relay at the other end.

DC track circuits

The components of the track circuit are :-

(1) Battery

(2) Adjustable Resistance

(3) Track Relay

(4) Track Lead Cables

(5) G.I.wires connecting cables to the rails

(6) Continuity rail bonds

(7) Insulated rail joints.

DC track circuit

Types of DC track circuits

One is continuously live and is called Closed track circuit.

the other one is made live only when occupied by the vehicles and is called Open track circuit.

Closed track circuit is further classified into two types

1. Double rail track circuit (Non RE area)

2. Single rail track circuit (RE area)

DC track circuits

Open track circuit

• Open track circuit is one in which the track relay is normally de-energized and picks up only when train comes on the track.

• The circuit gets completed when the track is occupied through the net resistance of the vehicle axles occupying the track circuit.

• The series resistance is so adjusted as to give sufficient voltage to the relay when track rails are shunted by axles with a high contact resistance.

• It is also to be ensured that the relay does not get enough voltage to pick up due to leakage currents through track bed ballast.

DC track circuits

• But in this if any connection breaks, its occupation goes undetected. Hence this track circuit can be used for limited purposes where its failure does not lead to unsafe conditions, short lengths like 26mts.

• In Open track circuits, the shunting resistance causes the track relay to pick up. If this shunting resistance is very high the track relay may not pick up properly. Hence the highest resistance value that can cause the track relay to pick up is called Pick up shunt resistance Value.

Open track circuit

DC track circuits

Closed track circuit

• In this type current is always flowing through the relay. When train comes over the track, the supply to the relay is shunted and the relay De-energizes.

• The feed is connected to one end of the track and the relay at the other end so that any breakage of rail continuity shall drop the relay.

• In this track circuit, the series resistance is called Regulating resistance. It regulates the relay voltage so that it falls below the drop away value when the track is shunted.

• The fall is caused by increased voltage drop across the regulating resistance due to rise in current when shunted by the vehicles.

DC track circuits

• The smallest closed track circuit is 26mts length. The longest workable track circuit depends on the ballast resistance.

Ballast resistance

• It is the net resistance of various leakage paths across track circuit rails offered by ballast & sleepers.

Train Shunt resistance (TSR)

• The highest resistance which when applied across the track can open the track relay front contact is known as its train shunt resistance value.

• A higher TSR is always aimed as to ensure safety in train working on these track circuits.

DC track circuits

• The resistance of a track relay shall be 9Ω for length of track circuit up to 100mts. More than 100mts 2.25Ω relay is used.

• The track relay is normally kept energized. Here the application of a proper shunting resistance drops the track relay. The highest value of shunting resistance that can cause the track relay to drop is referred as Drop shunt value.

• Once the track is dropped, it requires a considerable increase in its voltage to pick up again. This increase can be effected by increasing the shunting resistance. This least resistance value at which the track relay picks up again is called the Pick up shunt value.

DC track circuits

The circuit is designed so that most failures will cause a "track occupied" indication.

A broken rail or wire will break the circuit between the power supply and the relay will de-energize the relay.

A failure in the power supply will de-energize the relay.

A short across the rails or between adjacent track sections will de-energize the relay.

Closed track circuit

Track circuit Parameters

DC track circuits

Track circuit Parameters

Rr is the net resistance offered by the continuity rail bonds which is rather more than the resistance of the rail themselves.

RR is fixed for a relay and type of its coil connections.

RS is the resistance offered by the shunting vehicles axles.

RT is the regulating resistance which is adjustable when used with a fixed voltage battery and connected in series with battery.

RB is the resistance offered by the ballast and sleepers across the track to leakage of rail currents.

DC track circuits

Rfc is generally very low and Rrc is the main constituent of cable resistance.

Rs gets connected only when the track is occupied.

DC track circuits

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