Page 395 - IRSEM_Main Book

DC Track Circuit in RE Area (Single Rail)

(a) This track circuit can only be configured to work as single rail track circuit.

(b) Interference Mechanism, With reference to fig. above traction return
current flowing in the single traction return rails creates a longitudinal AC
voltage along its length, which is a function of the current value and the
impedance of the traction return rail. A wheel set at the feed end will
impress this interfering voltage on the relay, whilst a wheel set at the relay
end will similarly impress the interfering voltage on the feed set. The track
circuit equipment must be immune both to false operation and to damage
from such impressed voltages.

(c) Approved type of AC immunised track relays shall be used in AC traction
area.

(d) Length of DC track circuit is restricted depending upon rail return current.
With catenary current restricted to 300 Ampere on single track section and
600 Amperes on double track section, length of DC track circuits shall be
restricted depending upon the use of type of relay and sleeper.

(e) The length of track circuit can be extended upto 450 meters when QTA 2
plug-in relay is used.

(f) Restriction on the track circuit length due to use of concrete sleeper can be
relaxed upto 450 meters by PCSTE of Railway (if adequate ballast
resistance can be consistently obtained).

(g) In view of the increased AC immunity due to the presence of biased
magnetic arrangement, QBAT relays can be used upto a maximum length
of track circuit of 750 meters using one additional 'B' type choke at the
relay end, under minimum ballast resistance of 2 ohms/km. Operation of
track circuit with this type of relay will require four cells delivering 8.8 V.
QBAT relays shall be used in conjunction with QSPA1 relays conforming to
BRS 933A.

Chapter 17: Train Detection Track Circuits & Axle Counters Page 343 of 535