General Information on Track Circuits - RGS Online

<strong>General</strong> <strong>Information</strong> on Track Circuits
14 Jointless Track
Circuits
Withdrawn Document
Uncontrolled When Printed
Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page B19 of 25
Insulated rail joints can be expensive both to install and to maintain, especially on
tracks subjected to high speed, high axle weight traffic or where there is an
intensive service.
The use of audio frequencies permits the physical limits of an individual track
circuit to be defined by tuned short circuits between the rails rather than by
insulation in the rails themselves. Consider two jointless track circuits abutting at
a tuned zone as shown in Figure B15. Non–track mounted equipment has been
omitted for clarity.
RAILTRACK B19
Tuned
Zone
Feed F1 Tuning Tuning
Feed F2
Unit F1 Unit F2
Figure B15
The tuned zone comprises a measured length of track with a tuning unit across
the rails at each extremity. The track circuits operate at different audio
frequencies and each tuning unit is designed to its own track frequency, such
that the following criteria are obeyed:
a) Consider frequency F1:
The F2 tuning unit behaves as a short circuit between the rails, due to series
resonance of its inductive and capacitive components.
The F1 tuning unit tunes the two rails (inductive) and the F2 tuning unit short
circuit to parallel resonance, thus presenting a significant impedance to
frequency F1.
b) Consider frequency F2:
The F1 tuning unit behaves as a short circuit between the rails, due to series
resonance of its inductive and capacitive components.
The F2 tuning unit tunes the two rails (inductive) and the F1 tuning unit short
circuit to parallel resonance, thus presenting a significant impedance to
frequency F2.
A wheelset proceeding along track circuit F1 will shunt the track circuit, but when
it enters the tuned zone its effectiveness will reduce until, having passed tuning
unit F2 (short circuit at frequency F1), it will no longer shunt track circuit F1.
Similarly, the wheelset would not shunt track circuit F2 as long as it remained on
track circuit F1, due to tuning unit F1 presenting a short circuit to frequency F2.
As the wheelset passes F1 tuning unit, it commences to shunt frequency F2,
becoming more effective as it progresses towards the F2 tuning unit and beyond
into F2 track circuit proper.