General Information on Track Circuits - RGS Online

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<strong>General</strong> <strong>Information</strong> on Track Circuits
Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page D17 of 20
13.14.1 Fouling Point
This is a position a short distance away from the point of running line divergence
(crossing nose). Should any part of a vehicle on one track be between the
crossing nose and the fouling point, it will make physical contact with any
vehicles passing on the other route.
Where the angle of divergence is less than 45°, the fouling point occurs where
the distance between the running edges of the two rails is 1970mm, measured
at right angles from the track for which the fouling point is being determined.
Where the angle of divergence is greater than 45°, the fouling point occurs at
1970mm from the crossing nose, measured along the track for which the
clearance point is being determined.
In the case where tracks become parallel with a running edge separation of less
than 1970mm, the fouling point occurs where the tracks first become parallel.
13.14.2 Clearance Point
As track circuits detect the wheelsets of vehicles which are inboard of bodyshell
limits, the boundary of any track circuit designed to give assurance of clear
passage along the other route must be some distance beyond the actual fouling
point. This is defined as the clearance point.
In the absence of protecting trap points, the clearance point is 4880mm further
from the crossing nose than the fouling point. Where trap points are provided,
the clearance point is defined as the switch tips of the trap points. In both
cases, the IRJ defining the track circuit limit is positioned at the first suitable rail
joint beyond the clearance point. All clearance points shall be shown on the
bonding plans.
13.14.3 Minimum Length of Track Circuits
To prevent a vehicle bridging a short track circuit and consequently providing a
false clear condition, the minimum length of a track circuit is 18.3m.
13.14.4 Staggered IRJs
Where practical, IRJs should be positioned in each rail so that they are opposite
to each other. Where a physical stagger between opposite IRJs is unavoidable,
the maximum physical stagger must not exceed 2.6m. This is the minimum
wheelbase of vehicles which can work unattached.
To ensure that motor bogies do not become insulated from the traction return
path, the physical stagger for a signal rail overlap in electric traction areas must
not exceed 2.1m. On the Isle of Wight, vehicles used have smaller bogies and
as such, the maximum physical stagger is reduced to 1.7m.
In order to avoid loss of detection of a single car, four wheel vehicle, the
permitted distance between inner joints of staggered pairs of IRJs must not be
less than 11m, where the IRJs are staggered at a distance of less than 1.6m
(the minimum bogie wheelbase). If however, either pair of IRJs are staggered at
a distance greater than 1.6m, or provide a clearance point (see Clause 13.14.2),
then the distance between inner joints of staggered pairs must be not less than
the 18.3m minimum track circuit length.
Note: When consulting other documents, care should be taken to ascertain
whether “running edges” or “outside edges” of rails are being referred to.
RAILTRACK D17