General Information on Track Circuits - RGS Online

Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page H2 of 2
4 Full Test
Withdrawn Document
Uncontrolled When Printed
<strong>General</strong> <strong>Information</strong> on Track Circuits
shunt box may be connected across the track circuit relay links in the lineside
apparatus housing. The value of drop shunt obtained at the lineside apparatus
housing will usually be higher than that obtained at the rails, the resistance of the
relay end track leads being the most significant factor.
It is only permissible to undertake drop shunt tests at the track circuit relay links
in the lineside apparatus housing. where comparative shunts have previously
been carried out both at the rails and at the lineside apparatus housing, and the
two values are endorsed on the record card. It is then possible to judge any
value obtained at the lineside apparatus housing relative to its theoretical
railequivalent. Where such a theoretical value can be seen to approach the
minimum acceptable, the test should be verified at the rails.
The procedure for carrying out a drop shunt test is detailed in Part F 3.2.
A variation in the drop shunt value may be caused by variations in equipment
performance, or by expected variations in environmental conditions, with the
drop shunt reaching the upper end of its range in wet weather and the lower end
in dry weather.
If the drop shunt exceeds the maximum value for the track circuit (see the
relevant Code of Practice ), it is likely that the track circuit is being shunted by
poor ballast or debris. The track circuit should be examined for these faults. If it
appears to be in good physical condition, the track circuit equipment should be
regarded as failed and the cause of the failure investigated through the fault
finding procedure (see the relevant Code of Practice).
If the drop shunt is lower than the minimum value for the track circuit (see the
relevant Code of Practice), there has been an unexpected equipment failure.
The Engineering Supervisor is to be informed, and immediate investigations
undertaken to ascertain the cause of the low drop shunt value.
The full test must be applied whenever alterations are made (eg. relaying,
lead/jumper renewal, equipment replacement, adjustment, etc). The full test
comprises the following:
a) Carry out a Routine Examination.
b) At the feed end, measure and record the voltages, currents and other
parameters as required on the front of the record card. Any adjustments
required must be in accordance with the relevant section of this handbook for
the type of track circuit under test.
c) At the relay end, with the track circuit clear, measure and record the
voltages, currents and other parameters required and enter on the front of
the record card.
d) Perform a drop shunt test with the shunt box across the rails at the relay end
and enter details on the record card. Where the track circuit is of a type able
to be routine shunted at the relay links in the lineside apparatus housing,
perform a second drop shunt test at this position and endorse the record
card with that value.
e) Set the shunt box to the minimum for that track circuit and connect across
the rails at all extremities of the track circuit, confirming that the track circuit
occupies on each occasion. This test must obtain simultaneous track circuit
occupation where an overlapping section exists.
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