General Information on Track Circuits - RGS Online

Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page F2 of 5
4 Rail Clip Insulation
Tester
Withdrawn Document
Uncontrolled When Printed
<strong>General</strong> <strong>Information</strong> on Track Circuits
The shunt box has two resistance selection dials: 0 - 9Ω and 0 - 0.9Ω which are
additive to give a combined range of 0 - 9.9Ω in 0.1Ω steps. In order to avoid
overheating when left connected, the selected resistance is only placed between
the test leads when the button is pressed.
The unit comes complete with connecting leads fitted with 4mm plugs, and with
two rail clamps for making attachment to the rail foot. These clamps have upper
and lower contact points and should be attached to a section of cleaned rail foot
without over tightening. The contact points should be replaced if they become
blunt.
3.2 Drop Shunt Test
Test:
1 Connect the shunt box across the rails at the relay end of the track circuit.
2 To ensure that the clamps make a good electrical contact with the rails,
connect a voltmeter between the rail heads. With zero ohms set on the
shunt box and the button or switch operated, ensure the rail to rail voltage
falls to zero.
3 Set the shunt box to maximum resistance.
4 Whilst keeping the button depressed, steadily reduce the resistance value
until the track relay front contacts are fully open and remain open. The value
of drop shunt can then be read directly from the shunt box. The following
points should be borne in mind:
a) Track circuits do not react instantly to changes in shunt value and time
should be allowed for the relay to respond (2-3 seconds is adequate).
b) It is obviously impractical to start at 9.9Ω and reduce in 0.1Ω steps at 3
second intervals. Do a preliminary test by setting the 0 - 0.9Ω selector to
0Ω and stepping down the 0 - 9Ω selector to identify the approximate
value. Set the 0 - 0.9Ω selector to 0.9Ω and step down to obtain the
exact value.
For example, if the relay drops at 1.0Ω on the preliminary test, the actual
value lies between 1.0Ω - 1.9Ω.
3.3 Pick–up Shunt Test
Test:
1 Connect the shunt box as for the drop shunt test and set both resistance
selector switches to 0Ω..
2 Whilst keeping the button depressed, steadily increase the resistance value
until the track relay front contacts just close. The value of pick–up shunt can
then be read directly from the shunt box (as for the Drop Shunt Test, similar
techniques of delayed stepping and preliminary approximation should be
used).
The vast majority of concrete sleepered track comprises a resilient insulating pad
between the underside of the rail and the sleeper, with the rail secured on each
side by a clip bearing on the rail foot. Insulation is maintained by plastic pads
between the clip and the rail foot.
F2 R A I L T R A C K