General Information on Track Circuits - RGS Online

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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Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page F2 of 5 4 Rail Clip Insulation Tester Withdrawn Document Uncontrolled When Printed 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
Withdrawn Document Uncontrolled When Printed <strong>General</strong> <strong>Information</strong> on Track Circuits Railway Group Approved Code of Practice GK/RC0752 Issue Two Date December 1998 Page F3 of 5 Vibration causes the clip to wear through the pad, putting the rail in electrical contact with the concrete, which degrades the ballast resistance, and, if the track circuit is d.c. operated, may increase the levels of residual voltage. The degradation occurs gradually and identification of the failed insulations can be difficult. The Rail Clip Insulation Tester, sometimes known as “PRIT” or “K9”, consists of a unit with an extending handle, which can be rolled along the rail head (see Figure F2). Metal brushes mounted on each side “sweep” the rail clips, measuring the rail to clip insulation. Low values are indicated by an audible alarm. Description Catalogue No. Rail Clip Insulation Tester : 40/17741 4.1 Operation In d.c. traction areas, the machine must not be used on the rail adjacent to the conductor rail unless an isolation has been obtained. The brushes must be fully retracted and the insulated brush guards fitted before lifting over conductor rails. Figure F2 4.1.1 Preparation Set the brushes to the correct height. The unit is equipped with “on/off” and “polarity change” switches on the chassis and “test” and “silence alarm” buttons on the handle. Switch “on” and set the “polarity change” switch to +ve. Press the “test” button, note the continuous alarm tone and then silence it by pressing the “silence alarm” button. If no alarm is given, or if the alarm sounds only when the “test” button is depressed, the battery should be replaced with type PP9 or equivalent and the unit re–checked. 4.1.2 Use Push the unit along the rail. When an alarm is received, press the “silence alarm” button and check by re– sweeping the suspect fastening. Turn the “change polarity” switch to -ve and re–sweep: If an alarm is received, the clip is faulty; if there is no alarm, the clip on the other rail is suspect. R A I L T R A C K F3
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