TI21 Track Circuits in S&C

[minty625]

I was speakinh with a friend. As part of the project he is working on, they are putting in a new crossover (although clipped and padlocked out of use) in a TI21 track circuited area. He said the standard permits this (Its not an operational crossover). It got me thinking how it would work in terms of track circuit continuity. Does anyone know if any additional bonds would be required e.g. yellow bonds or switch reinforcing bonds? I can't say I've ever come across something like this.

 

[Chris Hamilton]

I’m assuming you mean its an Emergency Crossover that’s being installed and isn’t signalled (i.e. The signaller can move the points remotely, but can’t set routes and would have to talk the train through the protecting signal) or is controlled locally by a Ground Switch Panel?

If thats the case, then the need to ensure the reliability of the track circuit through the crossover is lessened. TI21 (like all RF TCs such as Reed and Aster) track circuits are incredibly intolerant to rusty rails, even more so than DC’s, so chances are the turnout wouldn't make good contact anyway if it was a rarely used switch. Yes, a single short loco or OTP could vanish in the middle, but as the signals can’t show anything other than Danger when the points are reverse, the risk is minimal. So providing the two opposing (Up and Down) tracks are of differing frequencies (which they should be in order to reduce interference), there isn’t really much of a need for any additional bonding beyond the usual ensuring of continuity across the +ve and -ve legs through the switch itself as would be done with any normal TC.

NB - Rarely used crossovers that are “routable” by the signaller usually use High Voltage Impulse track circuits in order to overcome the rust issue and make it less likely to show clear when occupied which could allow the protecting signal to clear to a Proceed aspect.

 

[minty625]

I’m assuming you mean its an Emergency Crossover that’s being installed and isn’t signalled (i.e. The signaller can move the points remotely, but can’t set routes and would have to talk the train through the protecting signal) or is controlled locally by a Ground Switch Panel?

If thats the case, then the need to ensure the reliability of the track circuit through the crossover is lessened. TI21 (like all RF TCs such as Reed and Aster) track circuits are incredibly intolerant to rusty rails, even more so than DC’s, so chances are the turnout wouldn't make good contact anyway if it was a rarely used switch. Yes, a single short loco or OTP could vanish in the middle, but as the signals can’t show anything other than Danger when the points are reverse, the risk is minimal. So providing the two opposing (Up and Down) tracks are of differing frequencies (which they should be in order to reduce interference), there isn’t really much of a need for any additional bonding beyond the usual ensuring of continuity across the +ve and -ve legs through the switch itself as would be done with any normal TC.

NB - Rarely used crossovers that are “routable” by the signaller usually use High Voltage Impulse track circuits in order to overcome the rust issue and make it less likely to show clear when occupied which could allow the protecting signal to clear to a Proceed aspect.

Yes I believe it is being installed but not actually being commissioned. I wonder if switch reinforcing bonds etc. would still be used?

 

[Annetts key]

Yes it is acceptable for TI21 / EBI track circuits to be used through points or switches that are either redundant and which are awaiting removal or new points or switches that are being laid in ready for future use after further work has been carried out.

Appropriate IRJs to keep the track circuits for the adjacent lines separate are required as is bonding to ensure continuity through the frog/crossing and switch rails as appropriate.

It happened once on the patch where I used to work.

All the exact details are in one of the Network Rail technical standards.

INB - Rarely used crossovers that are “routable” by the signaller usually use High Voltage Impulse track circuits in order to overcome the rust issue and make it less likely to show clear when occupied which could allow the protecting signal to clear to a Proceed aspect.

For the crossovers that were provided in the late 1980s for the SIMBIDs (SIMplified {"cut price"} BIDirectional signalling) normal DC (BR904 spec) track circuits were provided but stainless steel zig-zag was welded into the top of the rails of the points used for the reverse position movements.