01-08-2011, 04:53 PM
The TI21 track circuit acts as a giant transmission line between the rail-mounted tuning units. Here, the distance between the two running rails has a significant inductance which effectively becomes a "hidden" component of the track circuit. furthermore, the rails are carrying a very high level of current.
the connections to a track circuit interrupter would not be the right distance apart to have the correct impedance, and also the cabling would not be able to carry the current.
therefore, any track circuit interrupter on a TI21 track must be implemented as a separate DC circuit, operating an independent "interrupter" track circuit relay in the location. This can then be combined with the true track circuit relay in the TPR circuit, or sometimes may be reported separately to the interlocking.
I should also point out that the traditional "analogue" TI21 track can only be used in "jointless" mode in plain line, with the above characteristics. However, the new "EBI track 200" replacement can be used in "jointed" mode, in which the tuning units are omitted, and instead the digital TX/ RX units in the loc act as simple unique frequency transmitters and receivers. In this case the product can be used, with significant length limits, through pointwork, and it should be able to support TCIs wired in series with the RX unit in a similar way to a DC track circuit.
the connections to a track circuit interrupter would not be the right distance apart to have the correct impedance, and also the cabling would not be able to carry the current.
therefore, any track circuit interrupter on a TI21 track must be implemented as a separate DC circuit, operating an independent "interrupter" track circuit relay in the location. This can then be combined with the true track circuit relay in the TPR circuit, or sometimes may be reported separately to the interlocking.
I should also point out that the traditional "analogue" TI21 track can only be used in "jointless" mode in plain line, with the above characteristics. However, the new "EBI track 200" replacement can be used in "jointed" mode, in which the tuning units are omitted, and instead the digital TX/ RX units in the loc act as simple unique frequency transmitters and receivers. In this case the product can be used, with significant length limits, through pointwork, and it should be able to support TCIs wired in series with the RX unit in a similar way to a DC track circuit.