TI track circuits

[Peter]

Could someone please explain the operations if a TI track circuit, with regards to TTU's and ETU's.

Many thanks

The TTU and ETU are the trackside units that the rail connections attach to. The TTU carries out the electrical equivalent of the block joint. Obviously the function of the IBJ is to electrically separate the two track sections as far as the track circuits are concerned. To this end, if you understand a bit about transmission line theory and tuned circuits, you will be able to see that with the right component values, you can arrange for it to behave like short circuit shunt for the right frequency.

The key is not a single TTU but a pair of them separated by (for UK mainline rail sections on concrete sleepers) 20m of rail. The pairs in the case of TI21 are A and B, C and D, E and F, G and H (advanced users should note that A and E, B and F, D and G, D and F can be used but that is a different topic). The components in the TTUs, together with the complex impedance of the rails and sleepers forms a tuned zone.

For two TCs of ,say A and B frequency, when TC current at frequency A encounters the tuned zone, it circulates in there in such a way that the far end of the zone between the rails is effectively a short circuit hence very little of the current at that frequency leaks into the other TC. The reverse is true for the B frequency circulating from the other TC.

So, what does and ETU do? I said that a TTU is no use on its own because it needs the additional components of the companion TTU and the rail and sleepers. An ETU for a given frequency effectively has all of the components needed to look enough like a full tuned zone for the TC to work, assuming it is installed with the track leads 1m from the pair of block joints.

This means that for TCs in plain line you would see pairs of TTUs and for the last TC of this type before another type of track circuit which needs a block joint, you will have an ETU.

There is a useful property to save on equipment if you want to end a track circuited section where TI21s are used and that is that you can avoid the need to fit block joints by using a TTU and fitting a dead short across the rails 18.5m from the TTU. As far as the track circuit is concerned, the characteristics of the tuned zone are identical and in this case, rather than the companion TTU being the short circuit, there is a real one there, but on the TC side of the tuned zone, it appears invisible so you can measure the correct track circuit voltage.

As you ask about TTUs and ETUs, I guess you either understand or don't need to know about how / what the transmitter and receiver in the loc does.

I hope that gives you some idea of how they work, but please feel free to come back if you have other queries.

Peter