(12-02-2011, 03:11 PM)fil Wrote: Hi,
i have just read that the max distances for the above track is:
s=600m
d=2700m
is the above roughly correct?
the reason i ask is this, (talking AC TC only)
single rail tracks are primary choice for long stretchs of plain line.
reduces IRJ's and keeps traction guys happy.
single rail TC are serial bonded, which is preffered.
double rail TC are used over point work where parrallel bonding is needed.
(im thinking all point work must be parralel bonded, is this correct?)
it doesnt make sense to me that the TC to be used on the long stretches of line is limited to 600m while the double rail TC, which is to be used only through junctions is acceptable up to 2700m.
does anybody know why there is such a big difference in single v double distances?
as for bonding, i have not got a full understanding so feel free to point out mistakes.
thanks
As you have realised, the above doesn't make sense; I am wondering from where you obtained your source information. Definitely something seems to be the wrong way around!
The important thing is that you recognised this and asked the question you did.
There are in fact different types of ac track circuits; I am assuming initially that you are talking about 50Hz ac vane relays to provide immunity from dc traction via 3rd rail.
Traction engineers need double rail track circuits to be provided wherever practicable so that there are more parallel paths for the traction current to take to go back to substation and hence less volt drop. However single rail track circuits do have to be used over pointwork due to the bonding considerations (there need to be intermediate block joints within the track circuit since otherwise the lefthand and righthand running rails would be shorted together by virtue of the rails for the other diverging route)- generally though such tracks are quite short anyway so this is ok as not too much voltdrop along length.
However I think you are probably getting confused with different forms of track circuits. The types which allow the elimination of block joints are such as the Aster "U" or TI21; it is indeed true that these are double rail and also involve ac on rails but these are quite different to the ac vane tracks discussed above. The ac is at audio frequencies (e.g. 2000Hz) and there are electronic transmitter and receivers, tuning units etc. To refer to these as "ac track circuits" would be technically correct, although since this could cause potential confusion we don't- generically would be called "jointless track circuits". These are used predominantly on plain line, primarily because the rail joints otherwise needed can be eliminated. Maximum length depends on type, frequency, rails etc but in my experience 900m is a typical maximum limit. Having said this they can be used with a single central transmitter feeding two fundamentally separate track circuits to different receivers at the two far ends and thus the combination length can be almost double.
It would be interesting to know what type of track circuit will operate at 2700m long, for what ballast resistance and with hat drop shunt; I don't recall working on tracks of that length other than some peculiar ones for a very specific application that are normally de-energised so can the operate on the "pick-up shunt" when "sweeping" a track between two mechanical signalboxes.
The 600m figure you quote sounds very much like the maximum length of a dc (ac immune) single rail track circuit in a 25kV ac electrified area (although my memory says 650m). This limitation is dictated by the need for immunity to the traction; there can be a dc difference produced as a result of traction return current flowing in one rail and at greater lengths this could become great enough to interfere with the operation of the track circuit.
Re bonding- in order to ensure that any bond becoming disconnected etc will cause a right-side failure of the TC, you are correct that series bonding is preferred. Although we attempt to do so in pointwork, in some cases it isn't possible to achieve. Therefore we have to resort to parallel bonding and double up with "yellow bonds", the idea being to try to reduce possibility of wrong-side failure due to rail break or disconnected bond. There is almost always some parallel bonding in pointwork- but it is because there has to be rather than something we aim for.
