(28-02-2012, 03:02 PM)andyrail Wrote: When DC single rail track circuits are used due to one of the rails being used as a return conductor ,all the track circuit leads are still connected onto both rails how is it that the pos and neg from the abutted trackcircuits don,t affect each other.This is actually a very sensible question, but I am afraid the answer isn't simple but I have tried my best to express it without too much complication.
The first answer is: because they are not otherwise connected- they are separate batteries (or more likely the output of different transformer rectifiers). You are perfectly right that it wouldn't work if fed from one common N6 / B6 supply distributed from loc to loc via a cable!
Think of putting three 1.5V batteries in a torch to give a 4.5V supply; this entails connecting the positive of one battery onto the negative of the next and thus the three 1.5 sum to a total of 4.5V; think of the middle battery- this will be touching the negative end of the first battery and the positive end of the last battery, but it is not shorted out is it?
The positive and negative of a battery are relative to each other but have no absolute significance. No current flows if you simply put a wire between one end of a battery and one end of another- there is not a complete circuit.
So when you put a positive and a negative lead on the same rail without a block joint between them, this is the ONLY connection between them. What you will find though is that the voltage measured across the opposite insulated rail joint is approximately TWICE the voltage of the two separate tracks (just like the batteries in the torch summing to give the grand total).
Quote:What is the level of traction return current that flows though the return rail?
Why does it not effect the track circuit ?
Very large (many 100s if not 1000s of Amps) in the case of 3rd rail dc traction railway, generally shared across both rails of a track and indeed amongst other parallel lines as well.
Rather lower but still large in the case of 25kV ac overhead traction (since voltage is so much higher and the use of booster transformers sucks it up as soon as possible into the elevated return conductor).
To some extent it certainly DOES affect track circuits but this is minimised by careful design.
This is the reason why we use:
dc tracks in ac traction areas and
ac tracks in dc traction areas.
Effort also goes in to making the dc tracks as immune to ac as possible; the design of the relay encourages the changing magnetic field to be diverted rather than appear across the armature air gap and thus falsely operate the relay. This effect could eventually be overcome if the ac voltage was large enough, but this would cause so much heat dissipation that the relay coil would very rapidly burn out and thus act as a fuse so relay can't remain falsely energised for a significant time.
Conversely ac vane relays certainly require an ac flux to set up eddy currents in the vane in order for these to rotate and operate the relay and pure dc can't do this; the problem is that the dc isn't pure. Harmonics of the mains frequency from which the dc is produced is one problem, but the real killer is the interference that can be produced by modern trains with their semiconductor variable frequency drives. There have been various approaches (the slow to operate modifications to desensitise tracks when the Eurostar train was introduced) but really the solution is a more sophisticated track circuit such as HVI or TI21 / Ebitrack 200 that need a very specific waveform.
The greatest problem actually tends to be getting the impedance bonds to be as low resistance as possible for getting the traction current to flow around the insulated block joint, yet have a high enough impedance to the track circuit frequency that it isn't shorted out by the equivalent of very low ballast resistance.
So in short and make it as simple as possible-
IT IS DIFFICULT but the trick is to make the track circuit insensitive to the very much larger traction currents; this is only possible if they are of different nature,
it is a bit of a black art and the sort of thing that makes one think that axle counters seem a very good idea!
Quote:I've looked on line for a simple answer to this but have failed to find one, I install these single rail Track Circuits all the time and wonder, how can that pos and that neg from different track circuits be right next to each other without a block joint, and then add the traction return current into the mix.
Can anyone help with my confusion on this subject, please try to make as simple as possible. Thanks.
Hope this helps; can anyone explain better?
PJW