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TI track circuits
#1
Could someone please explain the operations if a TI track circuit, with regards to TTU's and ETU's.

Many thanks
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#2
(01-02-2010, 08:06 PM)Archie Wrote: 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
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#3
(01-02-2010, 11:16 PM)Peter Wrote:
(01-02-2010, 08:06 PM)Archie Wrote: 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 ......

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......
Peter

I thought I had a model answer to an exam queston on such a subject but can't immediately find it. Hopwever I did find two Powerpoint drawings that I did for it, so I attach these to illustrate Peter's description


Attached Files
.ppt   TI21.ppt (Size: 32 KB / Downloads: 194)
.ppt   inside TU.ppt (Size: 42 KB / Downloads: 171)
PJW
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#4
There are some caveats with TIs. Freq E-H are non-preferred. There are low power and mid-fed examples. TI - Traction Immune - is not a definate. They have limitations, i.e. not in P&C.

I'll try to find the circuit diagram for the internals and get someone far more knowledgeable than I to explain what makes them TI!

J
Ixion Ltd
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#5
(16-02-2010, 01:31 PM)Jerry1237 Wrote: There are some caveats with TIs. Freq E-H are non-preferred. There are low power and mid-fed examples. TI - Traction Immune - is not a definate. They have limitations, i.e. not in P&C.

I'll try to find the circuit diagram for the internals and get someone far more knowledgeable than I to explain what makes them TI!

J

The new digital ones will be apparently allowed in P&C; inbdeed I think there is already a trial site. Of course the French have used the predecessor UM71 in pointwork for many years and wonder why the UK refuse to do so.

Basically immunity is due to the fact that the signal oscillates between a pair of frequencies which ar 35Hz apart, both of which need to be present / absent appropriately over a suitable time period and they are slow to pick as a result. The logic is that interference of a certain freqiuency that persists for a matter of a second isn't a transient; conversely the probbaility of two frequencies which are not harmonically related to each other or the 50Hz mains supply is vanishingly improbable.
PJW
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#6
(02-02-2010, 08:00 AM)PJW Wrote: I thought I had a model answer to an exam queston on such a subject but can't immediately find it.
Still not located it, but I did find this Track Circuit Audit sheet used for recording values obtained during site testing but also having some useful info re expected values etc.


Attached Files
.doc   TI21 track audit sheet for MVAIR.doc (Size: 39.5 KB / Downloads: 123)
PJW
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#7
(16-02-2010, 01:47 PM)PJW Wrote: The new digital ones will be apparently allowed in P&C

It is not the fact that the are digital that makes them useable in P&C, it is the fact that at about the same time, Bombardier have developed the "track coupling unit" to replace the tuner units in single rail applications. Given that the output of the digital unit is 100% the same shape and form as the analogue, all units are supposed to work with TTU, ETU or TCU.
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#8
(01-02-2010, 11:16 PM)Peter Wrote: I hope that gives you some idea of how they work, but please feel free to come back if you have other queries.

Hi Peter,

could you tell me  how TCs is tested? and what could be the load for TCs for testing?

Secondly, as TCs is directly connected to track with what value of Inductor and Resistor to simulate the track for testing purpose.
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#9
Hi Peter,

could you tell me  how TCs is tested? and what could be the load for TCs for testing?

Secondly, as TCs is directly connected to track, with what value of Inductor and Resistor to simulate the track for testing purpose.


 am not sure quite how to interpret your questions; are these specifically asked for TI21 track circuits or more generally?

I am also not sure what level of existing knowledge you have, so I am afraid this post will be more asking you to clarify your question than give an answer! 


However I shall give some information that might help us refine your request.


TI21 track circuits are audio frequency track circuits, 
  1. primarily designed to be used in jointless configuration using a pair of tuning units separated by a length of rail to form a tuned zone to separate into different sections, therefore not needing Insulated Block Joints.  Peter has described this in more detail above,
  2. they can however be used in jointed configuration with IBJs more in the way of a dc track.
Testing is obviously a little different in each case.

I'll assume for now you are familiar with testing a dc track.  

Case 2 is quite similar to dc in some ways except that
  • there is more equipment to be checked to be of the correct type, installed in accordance with specification etc,.
  • voltages have to be taken at the transmitter end PSU, the Tx unit itself, the connections to the ETU (End Tuning Unit) as well as on the rails.  The voltages taken on the ETU and rails etc. must be taken with a frequency selective meter (aka blue  handbag) as need to differentiate "this" track circuit voltage from that of the "adjacent track" (though of course if the IBJ and other insulations are ok then there shouldn't actually be any other voltage)
  • voltages also have to be taken at similar placed at the Receiver end
  • unlike the dc track where any adjustment is made at the feed end, for the TI21 the track is adjusted by varying the gain setting at the Rx end. [Actually where the track is short there is a configuration change to Low Power by changing the connections on the ETU, but I'll ignore this].  For the traditional TI21 the gain is changed by altering strappings for 3 different coils of a transformer, altering the turns ratio by selecting a gain of 1, 2,3,.....13.  The modern form of the track Ebitrack200, is conceptually similar but the Rx is digital and it sets its own gain internally by being given two scenarios: track clear and a test shunt applied (1 ohm for Normal Power, 1.5 ohm for Low Power)
  • the track is current operated and so the tester needs to measure the voltage across a 1 ohm resistor in the input circuit, so as to get the numeric value of the current without interrupting the circuit; this has to be compared to the length of the track and gain setting to confirm that track operating in accordance with expectations.   For the Ebitrack, the current a can simply be read off the Rx unit display
  • broadly the rest of the process is analogous to the dc case, except that there is practically no hysteresis between the drop shunt and pick shunt and the TI21 is inherently slow to pick (2 seconds) so the shunt box must be operated far more slowly or a false result will be obtained

Case 1 needs more than case 2, because the rejection ration of the tuned zone must be checked.  
  • Adjacent track circuits use different frequencies in a frequency pair A/B, C/D, E/F or G/H; let us assume a boundary between a "A" track and a "B" track.  
  • The voltages on the rails at the "A" TU are taken both for frequency "A" and also for frequency "B"; similarly the voltages on the rails at the "B" TU are taken both for frequency "A" and also for frequency "B",
  • The rejection ratio for each frequency must be calculated; this involves dividing the voltage present at its own TU by the same frequency at the other TU,  This proves how good the zone is at preventing one track's voltage leaking through into the next track of the opposite frequency; the achieved figures must be checked to meet the relevant standard.
  • When testing the limits of the track, this is more involved in a tuned zone as the drop shunt in this area will be less than within the track itself, and it is important to check that there is an are in the middle where both tracks drop but the further track is not dropped by a dead short at its neighbouring TU.
The above is deliberately a bit simplified (and from memory- I don't think I have tested one myself for some 25 years!) but gives the most salient differences.

I think that covers the first part of your first question; the second part may also have been covered to some extent, depending on precisely what you meant.
  • Generally one would set up a TI21 track to drop at about 1 ohm; it is then likely to pick at 1.1ohm.  
  • However when testing to limits, the test is a 0.5ohm shunt.  In truth in a tunes zone between two Tx (not really a preferred configuration as would generally use an ETU as a centre-fed configuration of two track sections of the same frequency), then one is lucky to achieve 0.3 ohm throughout and in extremis if we could get the tracks to drop with a galvanised bond wire in the centre of such a TU we accepted it (though I suspect nowadays that the justification of this non-compliance would be very difficult- doubt whether the physics has changed, so I would bet that a "blind eye" is still turned at local level to avoid escalating as a problem!)

The second question is the one I don't really understand at all.
The rails are continuous through the tuned zone so yes electrically the two track circuits are separated by an equivalent circuit which has both resistance an inductance.  The dc resistance of 20m of rail is so low as to be negligible (however if it is jointed track with a fishplate joint bonded over by a couple of galvanised bonds then it isn't completely negligible); however the impedance presented at audio frequency is significant because of the inductance. I do not know numerically what it is; I suppose I could reverse engineer it from typical voltage readings, but really that is the concern of the product designer 30+  years ago; it is what it is and it is that value which dictates the TUs should be 20m apart on the typical 113 rail section.
There is no simulation involved in real applications; I guess that some training schools which don't have the luxury of the space etc to have a 30m length of railway line may well have got some simulated circuit of resistor and inductor to get sensible values for trainee's experience, but I do not know what those values would be.

Have I understood what you were actually asking?
PJW
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#10
Hi Peter ,Thank you for all information !!!

Let me tell you in more detail of my problem.1) TCs what you called we named it as TCU Track Coupling Unit. 
It consist of a tranformer a resistor and a capacitor which is frequency dependent. 
Principle Track Coupling Unit (TCU)  TCU is a passive device. It
can be used to terminate the track circuit. It consists of a capacitor, resistor and a transformer in series. It is
basically a band pass filter tuned at a particular frequency.It is a resonant circuit.

The major applications of TCU are as follows:
1. To terminate a track circuit.
2. In the point zone track circuit.
3. In the yards where it is difficult to form the tuned zone.

The advantages of using TCU as compared to ETU are as follows:
1. The main difference between TCU and ETU is that TCU can with stand 400 V DC / 275 V AC r.m.s. as
compared to 160 V DC / 110V AC r.m.s. for the ETU. Hence TCU can be used for Single rail
application.
2. TCU can be kept at a distance of 130 m from the rails. This facilitates the use of TCU in complex yards
where there is space constraint.
3. With TCU it is not necessary to use 35 mm sq. cable for connecting TCU to rail. 2 x 2.5 sq. mm can be
used.
4. Impedance Bonds are not used with TCU but are used with ETU.
3.5.2 Specificat ion
Maximum rail to rail volts : 400 V DC / 275 V AC (RMS)
Unit size
: 140 mm H x 142 mm W x 203 mm L
TCU to Track cable
: 2 x 2.5 mm Sq. Copper cable, 2C-19 x 1.53 sq. mm (35 Sq.mm)
copper cable or 50 Sq. mm equivalent aluminum.
Max. Distance of TCU rail Connection to
block joints.
: 1 meter


Now with the above information I am testing TCU but ideally i have no idea what should be the load,as it is directly connected to Track. Secondly as TCU has single side track its load should be half of TU.

I need more information regarding TCU testing in ideal condition.

Thanks
Hari.


Attached Files
.png   tcu.PNG (Size: 13.25 KB / Downloads: 12)
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