TI track circuits

[hariram]

I too have little knowledge of AFTC working. Here i am stuck up with TCU i.e Track Coupling Unit testing.
Attaching file for better understanding.  

TU/ETU/TCU either can be used at a time, depending upon the situation.
However as per my knowledge 
     TCU is used in Yards and it has one side of track 
     on other hand TU has track on both end. 
Hence it says that TCU has half load as of TU.

I have TU load spec.  that is 343uH inductor and 5ohm resistor. From this i have to derive  load for TCU.  

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3.4 Tuning Unit & End Termination Unit.
3.4.1 Tuning Unit :
Tuning Unit (TU). It is a passive device.
It consists of a tuned circuit, which is used to form the Electronic Separation Joint (ESJ) as explained below.

• The Electronic Separation Joint (ESJ) :
An Electronic Separation Joint (ESJ) comprises of two Tuning Units (TU) and is thus associated with two track
circuit frequencies.
Refer Drg. No.IN1D0069, which compares the conventional joint i.e. Insulation Rail Joint (IRJ) and the
Electronic Separation Joint (ESJ).
It is evident that the ideal separation joint will have following properties:
1. Minimum crossover length or overlap.
2. Minimum signal flow allowed in the reverse direction.
3. Detection of joint failure.

Each Electrical Electronic Separation Joint (ESJ) is associated with two track circuit frequencies, Drg. No.
IN1D0070 shows one A frequency track circuit and one B frequency track circuit. ‘A’ is for transmission to or
from left, while ‘B’ is for transmission to or from right.

Depending upon the application the joint may be associated with
(i) one Transmitter (Tx) and one Receiver (Rx)
(ii) Two Receivers (Rx)
(iii) Two Transmitters (Tx).

Each Tuning Unit (TU) presents low impedance to the frequency of the adjacent track circuit.
For example, 
the Tuning Unit (TU) of ‘A’ frequency will present low impedance via LA and C2A, to the ‘B’ frequency signal, 
whilst Tuning Unit (TU) of frequency ‘B’, via LB and C2B presents low impedance to the ‘A’ frequency signal.

The inductance of the rails between the two tuning units is tuned to high impedance for both the frequencies
present by means of net capacitive reactance in the Tuning Units. Each tuning unit tunes the rails to its
associated frequency. This results in the directional transmission or reception. The voltage appearing across
the low impedance LA, C2A or LB, C2B will be determined by the losses in these components alone. For a
particular frequency, there is a ratio between the voltage across the tuning unit of that frequency and voltage
across its companion tuning unit, the ratios for each frequency and for various Tx/Rx arrangements are given
in section 17.1 of Test G.

The low impedance circuits in the Tuning Units (TU) also serve the very important function of shorting the rail-to-rail traction harmonic voltages at the track circuit frequencies. Thus the track circuit frequency component of
the rail-to-rail traction voltage is kept low enough not to swamp the Receiver (Rx) and assist in preventing it
from de-energizing the Relay ® when the track circuit is clear.
The Transmitter (Tx) output and the Receiver (Rx) input provide low impedance to the track circuit, which is
essential for correct tuning of the tuned area.

On the Tuning Unit (TU), the Receiver (Rx) is always connected to the terminals 1 & 2.
For normal power mode (track circuit lengths of greater than 200 m), the transmitter is connected to the terminals 4 and 5, whilst for low power mode (track circuit length of 50 to 200 m), it is connected to terminals 1 & 2.
The overlap zone of the tuned track circuit is measured from the point midway between the Tuning Units (TU).
Within this region both the track circuit may be de-energized by a shunt. The length of the overlap zone
depends on ballast conditions.

For Broad Gauge (1676 mm) (BG) the length of tuned zone is 19.5 +/- 0.5 m and for Meter Gauge (1000 mm)
(MG) the length of tuned zone is 22 +/- 0.5 m.

3.4.2 End Termination Unit:
Refer Drg. No. IN1D0071 for the schematic of End termination Unit (ETU).
The end termination unit is a self contained tuned circuit for the electronic separation for track circuit isolation
is not required. Such applications are:
(a) end feed, or end receiver, adjacent to insulated rail joints or,
(TCU’s are now being used for such application instead of ETU).
(b) Center feed arrangements.

The End Termination Unit (ETU) employs the same housing as Tuning Unit (TU) and the same terminations.
i.e. output to track on T1 & T2, input from Transmitter (Tx) on 4 & 5 for normal power and 3 is the earth screen.
In low power mode the Transmitter (Tx) will feed in on 1 & 2 or output to Receiver (Rx) from 1 & 2 terminals for both normal and low power applications.



3.4.3 Specification of TU and ETU:
• Maximum permissible rail to rail voltage : 110 V AC or 160 V DC
• Mounting : Vertical
• Track to Tuning Unit (TU) /End Termination Unit (ETU) cable: 2C-19 x 1.53 sq. mm (35 Sq.mm) copper cable or 50 Sq. mm equivalent aluminum.
• Maximum length of Track to Tuning Unit (TU) / End Termination Unit (ETU) cable:
Long cable 3.95 m ± 0.02 m
Short cable 2.35 m ± 0.02 m
Weight
• Tuning Unit:               8.5 Kg
• End Termination Unit: 9.5 Kg

3.5 Track Coupling Unit
3.5.1 Principle:
Schematic diagram for Track Coupling Unit (TCU) is given in Drg. No. IN1D0194. 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.
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.

Hi Peter 
Thank you for all information !!!
..........................................................................
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.

From the picture you helpfully attached, I am a little bit confused and will need to check the literature.....

It does sound as if you are using the equipment in a configuration with which I am not familiar. 
You are probably right; in my days the TI21 could only be used as a double rail track circuit.
However I do know that it can now be used through pointwork as a single rail track circuit but do not know what components are required for this; I shall have to bring my knowledge up to date and respond to you later.

But in the meantime, I still don't actually understand precisely your use of the word "load". 
I agree that the track circuit would form the electrical load for the transmitter as seen via the various transformers etc. so I have no problem with the term in that context; however I do not comprehend why you would need to know its value when testing the track circuit set up.  Please explain further.