Track Circuit For Railway

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TEACHING NOTES ON TRACK CIRCUITS

SIGNAL & TELECOMMUNICATION TRAINING CENTRE, BYCULLA, MUMBAI ( I S O 9001-2000 CERTIFIED )

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CONTENTS 1. 2. 3. 4. 5. 6. 7.

Introduction Principle of working Types of track circuits Basic components Factors affecting working of track circuit Insulation joints D.C.track circuits a) In non RE area b) In RE area 8. Staggered polarity 9. Cut-section track circuits 10. Wiring of track circuits 11. Dead section 12. A.C.track circuits 13. Maintenance schedule 14. Trouble shooting

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3 Introduction: Track Circuit means an electrical circuit, provided to detect the presence of a vehicle on a portion of track, the rails of which forming a part of the circuit. Purpose: The track circuit is used to detect the occupation or vacancy of the track portion by the condition (i.e. track relay is down or pick up) of track relay. Principle of Working: The portion of the Track which is to be Track circuited, is first provided with wooden or RCC sleepers and then the rails are insulated from the rest of the Track by the provision of Insulated rail joints. At one end of the Track circuit feed is connected and at other end track relay is connected as shown when the track portion is not occupied by trains, the fed is available for track relay and the relay is in energized condition. When a train occupies the track circuit portion, the axel & wheels shunt the track much of the circuit current is passed through the wheels due to less resistive path and very less current is available for track relay, which is not sufficient enough to pick up the relay. Types of Track Circuits: 1. With respect to the Source

a) D.C. Track circuit b) A.C. 50Hz c) AC 83 1/3 Hz

2. With respect to type of functioning

a) Closed Track circuit b) Open Track circuit.

3. With respect to traction return current (In traction area)

a) Single

Rail

Track

Rail

Track

circuit. b) Double circuit .

4 4. With respect type of regulating device (only in a) Resistance fed Track AC Track circuit.)

circuit . b) Reactance fed T/ circuit c) Condenser fed T/ circuit

5) With respect to type of wiring (only in case of

a) Series Type

point & crossing tracks)

b) Multiple type

6) With respect to type of Independence Bond (only

a) Resonated

in case of AC double rail track circuit)

Track

circuit b) Auto coupled Track circuit

BASIC COMPONENTS OF TRACK CIRCUIT 1) Battery (Source of supply). 2) Regulating Resistance. 3) Track Relay. 4) Fuses (protective device). 5) Choke (protective device for AC RE Area). 6) Track lead cable. 7) Continuity Rail Bond. 8) Insulation Rail Joints. FACTORS AFFECTING WORKING OF TRACK CIRCUITS: TRc

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Track Lead Resistance at feed end.

TRr

-

Track lead Resistance at Relay end

RT

-

Regulating Resistance

Rr

-

Rail Resistance (Rail joint Resistance)

RB

-

Ballast Resistance

TSR -

Train Shunt Resistance

5 TRACK LEAD RESISTANCE AT FEED END & RELAY END: It is the total resistance of the Track Lead Cable from location to Track JB and resistance of Jacketed wire rope from Track JB to Rail connection at the feed end. It increases with the increase of the length of the Track lead cable. So more voltage drop across then resistance and voltage on rail will be reduced. RT

-

Regulating Resistance

It is the external variable resistance connected in series at the feed end. It has following function 1) To adjust and feed necessary voltage to track relay. 2) To prevent the short-circuiting of the supply by the axle and wheels of a train. 3) If the regulating device is not provided and if the internal resistance of the power supply is very low, then necessary voltage drop will not occur when track is occupied and track relay will not drop. 4) In case of AC Track circuit to get necessary phase angle. RB

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Ballast Resistance:

The effective resistance of various leakage paths between the rails caused by the sleepers and ballast in the track-circuited portion is known as ballast resistance. It is varying factor as it depends upon weather conditions. It value ins maximum in summer and minimum in monsoon. For an ideal Track Circuit, the value of RB of any Track Circuit should be as high as possible. It is inversely proportional to the length of the track Circuit. In order to get high Ballast Resistance: a) The ballast must be periodically screened. b) The clearance between Ballast & bottom of rails must be at least 50mm. c) Drainage should be properly maintained. Ballast Resistances value in case of wooden sleeper: a) 4 ohm/Km in Block Section. b) 2 ohm/Km in Station Section.

6 Ballast Resistances value in case of RCC sleepers. a) 1 ohm/Km in Block Section and Station Section. b) 0.6 ohm/km in Block Section and Station Section in RE Area. NOTE: As per latest Instructions of RDSO’s Circular No. STS/E/DC Track Circuit dated 25.01.1995, irrespective of type of sleepers used, the minimum Ballast resistance for DC Track circuits in RE area is 2 ohm/Km. FORMULA FOR CALCULATION OF BALLAST RESISTANCE RB IN CASE OF DC TRACK CIRCUIT: Ballast Resistance ‘RB’ Where VF VR IF IR

-

=

Average Voltage ---------------------Leakage Current

V F+ V ---------2(I F -IR)

Voltage at fed end Voltage at Relay end. Current at Feed end. Current at Relay end.

If the length of Track circuit is L meters and value of ‘RB’ Ballast Resistance calculated is ‘r’ then RB/Km = RxL/1000 ohms IN CASE OF AC TRACK CIRCUIT: In AC Track circuit, RB can be calculated by recording both Track current at feed end and track voltage at 0:4mtr length of the take circuit from feed end with Relay end open circuit Vol. Across rails at 0.4 lengths from feed end RB = ----------------------------------------------------------------------Feed end current when the relay end leads are open. RB = V/A There

7 Rail Resistance: The Resistance offered by the rails of a Track circuit is known as “Rail Resistance”. The rail bonds are provided to give electrical continuity at the fish plate Rail Joints. The Rail bonds shall be 8 SWG galvanized Iron wire. These bonds are inserted into 7mm holes drilled in the web of the rails and secured by tapered channel pins. The value of Resistance e should be as low as possible. It should not be more than 1.5 ohms/km for Track circuit up to 700 metre Length and 0.5 ohm/km for Track circuit more than 700 metre length. As per latest circular NO. STS/E/DC Track Circuit dated 25/27.1.95, Rail Resistance shall not be more than 1 ohm/1km 9wtih no reference to length of Tack Circuit).

FORMULA FOR CALCULATING RAIL RESISTANCE: Voltage droop in the Rails Rail Resistance

Rr

=

=

--------------------------------------Average track circuit Current (VF – VR) ----------2( IF + IR)

=

2(VF – VR) -------------( IF + IR)

Where VF

=

Feed end Track Voltage

VR

=

Relay end

IF

=

Feed end Track Circuit Current

IR

=

Relay end Track Circuit Current

Rail Resistance is directly proportional to the length of the Track circuit. Rail Resistance / KM

=

R/L x 1000 ohms.

Where ‘L’ is the length in meters and value of Resistance calculated in R ohm.

8 Train Shunt Resistance: The maximum value of Resistance which when connected across the rails of a Track circuit cause the front contact of “Track Relay” to open is called TSR. The value of TSR should not be less than 0.5 ohms for DC Track circuits and 0.15 ohms for AC 50Hz Track circuits. Where DC Track circuits are provided with RCC sleepers and 9 ohms DC Track Relay. TSR value has been reduced from 0.5 ohm – 0.25 ohm with the approval of rail Board. The value of “TSR” is inversely proportional to the value of Relay voltage and as well as ballast resistance. TSR is inversely proportional to Ballast Resistance for a given setting of Track circuit (i.e. as long as the Track circuit adjustment is not disturbed.

TESTING OF TSR: TSR must be checked: a)

During periodical maintenance.

b)

After carrying out any alteration or adjustment in the Track circuit.

c)

Not only at relay end but also at all parallel portions of the Track circuit.

Testing of ‘TSR” in parallel portion ensures that parallel portion has not become dead section.

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INSULATION JOINTS Insulation joints are provided to isolate the track circuited portion from the remaining portion of the track OR to isolate one-track circuit from adjacent track circuit electrically. TYPES OF INSULATION JOINTS: 1)

RDSO

2)

Glued Joint

-

a) 4 bolt type

-

b) 6 bolt type

RDSO JOINT COMPONENTS: Nylon End Posts

-1NO.

Nylon Left Hand side Channels

- 2 Nos.

Nylon RH side channel

-2 Nos.

Nylon Ferrules or bushes

-8 Nos.

Nylon backing plate with collar

-

4 Nos.

Iron backing plate with out caller -

4 Nos.

Here providing bushes in fish plate holes and side channels insulate fishplates and rails. Glued Joints Two types 1)

G3 (L) type having 6 bolt

2)

G3 (S) type having 4 bolt

COMPONENT END Posts

-

1 No.

Left hand side channel

-

2Nos.

R.H. side channel

-

2 Nos.

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6 Nos. for 6 bolt joint & 4 nos. for 4 bolt joint

(fiber glass) Formulas or Bushes

10 EPOXY SEALING MATERIAL: HOC fishplate and bolts are not isolated and both are isolates from rails by means of bushes (provided in Rail holes) and side fibers TESTING (INSULATION TEST): In Dry Season: Insulation resistance shall not be less than 25 mega ohms when a megger voltage of 100 VDC is applied across the joints. In Wet Season: Insulation resistance shall not be les than 3 kilo ohms. PRECAUTIONS NEEDED WHILE INSERTING: Glued Joints: 1)

At least 10 sleepers on either side of the joint must be well packed before the joint is inserted to avoid damage /fatigue of the joint.

2)

No damage shall be caused to the joint while inserting

3)

While welding the joint with adjoining rail the heat shall not spread to the joint. Heating appliances shall not be applied at a distance of 1m from the joint.

SLEEPERS For Track Circuit RCC sleepers are used. Rails are isolated from RCC sleepers by means of pads & liners. Rails are seated on sleepers through pads & sleepers and are holding in position by means of pendol clips. Both pendol clips and rails are isolated by means of liners. MINIMUM PERMISSIBLE RESISTANCE OF A CONCRETE SLEEPR: In Non-RE and AC RE Area, 500 ohms after 6 months from the date of manufacture. METHOD OF MEASUREMENT Measurement shall be made with a sensitive multi meter of not less than 20Kohm/voklt resistance of coil. Megger not be used. After cleaning a spot on the surface of each insert, measurement shall be made between insert A&B, A&C, A&D, B&C, B&D and C&D.

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D.C.TRACK CIRCUIT DC Track Circuit is one in which the source of supply is DC. These Track circuits are commonly used in non-electrified areas and also in 25KV AC electrified areas.

In non-RE Area: 1) 9 ohm DC neutral TR should be used for Track circuit up to 100 metre length and 2) 2.25 ohms DC neutral TR should be used for Track circuit more than 100mtr length. 3) When track circuits are provided with RCC sleepers in order to have economical working length of Track circuit equal to Train length (670M) use of 9 ohms relay is approved by Railway Board. 4) Staggering is done to detect the failure of block joint. FAIL SAFE ADJUSTMENT OF DC TRACK CIRCUIT. The following shall be kept in mind before adjusting DC track circuit. 1) Track voltage should not be more than 250% of its rated PV values. 2) TR should be excited at 125% of its rates pick value when RB is minimum and battery voltage is normal. 3) TR voltage should not be more than 85% of its DA value when 0.5 ohm TSR is connected across the track circuit. PROCEDURE 1) Note down the TR particulars a) Pick up value____________ b) Drop away value _________ 2) Calculate the following: a) 250% of P.U. value __________ b) 125% of P.U. value ___________ c) 85% of D.C. value_____________

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3) Disconnect the TR and bring it to the feed end connect it to the feed through the feed end junction box as shown isolating the rails. RB is maximum now. 4) Connect a 0.5 ohm TSR across the TR at the track lead JB adjust the TR voltage to 85% of its DA value. Now the TR should remain in dropped condition. 5) Disconnect the 0.5 ohm TSR 6) Measure the TR voltage. It should not be more than 250% of P.U. value. It more, readjust the regulating resistance and bring the TR voltage exactly to 250% of P.U. value. 7) Now connect the feed and TR to the rails in their respective places without disturbing the adjustment. 8) Calculate the value of ballast resistance and rail resistance by using the formula. They must be with in the permissible prescribed value. 9) Calculate value of an external resistance which when connected across the rails will bring down it value of ballast resistance to the minimum permissible value for that length of track circuit. 10) Connect the external resistance across the track.

The relay

voltage should not drop below 125% of P.U. value. If it drops, then check the battery.

IN 25 KV AC RE AREAS: The following types of Track Circuits are used in 25 KV AC RE area: 1) DC single rail track circuit. 2) 83 1/3 Hz AC single rail/double rail track circuit. 3) AFTC (Audio Frequency Track Circuit).

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DC SINGLE RAIL TRACK CIRUCIT IN RE AREA Components: 1) Track relay – AC immunized 9 ohm DC Track relay irrespective of length of Track Circuit. 2) Regulating device ‘F’ type (0-25 ohms) Rheostat type 3) CHOKE – ‘B’ type, Imp = 120 ohm Resistance = 3 ohms It is use to protect feed set from Traction return current. TRANSVERSE BONDING In case of isolated Track Circuit, insulated rail joints are provided on rails carrying +ve polarity of Track circuit voltage only. The other rail carrying –ve polarity is not provided with any insulated joints. However in between two consecutive track circuits, insulated joints are provided in both the rails so as to be able to maintain “stagger” tack circuit polarities.

Also the negative rails of adjoining track circuits are

provided with a cross connection-bonding strip in between, known as transverse ‘bond’. This transverse bond: 1) Facilitates; passing of traction return current ahead from one-track circuit to the other. 2) Helps in detecting a block joint (insulation rail joint) failure between the two track circuits. CORSS BONDING: In case there is a break in the traction return path of track circuit as shown, the heavy traction return current passes through the track feed source to the insulated rail and returning to the non-insulated rail through the track relay at the other end to go further ahead. This can cause unsafe condition in track circuit working. To avoid this, an alternate path shall be available for traction return current in such circumstances.

14 In multiple line section traction return rails in track circuits are cross-connected with bonding straps at an interval of about 100 metres in between them. LONGITUDINAL BONDS: There are also known as rail Bonds provided to maintain continuity of track circuit of the rail joint. This continuity is maintained by using 8 SWGGI wire with channel pins. In DC traction area thick copper wire roles known as rail bonds are used to maintain electrical continuity of heavy traction return current as well as track circuit current and these are provided by electrical department. In AC RE area the length of track circuit with: 1) Wooden Sleepers – 450 mtrs 2) RCC sleepers

– 350 mtrs.

3) With QTA2

_450 mtrs.

4) With QBAT

_750 mtrs

STAGGERED POLARITY: Where continues Track Circuits are installed, the polarity of the adjoining rails of the adjacent track circuits must be opposite, which is known as “staggered polarity”. If the staggered polarity is not maintained and if the insulation joint between the two adjacent track circuits fails, then there is a possibility of the track relay associated with an occupied track circuit being energized by the feed of the adjoining track circuit, especially when the resistance offered by the axle, wheels and contact resistance of rails is of high order. If the staggered polarity is maintained and whenever the insulation joint fails, both the feeds of the adjacent track circuits will get short circuited and track relay drops.

15 TESTING OF STAGGERED POLARITY: Short the insulation joints between the track circuit and observe the track relay. If the staggered polarity is correctly maintained, the relay will drop. The staggered polarity must be tested during periodical maintenance of the track circuit and also whenever any alteration or changes or adjustments are made in track circuits. CUT SECTION OF TRACK CIRCUITS: The term cut-section is defined as the sub-division of a track circuit of low ballast resistance. Whenever the track circuit required for the purpose of controlling a signal is very long, or the ballast resistance of the proposed track circuit is so low that it cannot be worked satisfactorily, cut section track circuits may be installed. Though the section is divided into parts, the circuit will have the characteristics of single-track circuit. In the above arrangement, 3TR pick up proves that both 1TR and 2TR are in picked up position, as the feed to 2T is controlled through the pick up contact of 1TR and the feed to 3T is controlled through the pick up contact of 2TR. Because of the above arrangement, only 3TR pick up is to be proved for clearing the signal. WIRING TRACK CIRCUIT For track circuiting turnouts, depending on the mode of connection between the rails of different track circuit portions three types of arrangement are possible. 1. Both positive and negative rails in series 2. Positive rail in series and negative in parallel. 3. Multiple wiring.

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Both positive and negative rails in series:

Feed End

Relay

Disadvantages: 1) Any disconnection in Jumpers or rails in the parallel portion is not getting detected as ‘TR’ remains up. 2) If a train occupies the parallel portion with a disconnection in it, then ‘TR’ will remain up which is unsafe.

DEAD SECTION: It is a portion of track circuit in which occupation of a vehicle cannot be detected. Due to same polarity on both rails of that portion, it is being bypassed by the track feed. It is happening due to difficulty in providing the block joints in square, which are because of engineering department’ s restrictions in making the cut in rails. Under any circumstances, the dead section should not be longer than the minimum distance between the wheels of a four-wheeler vehicle, so that the vehicle must shunt adjacent live portion of track circuit while being on dead section.

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A.C.TRACK CIRCUITS: AC Track circuits are provided exclusively in DC traction areas such as in Mumbai Division of W.R & C.R. It is possible to work A.C. Track circuits of frequency 83 1/3 Hz in AC traction area where traction power frequency is 50Hz. These Track circuits are of two types: 1) Single rail – Where traction return current is passes through one rail only. 2) Double Rail – where traction return current passes through both the rails of the Track circuit. Single rail Track circuits, as already seen in the case of DC Track circuits in AC RE area, are those in which traction return current passes through one of the two-track circuit’s rails.

In these, the

traction return current passes from one-track circuit to another through transverse bond provided by the traction power department. In DC traction areas, a 440/110-130V-track transformer is provided in the cabin for this purpose. 130 VAC 50Hz is supplied to the Tr. Relay as local supply each Tr. Relay having different set of phase allotment for local supply with reference to control phase. In case of 83

1/3

AC track circuit 165 VAC is used by local phase. The

phase distribution process is same as above. Another 110 V 3 Ph. Feed is taken as output from the trans and fed to a ring mains bus bar taken to various locations in the yard for track circuit feeding. The above phase distribution to local & control supplies is made for the following reasons 1) Load on each phase is to be balanced as far as possible. 2) Staggered phase connection is made on adjacent track circuits rails to avoid unsafe condition at the time of block joint failure.

18 110 V 50Hz of control supply or 110v 83 1/3 Hz Ac control supply between the nominated phases in the location is connected as input to the track feed transmission for stepping down the output from a suitable tapping on its secondary is connected with a 3 ohms damping resistance in series to the track lead Junction Box. sq.mm copper conductor or multiple

From JB 2.5

wire ropes (jacketed wire)

connects the feed to the rails. At relay end, track leads are taken similarly to JB from these 2.5 sq. mm. or 1.5 sq. mm cable takes this feed to the relay end step of transformer with a 2 ohms protective resistance in series from the secondary of this transformer feed is taken on a common cable from location to the cabin for feed relay control . In this way single rail AC track circuit is functioning AC DOUBLE RAIL TRACK CIRCUIT It has been already mentioned that double rail track circuit are those in which the traction return passes through both the track circuit rail by using Impedance Bond. Impedance Bond It is a device used for bonding the rails of a double rail track circuit with adjacent track circuit rails for the purpose of conducting traction return current while blocking the track circuit current to pass with in its relay coil. There are two copper cols in the bond wound on shell type core one is called the Main or buffer coil, which consists of about 8 to 10 turn of a heavy cross section. This carries the large traction return current when connected between adjacent rails across block joints at the end of a track circuit.

19 The second winding is called as the “Auxiliary Coils” which has a large number of turns of a smaller cross section with different tapping for connection. A 110/25-75v step down track fed trans draws its input from two nominated phases of supply in the location box. Its secondary output is taken on a cable to the impedance bond provided on the tack at feed end. The supply is connected to suitable tapping on the auxiliary coil of the bond with series resonating condenser of 10 mfd

this condenser

also acts as a regulating device for dropping a large voltage across ti under track occupation. The main coil of the Impedance bond is connected across the track rails with two multi-stand steel wire rope known as “side connections” of the bond. The voltage is stepped down on the bond at the feed end. The center tap of the bond main coil is connected to the adjoining track circuit by means of a neutral connection wire role for traction return current. At the relay end of the track, the main coil of another Imp. Bond is connected across the rails to draw about 1V from the track. This will be stepped up suitably on the auxiliary coil of the impedance bond. Also, the phase complying thin voltage gets connected as required by the connection of 10 uF bond condenser on suitable tapping of it auxiliary coil this output voltage at a phase angle above 60 degree with respect to source is fed. To the relay end track lead cable on the band where is laid up to the cabin relay room. This voltage is applied to the track relay control.

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MAINTENANCE SCHEDULE OF TRACK CIRCUITS: General: Track circuits shall be so maintained that: There is always a good connection between power feed and track, track relay and also between adjoining rails, through jumpers and rail bonds. The ballast resistance remains always high and does not fall below prescribed values. The limiting resistance shall be as high as possible. The insulating joints are of high resistance. The surface of the rail is clean and is free of dust, sand and foreign materials. The circuit is properly energized during wet weather on minimum ballast resistance conditions and during dry weather, on maximum ballast conditions. The track circuit shall not be over energized to such an extent that the shunting value drops below 0.5 ohms for all types of track circuits. However, the shunting value of double rail track circuits with AC track relays should not drop below 0.15 ohms. These values shall be obtained irrespective of whether the track is provided with concrete sleepers or wooden sleepers. The connection of DC track relay shall be quarterly interchanged to prevent permanent magnetization. Track relays: Pick up and drop away values shall be maintained within the limits specified by the manufacturer. Relays shall not be tilted or turned over to close the contacts. Insulated rail joints: a) Before opening a insulated rail joint, the components required for replacement, confirming to the rail section, shall be kept ready by the side of the track.

21 b) For replacement of end-post when there is no creep at the insulated rail joint, loosen the rail fastening and pull back the rail and insert the end-post between the rail ends. c) It is imperative that when an insulated rail joint is provided, at least three sleepers on either side of the insulated rail joint shall be packed properly. d) Fish bolts shall be kept tight. Nuts shall be tightened several times during the first two weeks after installation/ replacement, until components of insulated rail joints are firmly set. e) A metal flow is seen often at the rail-table at the joints. Such metal flow forms a lip and creates sharp burns at the rail ends. Projections formed at the rail ends shall be chiseled without damaging at the end-post so that these do not bridge the rail expansion gap and cause a short circuit. f) Brake-block dust, which may accumulate on the head and sides of the end-post and surface of the fish plates, shall be brushed frequently so that the possibility of electrical conductivity being established between the rail ends is eliminated. g) Opening and fixing of fishplates of nylon insulation joint for installation/replacement shall not be done by S&T staff as it is responsibility of Engineering department. h) Special type of pendol clips (‘J’ type) shall be provided at nylon insulation joints/ glued joints to avoid touching of pendol clips with the fishplate. i) Periodic coating by insulating varnish/ epoxy over the nylon insulated joints/ glued joint to avoid shorting due to brake dust shall be done. j) Take the voltage readings across the track relay terminals and note it. When the adjacent track circuit is shunted or disconnected, any change in the voltage reading will indicate a faulty insulated joint.

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