7.DC TRACK CIRCUITS - A CRITICAL REVIEW
7.1 / IntroductionDC track circuit is the oldest, most popular and commonly used vehicle detection device on Indian Railways. A review of DC track circuit design has been carried out with a view to update design to improve reliability and availability of DC track circuit and to reduce associated maintenance efforts. An attempt has also been made in this report to consolidate the improvement achieved through new technologies for installation and maintenance of track circuits.
7.1.1 / Working of Track Circuits
DC track circuits are simple in working, easy to maintain and consume less power. A closed type track circuit is normally used for vehicle detection purposes. Low power signal is transmitted from one end of track circuit along one rail to a distant end (relay), the other rail forms the return path. Track circuit limit is electrically defined by the provision of Insulation Rail Joint (IRJ) in the rails at either end. Whenever track circuit portion is free of any vehicle, the detector unit (relay) gets energised. This state of track circuit is called “Track Clear”. As soon as a train enters the track circuit section, its axles provide a low impedance path between the two running rails, shunting of the feed signal causes the detector to de-energise. This is the “Track Occupied” state of track circuit, which indicates the occupation of track circuit portion by a train. This information of ‘Track clear’ & ‘Track Occupied’ is used for signalling systems.
7.1.1.1 / Track circuits are normally used for –
Presence or absence of vehicles within the limits of the track circuit.
For detection of position of train including its complete arrival (Block Track Circuit).
For establishing the direction of movement of train (Directional Track Circuit).
For assessing speed or speed control (Hump Yard, Catch Siding).
Trolley protection circuit for axle counter
7.1.1.2 / The track circuit feed signal may be DC, AC 50 Hz or in audio range or may be of complex waveform. Operating voltages range from a few hundred milli-volt to impulses of 500V. Track circuits are supposed to be maintained in such a manner that detection of a vehicle is ensured by vehicle offering low shunt resistance through its axles.
Any accidental short circuit between the rails or disconnection in the circuit will cause the detector to show occupied condition of the track, thus likely to give a false alarm though on the safe side. Reliable working of track circuit depends upon good electrical contact between wheel sets of the train and the rails upon which they run. It also depends on a continuous low impedance path between wheel axles.
As the feed source may be DC, AC at power frequencies, AC at audio frequencies or series of impulses, the detector may be a simple relay, a more complex vane relay or a receiver tuned to a particular frequency or pattern of signals.
To overcome the problems arising from traction return currents, DC stray currents and other RF interference; suitable measures are required to be taken.
Double rail track circuits are employed in Non-RE areas. Single rail or double rail track circuits may be used in RE areas. Wherever double rail track circuit is used in RE-area, impedance bond is used to provide a path for traction return current.
7.1.1.3 / Requirements of Track Circuit:-
It should be able to detect any vehicle in track circuit section including lightweight vehicles.
It should work satisfactorily for all types of rolling stock in such a manner that no vehicle is left undetected.
All fast/ slow moving vehicles should get detected.
There should not be any failure on unsafe side.
It should be immune to interference due to traction return current flow, induced voltages, stray currents etc.
7.1.2 / Track Circuit Components
7.1.2.1 / Power Supply: - Depending on the type of source of supply, track circuit is categorised as DCTC, ACTC & Electronic TC. The supply for DC track circuit is fed from a low voltage source to limit the leakage across the rails. Primary or secondary cells are used for feeding the track circuits. Wherever primary cells are used, two cells in parallel are to be used. In RE-area, use of batteries prevents the rectification of AC voltages developed on account of flow of traction return current from rail through track feed charger. Separate batteries are required for each track circuit.
7.1.2.2 / Feed Resistance: - It is a resistance used for adjusting the voltage across rails. It ensures that during shunting of the track by a vehicle, high current may not be drawn from the source of supply. In absence of any feed resistance, if internal resistance of source is low, necessary drop in the rail voltage with the shunting of the track may not occur. A regulating type feed resistance permits adjustment of the track circuit for varying ballast resistance. For fail safe working of track side, feed resistance needs to be adjusted for maximum ballast resistance.
7.1.2.3 / ‘B’ type Choke: - ‘B’ type choke is used for blocking the flow of traction return current at feed end to prevent developing of DC potential across rail. This also prevents the flow of traction return current through track feed charger and battery. Thereby preventing burning of bridge rectifier diodes and damage to battery plates due to high AC currents. If used at relay end, it enhances AC immunity of track relay.
7.1.2.4 / Track Relay: - Relay used as “track relay" is more efficient and sensitive. Such relays have low resistance, restrict power loss & offer low leakage. AC immune Plug-in Type Track Relays (PITR) to BR specification have drop away values in the range of 0.7 Volts and thus work satisfactorily under normal conditions even in areas where limited stray currents are present. Shelf Type Relays (STR) have lower drop away values. Thus they are not suitable for areas where stray currents are likely. SEM Para 17.15.4 permits use of ‘Q’ type with higher over-energisation up to 300% against 250% as in case of STR. This enhanced over-energisation of Plug-in Type track relay permits working with lower ballast resistance and increased track circuit length.
7.1.2.5 / Train Shunt Resistance (TSR): - TSR is a misnomer. In fact, it is the highest value of resistance, which when connected across the rails, will cause the track relay to open its front contact. Under track occupied condition, there is another parallel leakage path through wheels & axles of vehicle. Resistance offered by the wheels/axles combination is a variable and critical factor for the fail-safe working of track circuit. Under normal condition, the resistance offered is a fraction of a thousand of an ohm but Signal Engineering Manual caters for a worst train shunting resistance of 0.5 . With the increase in ballast resistance, relay end voltage/current increases and the TSR falls correspondingly. Value of TSR is more at relay end in comparison to feed end. The difference increases with the increase in the rail resistance. Therefore, to achieve uniform TSR, rail resistance should be kept at minimum.
7.1.2.6 / Rail Resistance: - Rail resistance depends upon the cross section & composition of the rail and also on the effectiveness of bonding at the fishplate/welded rail joints. The DC resistance of rail is very low, around 0.035 /km, although this may increase to 0.25 /km by the relatively higher resistance offered by galvanised-iron bonds at the rail joints. The rail bonding is required to obtain electrical continuity. Resistance offered by such bonding is a variable factor at varying temperature conditions. At higher temperatures, rails flush at joints against another with force, while at low temperatures rail joint fastenings are pulled apart. Thus low contact resistance is results in both the cases. However to cater for the average temperature conditions, use of bond wire becomes necessary.
7.1.2.7 / Insulated Rail Joints: - Insulated Rail Joints (IRJs) are required to electrically insulate two rails at rail joints. IRJs serves the following purposes –
To define the limits of track circuits
To provide insulation between rails
To facilitate transposition
To provide traction return isolation (i.e. at impedance bond).
For high speed and high traffic density routes, use of glued joints, being more reliable and less maintenance intensive, is preferable.
7.1.2.8 / Ballast Resistance: - Ballast resistance is inversely proportional to the length of track circuit. There is a constant leakage across the two rails of the track circuit section as the rails are at different potential and electrical path is completed on account of dust, moist ballast & sleepers. Leakage depends on the condition of track, cleanliness & quality of ballast and drainage facilities. As per Signal Engineering Manual, a minimum ballast resistance of 2 /km in station section and 4 /km outside station section, is required for functioning of a track circuit. The variation in ballast resistance causes the current drawn from the supply source to vary, thereby altering the voltage drops and leakage in the track circuit portion. Under low ballast resistance condition, due to excessive leakage, detector relay may not energise at all, thus a false, although fail safe, alarm may be available even when track is clear. Under high ballast resistance condition, higher voltage at the relay end may cause a unsafe condition by lowering the value of “Train Shunt Resistance” and thereby track relay may not de-energise even in “Track Occupied” condition. Thus a balance is to be achieved by suitable adjustment of track circuit parameter to achieve fail safe working of track circuit. A reliable track circuit must therefore, be able to work over a wide variation of ballast resistance.
7.1.2.9 / PSC Sleepers: - As per SEM, wooden sleepers, concrete sleepers or any other approved type of insulated sleepers shall be provided for track circuiting. Concrete sleepers, where used, shall have a minimum resistance of 500 between insert to insert. PSC Sleepers are now being used extensively on Indian Railways. These sleepers have comparatively low insert to insert resistance in first year after casting. Based on the field measurement, it is observed variation in insert to insert resistance is observed as –
Table – 1
Period after manufacture / Insert to insert resistance of sleepers / Remarks
15 days after casting / 150 – 200
1 month / 200-400 / In wet condition falls to 300
6 months / 500 – 600 / In wet condition falls to 400-500
1 year / >1000 / In wet condition >700
2 years / 1200 – 3500 / In wet condition 1000 -2500
10 years / 10 K and above / In wet condition 6 K
Field observations have shown that damp PSC sleeper behaves like an electrochemical secondary cell when a DC voltage is applied across the two rails provided with such sleepers. Concrete sleepers have this special behaviour during first 3 monsoons. The chemical reaction takes place in sleepers during above period and concrete batteries are formed, hence PRC sleeper behaves like secondary cell(s), with charge voltage of the order of 20 to 30% of track circuit feed voltage. However AH capacity of such batteries formed on single sleepers is very low, as these are large number of such cells in parallel in a track circuit, combined effect causes residual voltage present in the sleepers may keep a track relay energised even after removal of feed battery. This effect is responsible for delayed release of track relays. This effect will be more prominent as over energisation of track relay increases.
As PSC sleepers are fitted with insulated liners & pad between rail & sleeper, effect of these concrete batteries can only appear due to absence or degradation of these insulations. Use of insulated rubber pad under the rail foot along with insulated liner combination increases the ballast resistance level significantly even higher than those obtained with wooden sleepers. Thus presence of insulated liners and pads is necessary for satisfactory working of track circuits.
7.1.3 / Provisions of Signal Engineering Manual (SEM) for fail safe and reliable working of track circuit
7.1.3.1 / SEM Para 17.15.4 - Operating Requirements
Minimum excitation of track relay shall not be less than 125% of its rated PU value at minimum ballast resistance.
Over excitation of the track relay shall be limited to 250% & 300% of its rated PU value for Shelf type and plug-in type track relay respectively.
7.1.3.2 / SEM Para 17.21.6 Fail Safety Requirement
Minimum value of TSR shall be 0.5 for all DC track circuits.
7.1.3.3 / SEM Para 17.15.2 – Uses of Track Relays
STR 2.25 is to be used in Non RE for track circuit length > 100 metres.
STR 9 is to be used in Non RE for track circuit length <= 100 metres.
QT2 type 4/ 9 is to be used in Non RE area.
Shelf type AC immunised track relay 9 is to be used in RE area.
Plug in type AC immunised track relay QTA2 type 9 is to be used in RE area.
Plug in type AC immunised track relay QBAT type 9 is to be used in RE area.
7.1.3.4 / Minimum percentage release of track relays should be 68% of its Rated pickup value. As per normal practice a deterioration of 15% in operating characteristics is to be considered for safety reasons Hence Drop away value shall be taken as 85% of 68 i.e. 57.8% of Rated pickup value.
7.1.3.5 / As per para 17.15.5 of SEM Pt. II, the maximum permissible track circuit length under different track parameters are:
Table –2
RE/Non-RE / Min. Ballast Resistance in / Type of Sleeper / Max. length of track circuit in metre
Non-RE / 4 / Wooden/PSC / 1000
Non-RE / 2 / Wooden/PSC / 670
RE / 4 / Wooden/PSC / 450
RE / 2 / Wooden / 450
RE / 2 / PSC / 350
7.2 / Short coming of DC Track circuit
7.2.1 / Presently for berthing tracks in RE areas, three track circuit sections are required. This is due to restriction on maximum permitted length of track circuit with PSC sleepers of 350 metres only. Use of three track circuit sections for one berthing track not only affects the reliability and availability of track circuit system but it involves extra installation cost, recurring maintenance cost and increased maintenance efforts. In ideal condition, one track circuit for berthing track should suffice.
7.2.2 / TSR reduces with increase in rail resistance. Thus track circuit connections play important role in working of track circuit. Conventional track jumper and rail bond suffers from the following –
Poor mechanical strength.
Time consuming process as drilling of holes in the rail is involved.
Double bond required to improve electrical continuity.
No firm contact for low electrical resistance.
Reduction in TSR value with increase in electrical resistance offered by bond wire/channel pin on account of rusting or loose connection.
Need periodic replacements almost twice a year. Deterioration starts after 3-4 months.
Wires get burnt due to flow of high traction return current.
Loose connection causes bobbing of track circuit.
Reluctance from Engineering Department as it weakens rails and rail fractures propagate through holes.
7.2.3 / Track charger is fed from power mains. Voltage variation inpower mains causes variation in track feed voltage (minimum value regulated by track battery). In the event of track battery disconnection during maintenance, track feed voltage increases. Present design of track feed charger has Trickle/ Boost setting with maximum charging voltage up to 2.5V. Even fully charged battery on trickle charge continues to get a charging current in excess of 3A. This continuous overcharging badly affects the life of track batteries. Due to varying track charger voltage, working of track circuit is affected and reliability of track circuit is reduced.
7.2.4 / Stray currents: - Due to nearby electrical system in the vicinity, DC stray current may be prevalent in a track circuit. The voltage of the foreign current in the rail may be due to the difference of earth potential at different points. A low rail resistance path will be available for flow of stray currents through the track relay causing the track relay to pickup even though a vehicle may occupy track. The effect of stray current may further be aggravated by broken bond wires, broken rail, high TSR due to rusty wheel which may prevent the shunting of track relay and thus resulting in failure on unsafe side.
7.3 / Determination of permissible Length of DC track circuit
7.3.1 / Factors affecting length of DC track circuit are:
Feed end resistance.
Relay end resistance.
Relay resistance and AC immunity.
Maximum permissible over-energisation of track relay.
Feed voltage, float charge voltage & end point voltage of secondary cell/nominal voltage of AD cell.
Type of insulated sleeper used.
Ballast resistance.
Rail resistance.
Traction return current.
Stray currents.
Train shunt resistance (TSR).
7.3.2 / Minimum length of track circuit: - Length of track circuit should not be less than the maximum distance between two consecutive axles of a vehicle or of two adjacent vehicles. It is to be ensured that while travelling upon track circuit portion, track repeater relay shall drop during the occupancy of track circuit. In case of trolley protection track circuit, relay shall drop before track device is influenced by first wheel of vehicle.
To achieve this, minimum length of the track circuit should be such that track circuit occupying time is more than the drop away time of (Track relay + TPR + safety margin) at the maximum permissible speed of train. It is also necessary that while travelling upon two consecutive track circuits, route do not get released inadvertently during movement of the train from first track to second track circuit. To meet this, it is to be ensured that after clearance of first track, track relay shall not pickup, until the track repeater relay of second track has dropped. Therefore drop-away time of track relay and its repeater relay shall always be less than -
T = (PU time of TR + TPR + Time required between occupation of second TC and last vehicle leaving the first track by vehicle with minimum wheel base at its maximum speed.
To achieve this it is necessary that proving circuit shall register the dropping and pickup of each TR. It means operating time of register relay will further limit the maximum speed of train in the section.
Calculation for minimum track length of TC
LT = SF SP [TDA of (TR+TPR)] – LV
Where
SF– Safety factor
SP – Speed in Metre/Second
TDA– DA time in Seconds
LV – Length of vehicle at outermost wheel centres.
Typical minimum track circuit lengths required shall be as under -
Table – 3
Maximum permissible speed (KMPH) / Wheel base in Metres / Minimum length of track circuit required in Metres