CH 17 Section 5 PDF

(n) Maximum length of track circuit depends upon various factors like ballast resistance, frequency adopted, track layout (whether any level crossing or bridge falls within track circuited area) and vendor of AFTC, and it cannot be laid down in absolute terms. Broadl...

(n) Maximum length of track circuit depends upon various factors like ballast resistance, frequency adopted, track layout (whether any level crossing or bridge falls within track circuited area) and vendor of AFTC, and it cannot be laid down in absolute terms. Broadly its length is limited to 700 meters in end fed mode. Note: (i) AFTC shall not be used for new works. Existing one shall continue to be maintained. (ii) Refer to Annexure: 17-MS3 for maintenance schedule of AFTC. Section 5: Precautions for DC Track Circuits & AFTC in RE Area 17.5.1 Track Circuits in RE Area Track circuit on AC electrified section may use IRJs (Insulated Rail Joints) or ESJs (electrical separation joint) and may be configured as single rail or double rail track circuit. Track circuit which use electric separation joint shall be configured only as double rail track circuit. 17.5.2 Single Rail DC Track Circuit (a) With single rail track circuits, one of the rails is reserved for the traction return current. This rail is referred to as the uninsulated rail. Any connection from the O.H.E. mast or other structure shall be made only to the uninsulated rail. Similarly, connections for the return current at feeding points as well as from booster transformers and return conductors shall be made only to the uninsulated rail. (b) As far as practicable, the rail adjacent to the O.H.E Mast shall be utilised as the uninsulated rail. However, this may not always be possible, particularly in yards where there are a large number of points and crossings or where the O.H.E masts are not always on the same side or where track circuits are staggered. In such case O.H.E. bonds crossing the track circuit rail shall be provided with suitable precaution by electrical department like insulating sleeve and clearance from bottom of the rail to avoid shorting of track circuit rail with uninsulated rail. (c) In single rail track circuits, in the event of a break in the uninsulated rail, very heavy current will flow through the track relay as well as the equipment at the feed point. To avoid this, the uninsulated rails of the adjacent tracks shall be cross-bonded at intervals of not more than 100 meters. In case the track circuit is less than 100 meters, the cross bonding shall be provided on the uninsulated rail at either end of the track circuit. (Ref: Drg no. 17-D3) (d) In the case of adjacent track circuits, the return rail shall be staggered. Chapter 17: Train Detection Track Circuits & Axle Counters Page 342 of 530 DC Track Circuit in RE Area (Single Rail) (a) This track circuit can only be configured to work as single rail track circuit. (b) Interference Mechanism, With reference to fig. above traction return current flowing in the single traction return rails creates a longitudinal AC voltage along its length, which is a function of the current value and the impedance of the traction return rail. A wheel set at the feed end will impress this interfering voltage on the relay, whilst a wheel set at the relay end will similarly impress the interfering voltage on the feed set. The track circuit equipment must be immune both to false operation and to damage from such impressed voltages. (c) Approved type of AC immunised track relays shall be used in AC traction area. (d) Length of DC track circuit is restricted depending upon rail return current. With catenary current restricted to 300 Ampere on single track section and 600 Amperes on double track section, length of DC track circuits shall be restricted depending upon the use of type of relay and sleeper. (e) The length of track circuit can be extended upto 450 meters when QTA 2 plug-in relay is used. (f) Restriction on the track circuit length due to use of concrete sleeper can be relaxed upto 450 meters by PCSTE of Railway (if adequate ballast resistance can be consistently obtained). (g) In view of the increased AC immunity due to the presence of biased magnetic arrangement, QBAT relays can be used upto a maximum length of track circuit of 750 meters using one additional 'B' type choke at the relay end, under minimum ballast resistance of 2 ohms/km. Operation of track circuit with this type of relay will require four cells delivering 8.8 V. QBAT relays shall be used in conjunction with QSPA1 relays conforming to BRS 933A. Chapter 17: Train Detection Track Circuits & Axle Counters Page 343 of 530  (h) As an interim measure, length of DC track circuit may be retained at the existing level at higher catenary current of 800 Amperes on single-track section and 1000 Amperes. On double track section by providing one additional ‘B’ type choke in series with track relay. Audio Frequency Track Circuit or any other approved type of device shall be used on sections where catenary current exceeds 300 Amperes on single- track section and 600 Amperes on double track- section. (i) Before installation of single rail DC track circuits in AC Electrified areas, measurements of stray DC currents shall be taken in accordance with the instructions in Annexure: 17-A1. The total stray current as measured, shall not exceed, (i) 10 milli-amperes, if length of track circuit is less than 100 meters. (ii) 100 milli-amperes, if length of track circuit is 100 meters and above. (iii) If measured value of current exceeds the specified limit as above, other type of track circuit suitable to work in A.C. traction area shall be used. (j) To protect the equipment from the effects of the rail voltage, a choke coil of approved type shall be provided in series with the feed resistance. 17.5.3 Audio Frequency Track Circuit for RE Area (a) In double rail track circuits, since both rails are used for traction return current, impedance bonds shall be provided at the IRJs. (b) No O.H.E mast or any other structure shall be connected to either of the rails. The Electrical Department shall run separate earth wire for earthing the O.H.E masts. (c) Connections at the feeding points and from booster transformers and return conductors shall be made to the center points of impedance bonds. (d) Only approved type Audio Frequency Track Circuit shall be used. (e) Audio Frequency Track Circuit shall be configured only as double rail track circuit and like any double rail track circuit it requires both rails to be balanced with respect to earth and with respect to current flow in each rail. (f) Impedance bond shall be used where there is no provision in design of Audio frequency track circuit to balance the traction rail return current. (g) Electric Traction Units shall be critically examined for harmonics generated by them before they are brought into service. Chapter 17: Train Detection Track Circuits & Axle Counters Page 344 of 530 17.5.4 Impedance Bonds (as applicable) in RE Area for AFTC (a) Impedance bonds used on 50 Hz electrified lines shall be of approved type. (b) The principles of operation of impedance bonds are equally applicable whether the traction return current is AC or DC The traction current rating of DC impedance bonds is usually much higher than those designed for AC traction systems. Thus, impedance bonds designed for DC traction current can usually be used on AC traction systems, although their bulk and cost is greater than a purposely designed AC impedance bond. Conversely, impedance bonds designed for AC traction systems are not suitable for use on DC traction systems. (c) Generally, AC impedance bond shall be capable of supporting a current of 400 Amperes continuously (200 Amperes in each rail), with a peak loading of 1000 Amperes for a period of up to five minutes, without suffering damage or overheating. (d) Leads between impedance bonds across IRJ on the same track shall be capable of carrying the traction current. It shall be PVC sheathed cables, terminated by compression lugs of an approved type and bolted to the impedance bond termination. (e) The design and installation of impedance bonds should take into account the requirements for mechanised track maintenance and should not be a hazard for men walking along the track. (f) The provision and maintenance of connections between rail and impedance bonds, and from one impedance bond to another on the same track is the responsibility of the S&T department. (g) Cross bonds, earth wires, connections to sub-station current return busbars and connections to booster transformers must be terminated on the centre tap of impedance bonds. The installation and maintenance of these connections is the responsibility of the electrical department. 17.5.5 Installation & Maintenance of Track Circuits in RE Area (a) RDSO’s guidelines shall be followed. (b) It is, however, important to recognize that the operation of track circuit is dependent upon the integrity of traction return bonding. It is therefore important that attention is given to the testing during installation and also during maintenance of traction return bonding. Chapter 17: Train Detection Track Circuits & Axle Counters Page 345 of 530 17.5.6 Track Bonding in RE Area (a) The Objectives of Track Bonding are: (i) To provide a path for traction return current, which ensures that no component of the track/traction return network rises above 25 V to remote earth, under normal traction load conditions and 430 V under traction short circuit conditions. (ii) To ensure that protective equipment operates satisfactorily. (iii) To minimize damage to installations due to traction short circuit. (iv) To maintain correct operation of track circuits. (b) Planning for Track Bonding (i) When planning electrification schemes, base plans will be provided. The plans will be issued to the signal department showing the track layout and proposed position of all electrification and associated structures. (ii) The signal department will then indicate on these plans the proposed position of insulated joints, impedance bonds, signal structures and track circuit bonding and in single rail track circuited areas, will identify the track circuit rail by marking with a thickened line. In double rail track circuited areas S&T department shall specifically indicate the provision of earth wire for structure earthing. (iii) The plans must then be returned to electrical department who will indicate the position of continuity bonds, cross bonds, structure bonds and all earth connections to the traction system. The plans will then be returned to the signal department for final approval and issue to concerned parties. (c) Bonding of Single Rail Track Circuits in RE Area (i) The track circuit rail must be series bonded in order to ensure that defective bonding cannot cause a wrong side failure of the track circuit. Accordingly, the bonding arrangement of the track circuit rail must ensure that the conductive path between the track circuit feed connection and the relay or receiver end connection is interrupted in the event of a disconnection. (ii) Because of traction return considerations, it is not possible for the traction return rail to be series bonded. It is therefore essential to avoid bonding disconnection to minimize the risk of loss of train shunt. (iii) In single rail track circuits, in the event of a break in the uninsulated rail, very heavy current will flow through the track relay as well as the equipment at the feed point. To avoid this, the uninsulated rails of the adjacent tracks shall be cross bonded at intervals of not more than 100 meters. In case the track circuit is less than 100 meters, the cross bonding shall be provided on the uninsulated rail at either end of the track circuit. (Ref: Drg no. 17-D3) Chapter 17: Train Detection Track Circuits & Axle Counters Page 346 of 530  (iv) On single line track circuited sections, a continuous earth wire is provided on the traction mast capable of carrying full traction return current. The uninsulated rail shall be connected to each of the traction masts by a structure bond, which shall be riveted at both ends. The arrangement for Double Line and Single Line Sections is as illustrated in given at Drg No.17-D3. (v) Typical bonding examples with single rail track circuits are shown in Drg No.17-D3. (d) Double Rail Track Circuits (i) No structure bonds, cross bonds or connections to earth wires or return conductors shall be connected to the running rails where double rail track circuits are in use. If necessary, an impedance bond must be specially installed to provide a neutral point for these connections. (e) Types of Bonds in RE Area (i) Rail Joint Bonds: Rail joints in track circuited areas shall be bonded using two 8 SWG bare galvanized steel wires or copper bonds/steel wires or flats secured by means of channel pins or welding/riveting. The S&T department is responsible for the installation and the maintenance of all rail joint bond. (ii) Rail bonds and Cross bonds: Where rail bonds and cross bonds are required for traction return purposes they shall be installed by and remain the responsibility of the electrical department. (iii) The longitudinal bonding on a non track circuited track adjacent to a track circuit shall be extended for a distance of 50 meters beyond the track circuit. (iv) In addition, the two rails of the non-track circuited track outside any track circuit or in between two track circuits shall be bonded together immediately after the block joints. (v) It is essential that the traction return and track circuit bonding is maintained in its design condition. The SSE/JE (Traction) and SSE/JE (Signal) of section will arrange for physical inspection at six months intervals to ensure conformity with approved Bonding Plan. In the event of faulty connection or bond being found immediate remedial action will be taken to restore system Integrity. The results of the joint inspections shall be entered in a register maintained by the SSE/JE (Signal) and put up within a month for the scrutiny of the Sr. Divisional Signal and Telecommunication Engineer/Sr.Divisional Electrical Engineer (OHE). Responsibility of maintenance of traction bonds is with electrical department. (vi) Detailed instructions for bonding of track are given in the AC Traction Manual. Chapter 17: Train Detection Track Circuits & Axle Counters Page 347 of 530