Chapter 21: Relay & Electronic Interlocking PDF

Chapter 21: Relay & Electronic Interlocking Section 1: Design Criteria of Signalling Circuits 21.1.1 Circuits - General (a) The Detailed designs for circuits of interlocking may be required for; (i) Route Setting Type i.e. on the basis of Entry/Exit principle....

Chapter 21: Relay & Electronic Interlocking Section 1: Design Criteria of Signalling Circuits 21.1.1 Circuits - General (a) The Detailed designs for circuits of interlocking may be required for; (i) Route Setting Type i.e. on the basis of Entry/Exit principle. (ii) Non-route Setting Type i.e. Route is set with individual operation of points for existing installations or alterations. (b) The designs shall be suitable for RE as per stipulations in Chapter 22 of SEM on "Requirements of signalling in 25 KV AC electrified areas". (c) The designs shall be in accordance with the approved signalling plans, Control Table, Control Panel diagram, Relay contact analysis and relay rack arrangements. (d) The designs shall be suitable for the specific type equipment to be installed such as Make of Electronic Interlocking (Distributed or Central), MSDAC, SSDAC, UFSBI, Block Working built into Electronic Interlocking etc and the type of Interfacing & features, they provide. Use of dual OFC in Distributed type of Electronic Interlocking, are desirable as they reduce extent of copper cables & provide an alternate path in case of Failures. (e) The drawings & designs shall be made in accordance with Typical templates, RDSO`s Standard circuits/templates, guidelines, Board guidelines, Industry`s best practices & principles of Reliability, Availability, Maintenance & Safety in mind. 21.1.2 Control Panels-General (a) The display of the layout on the front of the Control Panel referred to as the illuminated diagram shall be well proportioned. Where additional facilities are likely to be provided, domino type panel shall, preferably, be used. The areas covered by each track circuit shall be clearly distinguished by use of different colours. (b) The operating members, namely route switches/buttons, point switches etc., referred to in the following clauses shall normally be provided on the illuminated diagram itself in Geographical order. A separate illuminated diagram for indication and a separate 'console' containing all the operating members may also be provided. (c) At Non-route setting type installations, the route may be set with individual operation of points. (d) The Route setting shall be on "Entry/Exit" principle for installations of Route Setting type. Each route shall be controlled by means of two push buttons-one at the entrance and the other at the exit of the route. Chapter 21: Relay & Electronic Interlocking Page 433 of 530  (e) In Non-route setting type installations, after setting of the route by individual operation of points is completed, signal can be cleared by an individual push button in conjunction with a group button or by an individual switch controlling each signal or a common switch for conflicting signals which are not required at the same time, or one push button at the entrance and other at the exit end. (f) Individual push button in conjunction with a common push button or two/three position switches shall be provided for individual operation of points. (g) Where a route has more than one overlap, it shall be possible to select and set the desired overlap beyond the exit signal of the route. (h) Where the route has alternate approach routes, it shall be possible to select and set the desired route with desired overlap. (i) The switches/buttons shall have distinctive colours so that they can be readily distinguished, such as running signal red, shunt signal yellow, calling on signal red with white dot & exit button white. The alternate overlap white with black dot, alternate route Grey, point black, slot green, crank handle blue and point group button black with red dot etc. 21.1.3 The control panel shall be provided with: (a) Arrangement for individual operation of points. (b) For emergency operation of points (where provided, during point zone track circuit failures), each individual operation shall be recorded on a counter. (c) Necessary slotting facilities for adjoining cabins, ground-frames, level crossings and crank handles etc. (d) Arrangement so that slot shall be controlled by operation of two buttons or a switch similar to route setting principle for individual line. For cancellation of slot, a group slot cancellation button to be operated along with entrance/exit button shall be provided. Each such cancellation shall withdraw the slot but the route controlled by the slot shall be kept held for a predetermined time delay or till route locking if any, is effective. Each slot cancellation shall be recorded in a counter. (e) When SM's key is taken out, it shall not be possible to change the last operated position of any signalling gear from control panel. However, facility shall be provided to put back the signal to 'ON' position without altering the route. It shall be ensured that no command gets executed automatically on reinsertion of SM's key. (f) An indication panel shall be provided in major yards, giving position of the yard and important indications for maintenance staff. 21.1.4 Indications on Control Panel The control panel shall be provided with the following indications. Chapter 21: Relay & Electronic Interlocking Page 434 of 530 (a) Point Indications (i) The position of points shall be indicated either by white/yellow or green lights near each individual point switch and by white strip light on the point zone and by white strip light on the point zone in case of push button. During operation of the points, light/strip light for the intended position shall flash till points are correctly set and locked. (ii) The point locked indication in route shall be given by illumination of a small white light near the point or points switch/button which shall be extinguished when point is free. (b) Route Indications (i) Route indication lamps shall be provided to indicate setting and locking of the route. Indication that route is set and locked shall be given by a set of white lights (not less than two) on each track section. When any route is not set, route indicator lamps shall be extinguished. (ii) The complete route over which the movement is to take place shall be lit with a row of white lights when the route is correctly set and locked. (iii) As the train moves over the route, the portion of the route occupied shall change to red and after the train has cleared the particular track, it should change back to white lights until the sectional route/entire route is released, when the white light is extinguished. (c) Signal Indications (i) Indication that a stop signal is at 'ON' shall be given by a red light and a permissive signal is at 'ON' shall be given by a yellow light on the corresponding signal symbols on the control panel. (ii) Indication that stop/permissive signal is 'OFF' shall be given normally by a green, yellow or double yellow light as signal is seen in the field, on the corresponding signal symbol on the control panel. In case of signal „OFF‟ aspect failed to lit at site, the corresponding signal aspect indication on the control panel shall be indicated with flashing. However, in case of a domino panel where indication of all aspects is not provided, any 'OFF' aspect can be shown by green along with the following flashing indications for conditions as enumerated below:  Failure of green; green flashes red* blank;  Failure of yellow; green flashes, red lit;  Failure of double yellow, both green and red* flash; &  Failure of red*, green not lit (blank), red* flashing. *To be read as yellow for a permissive signal. (iii) Signal indication for the 'ON' aspect of a shunt signal on the same post as the running signal, is not required. In the case of shunt signal on independent post, the 'ON' indication shall be given by a white light strip or two miniature white lights in a horizontal position on the corresponding shunt signal symbol on the control panel. Chapter 21: Relay & Electronic Interlocking Page 435 of 530  (iv) Signal indication for 'OFF' aspect of a shunt signal shall be given by a slanting white light strip or two miniature slanting white lights. Such indication of a shunt signal located on the same post as a running signal, shall be given below the running signal indication. (v) A white light for 'A'/'AG' marker indication should be lit up on the control panel below the symbol of the signal when set for automatic working. (vi) Indication that "Calling on" signal is 'OFF' shall be given by a white light below the running signal indication of corresponding signal symbol. (vii) Starter Indicator`s indication shall be provided by a white strip light which should be lit when repeater of the signal has displayed 'OFF' aspect. This indication should be provided by a white strip over the running signal symbol on the control panel. (d) Track Circuit Indications (i) Indication that the track circuit is occupied shall be given by a row of red lights (not less than two) on the each track circuit portion. (ii) When the track is not occupied, the red lights shall be extinguished. (e) Power Supply Indications (i) An indication to indicate the availability of the supply from the Mains/Diesel Generator/Catenary 1 or 2 should be suitably indicated, if such indications are not provided in Change over Panel in SM room. (f) Other Indications (i) Approach track circuits shall be indicated on illuminated diagram. In continuous track circuit territory, the approach track circuits will cover all the track circuits in rear of the first stop signal up to the next signal in rear or up to the track circuit specified in the Control Table for approach locking. An approach track circuit controlling calling on signal shall be indicated separately with a distinct mark. (ii) Advance approach warning of trains if required shall be indicated on the control panel in the form of flashing lights or other type of indication and audible bell warning to attract attention of Station Master. These visual and audible warnings shall stop as soon as the approach track circuits are occupied or signals are taken 'OFF' for the train. It shall be possible to silence the audible warning by pressing a push button. This cancellation shall not apply to train approaching subsequently for which the push button must be pressed again. (iii) Signal Lamp Failure alarm: Failure of lighting of a signal aspect/route aspect at site shall light a red indication on the control panel and give an audible warning for the same. Such indication may cover a group of signals. A common audible warning can also be provided for a group of signals. It shall be possible to silence the audible warning by pressing a push button. Such cancellation of audible warning shall not apply to subsequent failures for which the push button must be pressed again. Chapter 21: Relay & Electronic Interlocking Page 436 of 530  (iv) Indication shall be provided on the control panel distinguishing between locked and free condition of crank handle as below -  When CH key is in & locked - Yellow Steady  When CH key released from control panel but CH key is not extracted/When CH key is inserted back but CH control is not taken back from control panel - Red Flashing.  When CH key released and CH key extracted - Red Steady  When Emergency CH key release control given from control panel with 120 Seconds timer in progress - Yellow Flashing (v) The respective signal lock indication (white light) shall start flashing when an emergency route cancellation is initiated, in an approach locked condition of the route. This indication will extinguish after a lapse of two minutes and cancellation of the route. (vi) If push buttons are used on control panel, a buzzer to indicate that push button/push buttons has/have been left pressed may be provided. (vii) Suitable indications for slots shall be provided (viii) Gate control shall be provided on the control panel as below-  When gate is closed & locked - Yellow Steady  When gate slot released from control panel to open but gate is not open/When gate is closed but gate slot is not taken back from control panel - Yellow Flashing  When gate is open - Red Steady (ix) The control panel indication lamps shall be operated by not more than 24V miniature type bulbs or light emitting diodes. (x) Return wires from indication lamps and relays shall be suitably bifurcated so that excessive current does not damage them. Any break in return wire shall not fail the indication/relay or give a wrong indication or pick-up a wrong relay. (xi) When an approved electronic flasher relay other than Mercury type is provided, a flashing white indication shall be provided at appropriate corner of the control panel to indicate to the operator the satisfactory working of electronic flasher relay. 21.1.5 Interlocking and Circuit Requirements: General (a) Before designing the detailed circuits, Control Table showing approach locking, back locking, overlap release, isolation, interlocking, dependence of signal aspect, grouping of crank handles and the condition for release of crank handles, route release, gate release etc. shall be drawn up and approved by the competent authority. Provisions of GR & SR shall be observed. (b) Signal circuits shall be so designed that the signal shall not change to a lesser restrictive aspect than intended one and route shall not be released because of fluctuations in power supply voltage or when the supply resume following its failure. Chapter 21: Relay & Electronic Interlocking Page 437 of 530  (c) Design of signal circuits and equipment shall be suitable for 25 KV AC electrified areas as per provisions of Chapter 22 of SEM on "Requirements of signalling in 25 KV AC electrified areas". (d) Common return shall not be provided in vital circuits. (e) Where relays other than 24 V, 1000 ohm relay with metal to carbon contacts are used, the circuits shall be so designed that not more than 45 such contacts are used in series in a circuit. (f) The proving of back contacts of metal to metal relays should be done in the circuitry. (g) All new installations shall be of route setting type interlocking. 21.1.6 Route Setting Type (a) The route setting and clearance of signal shall be with the following conditions: (i) SM's key is In (ii) The interlocking is free (b) The points are correctly set and locked in the route including overlap and isolation (if any) to desired position. (c) The crank handle keys for all the motor operated points in the route, overlap and isolation are locked and their control is not released, shall be proved in Signal Clearance only. The keys for all the key-locked points in the route, overlap and isolation are locked and their control is not released. (d) All interlocked level crossing gates are closed and locked against the road traffic in the route and overlap, if any, shall be proved in Signal Clearance only to allow gate closing in the last. (e) Slot if any, from other agency has been received. (f) The track circuits in the route up to the next signal/dead end, overlap and isolation and Fouling, if any, are clear. (g) Stop signal ahead is not blank (h) The interlocking between conflicting routes shall be achieved through route interlocking electric circuits at route initiation stage. (i) Approach locking or dead approach locking shall be provided for all manual stop signals. Approach locking shall be continuously effective from the predetermined point on approach of the signal. (j) Controls on level crossings, ground frames, cabins, sidings etc., shall be suitably interlocked. Chapter 21: Relay & Electronic Interlocking Page 438 of 530  (k) For clearing Calling On signals, proving of track circuits in the route and overlap are not required. Calling On signal shall detect all the points including interlocked level crossings in the route, which the main signal above it detects, except points in the overlap. Stop signal ahead is not blank is not needed. (l) For clearing of shunt signals, proving of berthing track circuits, points & LC gates in overlap, stop signal ahead is not blank is not needed. (m) A white indication for block control on the last stop signal should be provided on control panel, where- (i) Section ahead is worked on Absolute Block System and control panel and block instrument are placed at different locations. (ii) Block working is by means of axle counter or track circuit. 21.1.7 Non Route setting Type: (For existing installations & Alterations only. No new Installations of Non-Route setting type to be provided). The route setting and clearance of signal shall be with the following conditions: (a) The points in the selected route and if required in overlap and isolation are operated to required position by individual operation of switches or push buttons in conjunction with group push button. (b) Operation of entrance exit buttons/switches or one control switch clears the signal if; (i) SM's key is In (ii) The interlocking is free (c) The points including hand operated points in the route including overlap and isolation if any, are set, locked and detected. (d) Other conditions are same as para 21.1.6 (d) to 21.1.6 (m) above 21.1.8 Route Release Circuits (a) Complete route release including overlap shall be effective only after the signal governing the route is put back to 'ON' and corresponding route switch/button has been operated to normalize the route. Automatic route release by the passage of train and complete release of route through sequential route release shall be provided. (b) Where a route has got number of route sections, the circuit shall be so designed that the route section does not release only by picking up of the concerned track relay. Same is released only after the next track circuit is also dropped and picked up, except in case when the last track is a berthing track. In cases where the route is controlled by single track circuit, the route release shall be controlled after predetermined time delay. Chapter 21: Relay & Electronic Interlocking Page 439 of 530  (c) The route release circuit shall be so designed that it will release only when at least two track circuits drop and pick-up in sequence. (d) In installations provided with route setting system, sectional route release shall be provided if nos. of routes are more than 100. In such cases sub- route section already released shall permit setting up of other routes, if interlocking otherwise permits. (e) In case of installations provided with non-route setting system, sectional route release shall be provided if nos. of routes are more than 100. The complete route release shall be effective only after the signal governing the route is put back to 'ON' and corresponding route switch/push button has been operated to normalize the route. However, where sequence proving relays are provided to prove the authorized passage of a train, automatic route release shall be provided. (f) It shall be possible to release a route in emergency after suitable time delay as per Table-1, with the approach track occupied, provided the train has not passed the signal during the time interval. (g) It shall be possible to release a route in emergency after a suitable time delay as per Table-1, where approach track circuits have not been provided and after the signal has been put back to danger provided the train has not passed the signal during this time interval. (h) When the route is released by the passage of train, the overlap points shall be released only after the lapse of suitable time delay as per Table-1 after clearance of last point track circuit of the route i.e. the train is on berthing track. (However, in major yard this timing of route release can be reduced as per Table-1 with the approval of PCSTE). On cancellation, the overlap points shall be released simultaneously along with the main route. (i) Facility of emergency route/sub-route cancellation shall be provided and it shall be possible to release the route/sub-route with suitable time delay as per Table-1. Cancellation shall be operated after physical verification of the line by traffic representative. Every such cancellation shall be recorded on a non-resettable counter and in a register to be maintained by traffic representative. 21.1.9 Signal Control Circuits (a) It shall not be possible for a signal to assume 'OFF' aspect in installations provided with route setting facility unless the route switches/buttons have been operated and relevant route has been correctly set and locked and relevant track circuits are clear. In installation provided with non-route setting facility, the signal shall not assume 'OFF' aspect unless the route is set and locked, relevant track circuits are clear and signal switch/button has been operated. In case self-restoring type of push buttons are used, signal shall not assume 'OFF' aspect unless concerned push buttons are pressed simultaneously for 3 seconds and released. (b) Circuits shall be so designed that the failure of any part of a circuit affecting the control of the signal shall cause the signal to display a most restrictive aspect. Chapter 21: Relay & Electronic Interlocking Page 440 of 530  (c) The circuit shall be so designed that in case of failure of lighting of a Main signal aspect at site, the more restrictive aspect is lit automatically and in case of failure of lighting of ON aspect, it shall not be possible to clear the signal in rear. (d) Fouling protection, approach locking, indication locking, route locking, siding control key locking, crank handle locking and track locking shall be incorporated in the relevant control circuits. (e) Each aspect light of a signal may be proved where necessary and the aspect indication shall be provided. (f) Wherever required, necessary control on the level crossing, ground frames, cabins, siding control key, crank handles etc. shall be provided. (g) The correspondence of the point control relays and point indication relays may be proved in signal circuits before the signal displays an 'OFF' aspect. 21.1.10 Locking of Advanced Starter and Starter Signal at Junction Stations (a) Starter released by Advance starter is not required at way side stations having single & double line block working, where track circuiting have been completed. (b) Starter released by Advance starter is required at all diverging ends of junction stations, stations having twin single line block working and also at stations where track circuiting has not been provided between starter and Advance Starter. (c) All signals shall be automatically replaced to 'ON' by the passage of a train past the signal. However, starter signals shall be automatically replaced to 'ON' by the passage of a train (after a specified time delay, where required as per Table-1). 21.1.11 Level Crossing Gates circuits (a) It shall not be possible for a main signal to assume 'OFF' position unless all interlocked level crossing gates in its route and overlap are closed and locked against the road traffic. (b) It shall not be possible to open any such interlocked gate at a station until the concerned signal is replaced to 'ON' position and the route up to the gate is released or it is proved by suitable circuit arrangement that train has cleared the gate. (c) Mid section Gate signals shall be provided with Dead approach locking with Time delay as per Table-1. (i) Back locking for Gate signals in automatic signalling territory shall be provided. (ii) Back Locking may also be provided, if the distance of gate of a signal to LC gate is more than 180 meters, by two track circuits (where required) based on risk assessment. Chapter 21: Relay & Electronic Interlocking Page 441 of 530 21.1.12 Point Control Circuits (a) The points once set shall remain in the last operated position until these are operated by different route setting or by individual operation of points, except when stipulated otherwise. However, isolation points of catch siding, slip siding may operate automatically after the passage of train to their isolation position where sectional route release is provided. (b) Moreover, soon after CH/NX control is released, points shall not be operated by route setting operation. However, the points can be operated individually from the control panel/Control Terminal till such time the CH/NX key is not inserted in point machine. (c) Operation of points shall be possible only when the interlocking is free and relevant point track circuit is clear. Emergency operations, where provided when point zone track circuit fails, it shall be possible to operate the concerned point provided interlocking is free by releasing an emergency control. Each such operation shall be recorded on a counter. (d) Interlocking between points shall be provided only to the minimum extent necessary. (e) Point control circuits shall be so designed that a cross connection or a short circuit cannot operate a point or give a false indication of the same. (f) Hand operated points shall be equipped with circuit controllers that are operated by the switches when closed. Keys controlling such switches shall be electrically locked by the approach track back locking track circuit so that it's not possible to work them in the face of an approaching train. (g) Means shall be provided to cut off the motor feed in case of obstruction to the point movement after a predetermined time lag, based on the type of point machine used. (h) The point operating control circuits shall have track circuit control, overload protection and cross protection. 21.1.13 Crank Handle & Siding Control Keys (a) Crank handle/point NX key provided for manual operation of the points worked by electric point machine must be interlocked with signals. (b) The crank handles shall be provided to facilitate operation of points in case of failure of point machines. For this purpose, if necessary, the points in the yard may be divided into convenient groups and to distinguish particular groups, crank handles with different wards shall be used. The crank handles shall, however, normally be locked. It shall be possible to release them for use in point machine by releasing a control from the control panel. With the crank handle released, it shall not be possible to clear the signal for the concerned routes. The interlocking of crank handle can also be achieved by using NX key of different wards. Chapter 21: Relay & Electronic Interlocking Page 442 of 530  (c) It must not be possible to release the crank handle/point NX key unless the signals have been put back to 'ON' position and concerned route is released. However, if the route remains locked due to what so ever reason, it shall be possible to release the crank handle/point NX key after a time delay of 120 seconds from the time the signals have been put back to 'ON' position. Each such CH/NX keys release operation shall be recorded on non-resettable counter. (d) In major yard where number of points are more, these points should be grouped in different zones maintaining the yard flexibility. The crank handle/point NX key for each group of point machines should be so arranged that they cannot be interchanged. (e) Siding control key for manual operation of siding points, taking off from running lines, shall be so interlocked that it may not be possible to clear the signal leading over to that line when the key is released for operating siding points for shunting purposes. (f) It must not be possible to release the crank handle/point NX key unless the signals have been put back to 'ON' position and concerned route is released. However, in case of failure of Interlocking or for any other emergency case (E.I/both Control Terminal& Control Panel/both Control Terminals fails) an „Extreme Emergency Key‟ and self restoring „Extreme Emergency Button‟ shall be provided. Turning of key along with pressing of „Extreme Emergency Button‟, shall restore all signals to „ON‟ aspect and after a time delay as per Table-1, all CH/NX Keys shall be free and it shall be possible to release the CH/NX key. To normalize this emergency operation, ASM shall restore back the „Extreme Emergency Key‟. Each such „Extreme Emergency key‟ release operation shall be recorded on a non-resettable counter. 21.1.14 Cross Protection features (a) For purely internal circuits including vital circuits, double cutting or cross protection may not be provided, unless specifically required. (b) All external circuits shall be suitably protected from cross connections and rendered immune to false operation by induced voltages in RE areas. 21.1.15 Control Panel/Control Terminal [Video Display Unit (VDU)] (a) Stipulations given in above paras shall apply also apply to Control Panel for Electronic Interlocking Installations. These Control Panels shall not be used in future E.I installations. (b) Control Terminal/multiple Control Terminals shall be of appropriate size to display layout of the Yard in well-proportioned dimensions and shall display all functions clearly distinguishable. Normally Control Terminal/multiple Control Terminals shall be provided in redundant mode. (c) Video Display indications on Control Terminal shall conform to Control Panel indications and UFSBI Block panel as Block working built into Electronic Interlocking (EI) as covered in Section above or other approved type. Chapter 21: Relay & Electronic Interlocking Page 443 of 530 (d) Operation of Signals, Points and other Controls such as Gate, Crank handle, Siding, Slot, Block Working etc. shall be carried out through appropriate dropdown menus or other approved means. (e) Control Terminal shall have suitable protection facility against unauthorized operation which may be provided by physical „SM Key‟ in a separate box and key contacts are taken to pick up two separate „SM Key‟ relays in relay room for redundancy purpose. These relay contacts are taken as input through two different input cards preferable in different OC‟s and „SM Key IN‟ bit shall be made by their parallel proving inside E.I. (f) All Emergency operations shall be two-step process protected against unintended operations. (g) Control Terminal shall have provision for Signal, Point and Line Block Collars equivalent of Control Panel in the form of individual „Block‟ & „Un- Block‟ feature which shall remain effective even during changeover from one Control Terminal to other Control Terminal. „All Un Block‟ option shall be provided while E.I starts up to un-block all signals, points & lines after a time delay of at least 120 seconds. SM‟s key shall be required for unblocking of a function whereas no SM‟s key required for blocking. There should not be any automatic unblocking of functions. Blocking of a function shall be indicated on Control Terminal by different mark or colour over the concerned function. (h) Where Control Panel and Control Terminal are provided, it shall be possible to switchover the controls from Control Panel to Control Terminal and vice-versa, and where two Control Terminals are provided, it shall be possible to switch the controls from one Control Terminal to other Control Terminal through suitable means by either manual switch or password. (i) Indication that E.I system is healthy shall be displayed on the Control Panel using a blinking green LED or blinking green indicator on Control Terminal. (j) Operation of points shall be possible only, when the interlocking is free and relevant point track circuit is clear. Emergency operations provided when point zone track circuit fails and after line verification from site by traffic personnel, it shall be possible to operate the concerned point by inserting an emergency key in a separate box provided interlocking is free. Each such operation shall be recorded on a non-resettable counter. (k) Each „Normal‟ or „Reverse‟ operation of point shall be controlled through two output relays derived from two output bits should be from different output cards of E.I. (l) Last controlled position of points shall be available by using latch relay in point control circuits. (m) Mid stroke point reversal preventing feature shall be provided to prevent starting of operation when the point is under operation or tried for other position during mid stroke. Chapter 21: Relay & Electronic Interlocking Page 444 of 530  (n) Maintenance Terminal of Electronic Interlocking system shall be provided for the Signal Technician. (o) Block Working built into EI shall be provided for line clear purpose. It shall be possible to take a line clear when block section is clear upto first stop signal including Block overlap. Indication of Block built into VDU and indication of TCF, TGT, TOL, Bell, Block clear, SNK, SNOEK shall be as per drawing no. RDSO/S/32019. 21.1.16 Precautions for design of circuits using Electronic devices The following fail-safe principles shall be incorporated in the design of the equipment. (a) Component failure shall be self-detecting by way of causing a signal to display a most restrictive aspect as far as practicable. (b) All fail-safe circuits shall work on continuous energisation principle such that open circuits in wiring, relay contacts, etc., or loss of power supplies shall not cause unsafe conditions. (c) Due consideration shall be given to the effects of faults in fail-safe electronic equipment to allow open or short circuit or earthling conditions and variation in component values due to ageing, replacement of faulty component with new components of specified tolerance, etc. Safety shall not be impaired as a result of multi-terminal devices failing-either open circuit, short circuit or with partial short circuit between any pair of terminals or earthing. (d) The physical construction of fail-safe equipment shall be designed to eliminate the possibility of external objects causing short circuits between combinations of terminals in vital circuits. This may be achieved for example, by adequate separation of terminals, and by the fitting of protective shrouds where necessary. (e) Stipulated guidelines shall be followed in the design, Installation, Testing, Commissioning & Maintenance of Electronic Equipments/Systems. 21.1.17 Train Detection (Track Circuits & Axle Counters) (a) Provisions of SEM Chapter 17 on Train Detection (Track Circuits & Axle Counters) shall be followed. (b) DC track relays shall be centralized in location/relay room only by using separate cables of adequate cross section. (c) Track circuiting of the entire station section including all lines where direct reception is provided shall be done. (d) Type of Axle Counter – SSDAC, HA-SSDAC, MSDAC and Location of Track section Proving Relays and Type of Interface - whether Signal Cable or Quad cable or OFC shall be considered. Chapter 21: Relay & Electronic Interlocking Page 445 of 530 21.1.18 Signalling Cables. (a) Provisions of Chapter 15 of SEM on Cables shall be followed. (b) At least two cores of cable shall be provided between two ends of the yard to cater for telephone communication for maintenance purposes in non RE areas. Where the yard is extending over a large area, the location boxes may be grouped and one socket may be provided in one of the location boxes in the group so that telephone communication is conveniently available for co-ordination with the cabin during maintenance or rectification of failures of various ground equipments. Where such groups are situated in different directions from the cabin, a separate pair of conductors shall be used for each direction connected to the cabin. In RE area, provision shall be made for telephone communication through a telecommunication cable if already available or a separate telecom cable shall be laid for the purpose. (c) Earth leakage detectors of approved type may be provided to detect any leakage to earth in cable conductors. 21.1.19 Signals in Outdoor (a) The main signals shall be of multi unit colour light type. Shunt signals shall be of position light type. LED signals of approved type shall be used. (b) Route indicators shall be either of the direction type or multi lamp unit type or of Stencil type conforming to approved design. (c) 'A' & „AG‟ marker lights for semi-automatic signals shall be of approved type. 21.1.20 Point Machines - Operation a) The point operating control circuits shall have track circuit control, overload protection and cross protection. b) Crank handles with different wards shall be used. The slots in the point machines provided to take the crank handles would also be suitably made to take only the crank handle applicable to the group. The crank handles shall, however, normally be locked. It shall be possible to release them for use in point machine by releasing a control from the control panel. With the crank handle released, it shall not be possible to clear the signal for the concerned routes. The interlocking of crank handle can also be achieved by using NX key of different wards. Notes:- (i) For more details of Installation of Outdoor signalling Equipment, Refer to Chapter 19. (ii) In case of alterations to existing installations, Detailed documentation for alterations, duration of Non-Interlocking/Disconnection & sequence of working shall be made carefully and concerned staff to be briefed. 21.1.21 Various Timers in Signalling circuits shall be as given below in Table-1. Chapter 21: Relay & Electronic Interlocking Page 446 of 530