Signalling and telecommunication

A
PROJECT REPORT
ON
SIGNALLING AND TELLECOMMUNICATION
SUBMITTED BY
ADRITA MAJUMDER
EMAIL ID- adrita.majumder@gmail.com
SUPERVISED BY MR.ATANU DEY
DEPARTMENT-ELECTRONICS AND COMMUNICATION
JUNE, 2015

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CERTIFICATE
MR.ATANU DEY
DY. CHIEF SIGNAL AND TELECOM ENGINEER
EASTERN RAILWAYS, KOLKATA
This is to certify that project report of B.Tech, held during the 6th
-7th
semester break entitled-SIGNALLING
AND TELECOMMUNICATION is a document of work done by Adrita Majumder of ACADEMY OF
TECHNOLOGY under my guidance and supervision during the period, June, 2015.
............................................
MR.ATANU DEY
DY. CHIEF SIGNAL AND TELECOM ENGINEER
EASTERN RAILWAYS, KOLKATA

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STATEMENT BY THE CANDIDATE
ADRITA MAJUMDER
B.Tech, 7th
Semester
Department of ECE, Roll Number 08
Academy Of Technology
I hereby state that the technical presentation entitled signaling and telecommunication has been prepared by
me to fulfill the requirement of the vocational training during the period JUNE 2015.
....................................................
ADRITA MAJUMDER

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ACKNOWLEDGEMENT
I would really like to thank every person who has helped me to complete my report successfully. All the
websites where I have taken help from and all my friends who have helped me to chose this topic and collect
every bit of information about the topic. Special thanks to my project mentor MR ATANU DEY without
whom completion of this very report would have been just impossible. He has given me his valuable time
and worthy opinion to create my project successfully. Definitely my parents are worth mentioning who have
kept supporting me throughout and have kept faith that I could do it.

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ABSTRACT
In this report I have given an overview of the signal and telecommunication systems that have been used and
are presently being used in the Indian Railways as a part of the day-to-day signalling and communication
procedures.
I have covered in this report the history and the latest developments in railway signal and communication as
well as related fields. I have made an elaborate study on the various equipments that have been used and are
currently being used as part of communication in the railways.

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TABLE OF CONTENTS
TITLE PAGE NUMBER
CERTIFICATE BY THE SUPERVISORS I
STATEMENT BY CANDIDATE II
ACKNOWLEDGEMENT III
ABSTRACT IV
SOLID STATE INTERLOCKING 1
INTEGRATED POWER SUPPLY 5
SINGLE SECTION DIGITAL AXLE COUNTER 9
DATA LOGGER 15
OPTIC FIBRE 22
SIGNALLING RELAYS 34
CONCLUSION 40
BIBLIOGRAPHY AND REFERENCES 41

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SOLID STATE INTERLOCKING
INPUT CARDS
All the field conditions (i.e. Field relay contacts) are connected to these input cards of EI system.
The maximum inputs capacity of each RI card will depend on design of the RI cards by different
manufacturers. The total number of inputs will depend on the yard layout.
Total inputs means:
 Field inputs : ECRs, TPRs, NWKR etc.
 Panel inputs : GNs , UNs, NWNs, RWNs, etc.
 Read back inputs : HR, DR, WNR, WRR etc
Opto couplers are provided to isolate field optically from the system in Input cards. These cards will read the
conditions of inputs and passes the information to EI system.
PROCESSOR CARD
This card is also called as central processing unit card of the System. This is provided with microprocessor,
RAM, ROM, EPROM, EEPROM Memory IC’s. These EEPROMS or EPROM’s (ROM’s) are programmed
with software required for executing the system commands.
System software consists of the following:
- Executive software programmed in system EPROM’s
- Application software programmed in DATA EPROM’s.
-
EXECUTIVE SOFTWARE
 This software is common to all EI’s for the same company manufacturing.
 This is a factory installed software.
 Performs all operations.
 Cuts off vital supply voltage to output relays, in case of unsafe failures.


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APPLICATION SOFTWARE
 This software is specific to each station and different for different stations.
 This is as per table of control of specific station.
 Can be installed at site by signal engineers.
 Logic installed through Boolean expressions or user-friendly equations.
OUTPUT CARD (RELAY DRIVE CARD)
This card receives the output of CPU card as input and picks up relevant output relay as per the panel
operators’ request. The output of this card is terminated on phoenix terminals from there the output relays are
connected.
The essential modules of an E.I. is as follows.
 Hardware module
 Software module
HARDWARE MODULES USED IN THIS SYSTEM:
Equipment consists of :
 CARD FILE:
Each card file is like a shelf having 20 Slots to accommodate various PCBs that are used in a system.
Slot nos.1 to 15 and 20 are used to accommodate Non-vital Input-output or Vital Input or Vital Output PCBs.
Slot no.16&17 are used to accommodate Power supply PCB. Slot no.18&19 are used to accommodate CPU
PCB. In this cardfile a mother board is available in the rear side connecting all the 20 Slots. This cardfile is
suitable to mount on a 19” rack.
Power
Supply
Card
CPU
Card
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20
 CPU PCB
Each card file to have one CPU PCB and always placed in slot no.18&19. In this card Micro
Controller used is Motorola 68332 and its speed is 21 MHz. In this card, 4 nos. of flash EPROMs of 8 MB
are used to store executive and application software, Two nos. of fast Static RAM (each 64KB) are used to
process the vital data and Four nos. of Static RAM (each 64KB) are used to store events and errors.
The main functions of CPU is, it monitors continuously status of Vital Boards. It also monitors
system internal operation for faults and responds to detected faults. It processes application logic based on
inputs
received and deliver outputs to drive external gears. It records system faults and routine events in user-
accessible memory. It monitors and controls the serial communication ports. It controls power to vital outputs
through external VCOR relay.
 POWER SUPPLY PCB

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Each card file to have one Power Supply PCB and always placed in slot no.16&17. Power supply
PCB is basically a DC-DC converter that converts 12V DC input supply is +12V, -12V and +5V required for
various board functioning. Based on diagnostic check by CPU, Power Supply Card receives 250Hz signal
from CPU and extends supply to VCOR relay. This card provides isolated supply to internal circuit.
 VITAL OUTPUT PCB
Each Vital Output PCB has 16 Outputs. It is available in 12V and 24V DC applications. Each Vital
Output can drive an output device such as any Q-series relay. This output relay in turn controls signals,
points, crank handle, siding control, level crossing etc. Since Vital Output drives the relay, which controls
important outdoor gears, all the Vital Output boards are continuously diagnosed by a CPU. Any abnormality
in any of the outputs will shut down the system to ensure safety.
 VITAL INPUT PCB
Each Vital Input PCB has 16 Inputs. It is available in 12V and 24V DC applications. Each Vital Input
is assigned to read the status of outdoor gears such as Track circuits, Point detectors, Crank handles, Siding
controls, level crossing etc. Since the Vital Inputs read the status of outdoor gears, they are normally
configured with double cutting arrangement using relay contacts.
 NON-VITAL INPUT/OUTPUT PCB
Each Non-vital I/O has 32 inputs and 32 outputs in one PCB. It is available in 12V and 24V DC
applications. Non-vital inputs are Panel push buttons and keys. Non-vital outputs are Panel indication LEDs,
counters and buzzers. The status of Non-vital Input/output is known from LED indications available in front
of the card.
 VITAL CUT OFF RELAY- VCOR
Each card file will have one VCOR to ensure the healthiness of the system. VCOR has 6 F/B
dependent contacts each rated for 3 Amps. When system is healthy the coil receives voltage from Power
Supply PCB, which in turn controlled by CPU. Power to Vital output board is controlled by VCOR, thus
ensuring safety.
 WIRING HARDWARE
48 Pin Address select PCB and Connector assembly is provided for Vital Input and Vital Output
cards. 96 Pin Address select PCB and Connector assembly is provided for Non-Vital I/O cards. 48 Pin
Connector Assembly is provided for PS and CPU PCB. EEPROM PCB which is provided on rear side of the
CPU connector to configure various serial communication ports. Keying plugs are provided in the cardfile to
ensure coding to each type of cards.
SOFTWARE MODULES USED IN THIS SYSTEM:
System software consists of the following:
EXECUTIVE SOFTWARE
 This software is common to all EI’s for the same company manufacturing.
 This is a factory installed software.
 Performs all operations.
 Cuts off vital supply voltage to output relays, in case of unsafe failures.
APPLICATION SOFTWARE
 This software is specific to each station and different for different stations.
 This is as per table of control of specific station.
 Can be installed at site by signal engineers.
 Logic installed through Boolean expressions or user-friendly equations.

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A GEC-MANUFACTURED SSI INTERLOCKING CUBICLE
A PAIR OF DATA LINK MODULES
TRACKSIDE FUNCTIONAL MODULE

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INTEGRATED POWER SUPPLY
INTRODUCTION
A typical 4 line station requires power supplies of 24 V D.C( 5 nos ) , 12 V.DC ( 5nos ), 6V (2 nos),
110 V DC and 110 V AC for signalling. These require as many chargers and Secondary cells & Invertors
requiring more maintenance & spares. Can they be Integrated in to one system.
Thus the concept of Integrated Power Supply has been evolved by integrating concepts One Charger,
One set of Battery Bank feeding Invertors and D.C- D.C converters for deriving various D.C & A.C.
voltages. Integrated power supply system delivers both AC & DC Power supplies as an output with the
output voltage tolerance of ± 2 %.
ADVANTAGES
 Reduces maintenance on Batteries, Battery charger & overall maintenance.
 Its construction is in modules and hence occupies less space. Reduced space requirement, resulting in
saving of space for power supply rooms.
 Provides centralized power system for complete signaling installation with continuous display of
working status of system for easier monitoring.
 Defect in sub-units of system is shown both by visual & audible indication. Reflects the condition of
battery with warning.
 Replacement of defective modules is quick & easy without disturbing the working of the system.
 It uses (n+1) modular technology hot standby arrangement and hence high reliability and more
availability of the system.
 The system provides uninterrupted supply to all signalling system even during the power failures.
Thus, No blank Signal for the approaching drivers.
 System can be easily configured to suit load requirement.
 The diesel generator set running (Non-RE area) is reduced almost to ‘NIL’. Hence, low wear and tear
of D.G. set components & reduced diesel oil consumption.
COMPONENTS
(a) Un-interrupted power supply (U P S)
(i) SMPS Battery chargers with Hot stand-by mode.
(ii) Hot Standby PWM Inverters with auto changeover
(iii) CVT Regulator [FRVS]
(b) AC distribution board [ACDB]
(i) STEP DOWN TRANSFORMERS
(c) DC distribution board [DCDB]
(i) DC-DC converters.
WORKING
IPS works satisfactorily for A.C input variation of 150V AC to 275V AC with single-phase power supply
and frequency variation from 48 Hz to 52 Hz. The input is fed to SMPS charger, which converts in to 110
V.D.C as output. It is fed as input to three sub units.
 To battery bank charging the batteries.
 To ON line inverters that converts 110 V.D.C in to 230 VAC ± 2%as output.

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 As 110 V.D.C bus bar to D.C Distribution Panel as an input to various D.C-D.C converters located in
it.
 A 110 V Battery Bank of VRLA cells are connected to SMPS Panel. IPS Status Monitoring Panel is
located at ASM room or at S&T staff room if round the clock S&T staff is available at Station.
CONSTRUCTION
IPS mainly consists of:
 SMR (Switch Mode Rectifier) Panel / SMPS based Float cum Boost Charger (FRBC) Panel.
 A.C. Distribution Panel.
 D.C. Distribution Panel.
 Battery Bank. (110V DC).
 Status Monitoring Panel.
SMR (SWITCH MODE RECTIFIER) PANEL / SMPS BASED FLOAT
CUM BOOST CHARGER (FRBC) PANEL
It consists of SMR / FRBC modules and Supervisory & Control Unit. SMPS based SMRs
(converters)/ SMPS based Float cum Boost Chargers (FRBC) modules are provided with (n+1) modular
technology hot standby arrangement with active load sharing basis and 1 additional module as a cold standby
(n+2). Supervisory & Control Unit, which controls and monitor the complete system. It has various
indications on the panel reflecting the working of the panel.
FRBCs are suitable for operating in parallel on active load sharing basis with one or more modules of similar
type, make and rating.
n = required no. of modules to cater for actual current requirement.
A.C DISTRIBUTION PANEL
It is made of ON-Line inverters with (1+1) modular technology hot standby arrangement & CVT
(Constant Voltage Transformer) / AVR (Automatic Voltage Regulator) and set of step down
transformers.The inverter is protected against overload and short circuit with auto reset facility. Whenever
the failure occurs, it trips and restart automatically after about 10 to 20 sec. But if the problem persists, the
protection is permanently gets latched and it will not be switched ON again unless the fault is cleared
followed by pressing of reset button. The output of inverters is regulated to 230V AC ± 2%, 50Hz ±1Hz for
an input voltage variation of 90V DC to 140V DC. Normally both the Inverters are powered ON and both are
delivering the Output voltage but only one (main) inverter is connected to the Load. If main inverter is failed
then only the stand-by inverter will come on Load automatically with in 500msec. At 70% Depth of
Discharge (DOD) of the battery bank 110VDC supply to the inverters will be cut-off. So the Signals feed will
be cut-off. The auto-change over arrangement is also provided for bringing the CVT in circuit with in
500msec, when the both the inverters output is failed. It has various indications on the panel reflecting the
working of the panel.
D.C DISTRIBUTION PANEL
It takes care of D.C Power supply requirements of our signalling. It consists of sets of D.C-D.C
converters for individual D.C power requirements with (n+1) modular technology hot standby arrangement
with active load sharing basis. The DC-DC converters of Relay Internal are provided with (n+1) modular
technology hot standby arrangement with active load sharing basis and 1 additional module as a cold standby
(n+2). The DC-DC converter works satisfactorily with the input voltage variation of 98VDC to 138VDC. At
90% Depth of Discharge (DOD) of the battery bank all the DC-DC converters 110VDC Input supply will be
cut-off, except for Block Tele DC-DC converters. The supply for Point operation is also catered through a
20A fuse by this unit. It is also provided with various indications that reflect its working.

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STATUS MONITORING PANEL
IPS status monitoring panel has been provided in the ASM room for giving the important alarms and
indications to ASM. Status Panel tells present working status of IPS displaying battery voltage continuously
and five other indications, which will light according to IPS status. During normal working these indications
will not lit. Whenever the battery has come on to the load and has discharged by 50% D.O.D. (Depth of
Discharge) then first Red indication lit with description “START GENERATOR” with audio Alarm. i.e. DG
set is to be started and put on the load. If DG set is not started with this warning, then if battery gets further
discharged to 60% D.O.D and second Red indications appears with description “Emergency Start generator”
with audio alarm, even now if DG set is failed to be started, the battery further gets discharged to 70 %
D.O.D and 3rd Red indications appear with description “ System shut down” with audio alarm, which will
continue till Generator is started, resulting in A.C output from IPS is automatically cut off, results all the
signals will become blank.
When there is any defect in any sub module of IPS even without affecting working of system, the 4th
Red indication appears with description “Call S&T Staff” with audio alarm, so the ASM advises S&T staff
accordingly. Green LED 5th indication comes with the description “Stop Generator” with audio alarm, when
the DG set is running and if the Battery bank is fully charged condition.
EARTHING
The IPS systems and its individual modules are having earth terminals and all these are properly
earthed with earth resistance of less than 1 ohm. Earth provided shall preferably be maintenance free using
ground resistance improvement compound. (The acceptable Earth Resistance at earth busbar shall not be
more than 1 ohm Code of practice for earthing and bonding system for signalling equipments.)
LIGHTNING AND TRANSIENT PROTECTION IN IPS
Manufacturer will provide Stage1 & Stage 2 protection along with the IPS. These are described
below.
Stage 1 protection is of Class B type, against Lightning Electro-Magnetic Impulse (LEMP) & other
high surges, provided at Power Distribution Panel. It is provided with a 63 Amp fuse in phase line and is
connected between Line and the Neutral and also between the Neutral and Earth.
Stage 2 protection (Power line protection at Equipment level) is of Class C type, against low voltage
surges, provided at the equipment input level. This is thermal disconnecting type and equipped with
protection against SPD (surge protection device) failure due to open & short circuit of SPDs and is connected
between the Line and the Neutral. If supply / data / signalling lines (AC/DC) are carried through overhead
wires or cables above ground to any nearby building or any location outside the equipment room, additional
protection of Stage 2 type shall be provided at such locations. Class B & Class C arrestor is provided on a
separate wall mounting type enclosure in IPS room.
Stage3 protection (Protection for signalling/data line) is of Class D type. All external data/ signalling
lines (AC/DC) shall be protected by using this Class D type device. It consists of a combination of Varistors
and Gas Discharge Tube with voltage and current limiting facilities.
FEATURES
 Chargers used in this system are of SMPS technology chargers with 90% efficiency. These chargers
are supported with hot standby mode with (n+1) modular technology.
 One/two sets of Maintenance free Battery banks (110VDC). Normally one set (110VDC) of Battery
bank is used. Conventional flooded type Lead Acid Batteries or Low Maintenance Lead Acid
batteries can also be used. (SMRs settings are required to be adjusted depending on the type of
Batteries used.) Various voltage levels of

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 battery banks are avoided. Reduction in Battery maintenance & less flour area required.
 DC-DC Converters working from 110V Central battery have been used for all dc supplies. This has
improved overall efficiency of the system since number of conversion from AC to DC have been
reduced to 2 stage as compared to 3 stage conversion in case of transformer-rectifier system.
 DC-DC converters are available in modules. Easy replacement of defective modules. This ensures
less down time.
 DC-DC Converters are used in load sharing N+1 configuration (i.e. with hot standby with N+1
modular technology) to improve the reliability & availability of the system.
 Capacity of inverter has been brought down to 1.5 KVA from 5 KVA and used for feeding only
Signals supply. Hot standby inverter is provided with auto changeover facility. This improves the
availability of the overall system.
 High efficiency inverter is used with PWM (Pulse Width Modulation) technology in place of Ferro-
resonant technology based inverter. This improves the efficiency of the overall system.
 Continuous power to Signal Circuits even in absence of DG set/Local Power Supply.
 Generators need not be switched ON every time during train movement.
 Metal-to-metal relay installations and block working by axle counters have also been covered.
 Supply of spare modules/Components/Cells have been included as part of main supply.
 Provides highly regulated voltage to all signal relays & lamps for better life.

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SINGLE SECTION DIGITAL AXLE COUNTER
INTRODUCTION:-
The axle counter equipment is working on high frequency and using amplitude modulation or phase
modulation for detection of presence of wheel. The equipment described in this notes is Phase modulation type
for the detection of presence of wheel. In the Phase modulation type track device, the detection of presence of
wheel is with the phase reversal of 1800 out of phase, which enables this system to be more healthy and safe.
In Phase Reversal Modulation technique trolley suppression arrangements, to prevent the counting of
wheels caused by push trolley passing over the track device, are not required as the system will take care of
validation of generated pulses caused by passage of wheel over the track device, depending up on the phase
shift of the pulse. This phase shift of the pulse may be normally 160° to 180° for a train wheel and it may be
approximately 100° to 120° for a push trolley wheel.
This is Digital Axle Counter equipment for single track sections containing 2 out of 2 micro-
controllers to count the axles, establish the track occupancy of a track section and to provide this information
to the block or the interlocking equipment.
In this system no separate evaluator is required and no analog data is being transmitted. One set of
Axle counter equipment is provided at entry end and other set provided at exit end. Both sets are being
connected through a twisted pair of telecom cable i.e. existing RE cable one PET quad is used for both UP
and DN Axle Counters. Digital DATA is being transmitted between two ends of Axle counters (Outdoor
track side Detection points).This system is a fully duplex capable of operating according to CCITT V.21 and
the Data will be transmitted at the rate of 300bit/sec. This data Transmitted ensure negligible interference of
the noise. The system is highly reliable.
FEATURES
(a) The system consists of
(i) Single Section Digital Axle Counter (SSDAC) units.
(ii) Tx / Rx coils.
(iii) Vital Relays.
(b) Tx/Rx coil axle detectors are mounted to the web of the rails. The design of system consists of 21 KHz &
23 KHz High frequency Phase Reversal type axle detectors.
(c) Compatible with 90R, 52Kg & 60Kg rail profiles. Easy to install, commission & maintain.
(d) Track devices at both (entry & exit) points of the section, should be fixed on the same rail.
(e) System is designed to detect the solid wheels with diameter > 400mm with standard wheel flange.
(f) The system works in pairs. For monitoring single-track section one pair of SSDAC units are required and
to be installed near the trackside one at the beginning and another at the end of the track section. i.e
Trackside electronic counting equipment.
(g) The basic design of the system is based on counting the number of axles passing at each detection point.
These stored counts are transmitted to the second unit of the system and vice versa by means of modem
communication.
(h) The communication consists of digital packets having details of Counts & Health.

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(i) If counts registered at both detection points are equal, the section is cleared otherwise the section is shown
as occupied. The system ensures no error condition to arrive at the decision of clearance.
(j) System is designed as per CENELEC, SIL-4 (European standard), using micro controller along with other
electronic circuits and programmed using dedicated software. When any of these circuits fail, the system
goes to fail safe condition.
(k) It is programmable for either Preparatory Reset or Conditional Hard Reset as per requirement.
(l) Micro controller based design with 2 out of 2 decisions and counting through software.
(m) V.21 Modem communication (2-wire) on ½ quad cables and also compatible to work on voice channel of
OFC & Radio.
(n) Opto isolated vital relay drive for Q-style 24V, 1000 _ and Vital Relay output can be giving at both ends
of the system.
APPLICATIONS
The system can be widely used in Railways for Block Working (BPAC), Intermediate Block Signaling, Auto
signalling and Track circuiting for: i) Loop line ii) Main line iii) yard lines
SYSTEM DESCRIPTION
This system comprises of
Tx coils-2 nos
Rx coils-2 nos
Reset Box (RB 258A)
Card 1: Signal Conditioning Card – 1
Card 2: Signal Conditioning Card - 2
Card 3: Micro controller Logic Board – 1
Card 4: Micro controller Logic Board – 2
2 nos. for independent resetting – when used in block sections.
1 no. for common resetting – when used for Track circuiting at stations
Card 5: Event Logger Card.
Card 6: Modem Card.
Card 7: Relay Driver Card.
Card 8: DC-DC Converter Card.
SSDAC (DACF 700AP) UNIT
(A) SIGNAL CONDITIONING CARD (CARD 1&2) (SCC)
-1 (SCC-1) generates 21 KHz carrier
signals,
-2 (SCC-2) generates 23 KHz carrier signals, which is transmitted to 2nd
set of Tx coils.
s receive these signals.
modulated.
train pulses.

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(B) MICRO-CONTROLLER LOGIC BOARD/ CARD (CARD 3&4) (MLB)
The Micro-controller Logic Board (MLB) is the heart of the system.
o 8051 Based Architecture
o 8 Bit Micro Controller
o ATMEL AT89S8252 Micro controller
o 2 out of 2 Decision
o Uses C subset language
- TOOLS
o KEIL μ Vision Development System
o Universal Programmer
o 2KB Program Memory
o 256 bytes RAM
o 8KB Flash memory
o 32 Programmable I/O lines
o Wide Operating Voltage range of 4V-6V
o Full Duplex Serial Port
o Programmable Watch Dog Timer
o Fully Static operation up to 24MHz (Upgraded to 40MHz)
o Operating Temperature of –40°C - +85°C
o Use of State Machine for Axle Counting
o Use of ASCII MODBUS protocol
o Use of CRC16 technique for Error Checking during communication
o Wheel detection,
o Train direction checking and
o Wheel counting functions.
o It receives the remote wheel count and computes the status of the section for clear or occupied.
o It also checks various supervisory signal levels like supervisory of Tx/Rx coils, presence of various
cards, communication link failure etc.These cards communicate with each other for wheel count.
At Entry-end if train enters into section (1st detection), the counts are incremented and when train
shunts back from the same detection i.e, if train exits from the section from the same detection, the counts are
decremented. At Exit-end if train enters into section (2nd detection), the counts are decremented and when
train shunts back from the same detection i.e, if train exits from the section from the same detection, the
counts are incremented. Both the track devices at Entry and Exit ends must be fixed on same side of the
track.
This MLB card is having Extensive LED display.
o A block of 8 LED indicators for count progress / error display,
o 2 independent LED indicators for section status.
o The errors occurring in the system during the operation of the SSDAC are encoded and are indicated by
means of the 8-LED block present on the front panel of the MLB cards.
(c) EVENT LOGGER CARD (CARD 5)
Event logger card is designed to capture and store important signals from the remote and local SSDAC units.
The stored data can be downloaded from the event logger card for the purpose of analyzing the events
occurring during the operations of the SSDAC. The data can be analyzed with the help of CEL data analyzer
software.

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The event logger card captures following signals
(i) Pulse signals.
(ii) Supervisory signals.
(iii) Card removal information
(iv) Serial packets from:
Event Logger card has a Rabbit processor and 2 MB FLASH MEMORY to store packets. The data is initially
stored in the buffer and subsequently transferred to FLASH memory every two minutes. Normally 4096
pages of the data can be stored in flash memory on FIFO (first in first out) basis.
Run: This LED blinks continuously indicating the normal working of the event
Log: This LED blinks whenever data is being logged into the flash memory. (Approx, after every 2 minutes)
Dnld: This LED is ON when data is being downloaded from the flash memory of the card and becomes OFF
when download is complete.
(D) MODEM CARD (CARD 6)
(i) The modem card transmits and receives the digital packet information form one counting unit to the other.
The packet will appear after every 1.8 sec. and the packet carries the latest information such as:
(ii) The modem card being used is V.21 type (2-wire) in SSDAC.
(iii) This card interfaces with serial RS232C port of both Micro-controller Logic Boards.
(iv) It multiplexes the two RS232C inputs and selects one of the two channels and provides signal conversion
from digital to analog (FSK modulation) and vice-versa.
(v) Data transmission rate is 300 bits/sec.
(vi) Automatic Gain Control circuit is incorporated, hence no gain adjustments required.
(vii) Mode selection on Modem card. The modem has been set in ‘ORIGINATOR’ mode for entry and in the
‘ANSWER’ mode for exit in the factory.
(viii) LED Indications provided on Modem card
-Transmitting the signal when LED is flashing.
- Receiving the signal when LED is flashing.
-Remains OFF in SSDAC.
-Carrier is detected when LED is glowing.
(E) RELAY DRIVER CARD (CARD 7)
(i) The Relay Driver card (RD) provides the 24V DC output required for driving Vital Relay.
(ii) One RD card is used in each SSDAC counting unit. The RD card receives the command of clear and
clock signals from MLB1 & MLB2 cards and drives the vital relay ‘ON’ when section is NOT OCCUPIED
through opto- isolator circuit.
(iii) If a train occupies the section, the vital relay is dropped. The vital relay status is read back by the system
as per the driving output.
(iv) It has

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– LED flashes when the section is clear.
– LED flashes when the section is clear.
All the above LED’s are lit for section clear condition.
(F) DC-DC CONVERTER CARD (CARD 8)
INPUT VOLTAGE
Nominal Voltage : voltage 24V DC, Maximum current drain= 1.2A
Required voltage : 18V DC to 30V DC.
Output voltage
Nominal Voltage : +5 V DC @ 2 A, Required voltage : 4 . 7 5 t o 5.25V DC
Nominal Voltage : +12V DC @ 200 mA, Required voltage : 11.75 to 12.25V DC
Nominal Voltage : +24V DC @ 300 mA, with common ground, Required voltage : 23.5 to 24.5V DC
Nominal Voltage : +15V DC @ 100 mA with isolated ground, Required voltage : 14.5 to 15.5V DC
SURGE VOLTAGE AND LIGHTNING PROTECTION
Transient surge voltages arise as a result of Lightning discharge, switching operations in electrical
systems and electrostatic discharge. These surge voltages often destroy the electronic equipment to a large
extent. In order to prevent surge voltages from destroying the equipment, all the input lines of SSDAC i.e.
Power Supply (24V), Reset (48V) & Modem is to be routed through surge voltage protection devices for
effectively protecting the system. These devices (3 numbers) are mounted in a box and supplied along with
the system. One number of box is to be installed at each location and wired to the SSDAC.
Each surge voltage protection device consists of two parts.
(a) Base
(b) Plug Trab
The Base of the device is used for wiring the input and output signals. The connection details from relay
room to the box and from box to SSDAC unit are provided on the box. The Plug Trab consists of MOV and
GD Tube and diverts the excess energy during surge voltages or lightning into the ground connection. The
operation of these devices relies on a high quality ground connection in order to safely shunt away the
unwanted energy. The impedance of the ground connection is critical and it should be less than 2 Ohms.
NOTE: The 3 Plug Trab connections are not to be interchanged with one another. The plug Trab is a
detachable device and can be replaced with SPARE unit in case of blown device, which is indicated by
means of LED for 24 V.
EFFECTIVENESS OF PLUG TRABS
The effectiveness of plug Trab depends wholly on the Earth connection provided to the system. The earth
provided to the system should be less than 1 ohms and connections should be firm and proper to the SSDAC
unit, VR box, etc.
SURGE VOLTAGE AND PROTECTION DEVICE SV-120
The Surge Voltage protection device is to be installed at each location along with every SSDAC unit.

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EARTHING:-
The lead wires connecting the installation and the earth electrode shall ordinarily be of stranded
copper wire of 29 sq. mm (19 strand wires of 1.4 mm diameter). Copper wire has been specified because GI
wires usually are having greater corrosion. However, in areas where copper wire may be frequently stolen
due to theft, ACSR of size 64 sq. mm (19 strands of 2.11 mm diameter) may be used.
LIMITS OF EARTH RESISTANCE
(a) Apparatus case connected to Earth (SSDAC and vital Relay Box is housed in Apparatus case and
Connected to earth at outdoor) shall be less than or equal to 1 ohm
(b) All cable connected to same earth shall be less than or equal to 1 ohm
(C) Reset box connected to earth near SM s Room shall be less than or equal to 1ohm
EQUIPMENT TO BE EARTHED
A Common Earth should be provided for SSDAC for items 1(a) & (b) of the above at the outdoor.
(a) The Apparatus Case is to be connected to earth (the chassis of SSDAC & Vital Relay Box should be
properly connected to apparatus case).
(b) Metallic sheath and armouring of all the underground main cables are to be earthed
(i) In R.E area, the metallic sheath and armouring of main telecom cables are earthed at both ends.
(ii) In R.E area, the armouring of Jelly filled cable shall be earthed at both ends.
(c) The Earthing shall be provided at every location box where cables are terminated.
(d) Earth already available for other equipment may be used for earthing of Reset Box near SM s Room /
Cabin etc.

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DATALOGGER
INTRODUCTION
Datalogger is a Microprocessor based system, which helps in analysing the failures of relay inter
locking system / Electronic Interlocking system. This is like a black box, which stores all the information
regarding the changes take place in relays , AC / DC Voltages and DC currents along with date and time. The
same information / data can be transferred to the computer to analyse further “on line" / “off line” analysis of
stored date. A print out also can be obtained through a printer by connecting directly to the datalogger unit.
The data belongs to Relay contacts is considered as digital inputs and the data belongs to voltage
levels / currents is considered as Analog inputs. Datalogger ‘s are mandatory for all new relay interlocking
(PI/RRI) , EI installations and it is also recommended to provide in all existing PIs / RRIs. To increase the
line capacity, mechanical signalling equipments are upgraded to PI /RRI or EI. Due to complexity in the
circuits and wiring sometimes it is very difficult to rectify the failures. So datalogger can monitor these
systems with real time clock. Thus, it can be named as black box of S& T equipments and hence it is a vital
tool for accident investigation. Datalogger is used at Stations / yards. Whereas in case of Auto Section & IBH
Mini dataloggers, called as Remote Terminal Unit (RTU), are used.
ADVANTAGES OF DATALOGGERS
(a) Dataloggers helps in monitoring the typical failures such as intermittent, auto right failures.
(b) It helps in analyzing the cause of the accidents.
(c) It helps in detecting the human failures / errors such as :
(i) Drivers passing signal at Danger.
(ii) Operational mistakes done by panel operators / ASM’s of operating department.
(iii) Signal and telecom engineering interferences in safety circuits.
(iv) Engineering and electrical department interferences / failures.
(v) It helps as a “TOOL” in preventive maintenance of signaling gears.
(d) Dataloggers can be connected in network. Networked dataloggers helps to monitorthe PI/RRI/EI remotely
(e) Failure reports can be generated remotely with help of datalogger network
(f) On line and Off line track simulation is possible.
(g) Speed of the train on point zones can be calculated.
(h) Age of the equipment in terms of number of operations. etc..
COMMON EQUIPMENT FOR ALL DATALOGGERS ARE GIVEN
BELOW:
(a) CPU card .
(b) Digital and Analog input cards.
(c) Local terminal.(PC).
(d) communication links.
(e) Printer.
All the dataloggers requires a potential free ( spare ) relay contact for monitoring digital inputs
through Digital input cards & for monitoring Analog inputs such AC/DC bus bar voltage levels through
Analog input cards. Digital and Analog inputs are connected to the Processor card. Processor card consists of
memory IC’s. Memory IC’s are programmed as per requirement of the signal engineers.
Provision of Dataloggers is mandatory with RRI systems and is optional for PI systems as per
amendment to the specification for Relay Interlocking systems ( IRS/S-36 ). The data collected by the
datalogger can be used for failure analysis, repetitive discrepancies, and for accident investigations.

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Note:
If the serial communications is more than 50m then line drivers shall be used up to 3 Kms.
4wire leased line Modems shall be used if the serial communication is more than 3 Kms.
STUDY OF EFFTRONICS DATALOGGER
TECHNICAL DETAILS
(a) 24V / 12VDC Power Supply.
(b) Total Storage Capacity of 10 Lakh events.
(c) In-built Temperature sensors.
(d) Internal Buzzer for alarming during failures.
(e) Real Time clock with internal battery backup with data retention up to 10 years.
(f) 512 LED matrix to indicate the status of 512 Digital inputs at a time page wise.
(g) Seven segment LCD screen (2x24) to display the status of digital/analog signals,Time, Temperature etc.,
(h) Using the keyboard, various functions can be viewed in the LCD panel.
(i) Max Digital Inputs 4096.
(j) Max Analog Inputs 96.
(k) Digital Input Scanning Time 16 millisecond
(l) Analog Input Scanning Time is less than 1 Sec
HARDWARE (EQUIPMENT)
Datalogger system consists of:
(a) Datalogger (CPU - with Microprocessor 68000)
(b) Digital input cards.
(c) Dual modem card.
(d) Digital Scanner units (DSU)
(e) Analog Scanner units (ASU)
CPU CARD
It is provided with Motorola microprocessor M 68000. It performs all the activities pertaining to the
datalogger. It continuously scans (check) the Digital inputs(inbuilt), Digital Scanner Units and Analog
Scanner Units. i.e., scanning of digital signals (Relay operations) for every 16-milli seconds and scanning of
analog signals (i.e. AC/DC voltages & DC currents) for less than 1 second.
This card will support the I/O interfaces of LCD (Liquid Crystal Display) - 2X24 alphanumeric, Key
Board, LED Matrix Display, Real Time Clock. LCD display and keyboard: This will acts as man machine
interface between the datalogger and the signal engineer. All the operations (Software) can be performed
using this LCD and keyboard.
Real time display with 7 Segments: This is built in real time clock within Datalogger and its current
time will be displayed on six 7-segment display provided. (Real time clock depend upon DALLAS 1286
chip). This IC will come with internal battery backup; hence there is no need to add external batteries.
CPU card continuously scans (checks) the DSUs and ASUs. Each input connected to digital scanner
units are optically isolated by Opto couplers. When CPU card scans the digital inputs, it compares with the
previous stored data and if there is any change from the previous status then only that data will be stored (the
status / conditions of relay) with date and real time. A total of minimum 10 Lac events can be stored in
memory on first in first out basis so that latest data is available in the system. There is no loss of data from
datalogger memory in case of power supply failure of datalogger.
DIGITAL INPUT CARDS (IN-BUILT)
This system is having maximum 8nos. of inbuilt Digital inputs cards. Maximum 64nos. of digital
inputs can be connected to each digital input card. The potential free relay contact, may be front or back
contact, terminated at the Tag Block from the relay of signals, tracks, points, Buttons etc. and are

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subsequently connected to Digital input cards through Flat Ribbon Cable (FRC) connectors. These in-built
digital input cards can monitor a total 512 nos. of relays status.
DIGITAL SCANNER UNIT (DSU)
Each DSU contains 8 nos. of Digital Input cards. Each input card can be connected with 64 inputs.
Total input capacity of DSU unit is 512 inputs. These scanner cards contain Optocouplers and Multiplexer.
Inputs are connected to Stag card. The stag card out put is connected to DSU through FRC connectors.
Maximum 7 nos. of DSUs can be connected to the system. So, Digital input capacity of the system is 4096.
All these digital inputs are scanned at rate of 16 m.sec.
ANALOG SCANNER UNIT (ASU)
ASU contains maximum 3 nos. of Analog input cards. Each input card can be connected with 8nos. of
Analog inputs. Total input capacity of the ASU is 24 analog input channels. Maximum 4nos. of ASUs can be
connected to the system. Analog input channel capacity of the system is 96. All these analog inputs are
scanned at a rate of less than 1 sec.
PARALLEL PORT
Parallel port is provided for connecting printer.
RS-232 SERIAL PORTS
At least 6 Serial communication ports are provided for communication with other dataloggers,
Central Monitoring Unit, Remote Terminal Unit, Electronic Interlocking system, Integrated Power Supply
system etc.
EXTERNAL NON-VITAL RELAY CONTACTS
These relays provided in the system where 16 number of the Relay contacts are provided on the rear
panel through Tele control port to extend alarms and to control the power equipment from remote or local
locations through computer in case of any occurrence of failures. Each control can sink or source 100 m.
amps of current.
INTERNAL MODEM CARD / DUAL MODEM CARD (IN-BUILT)
It is fixed in datalogger Euro rack itself. One card contains two modems. The top modem is called
ANS (answer) modem and the bottom modem is called as ORG (originate) modem. It is used in case of
networking of Dataloggers. In network, connect ‘ANS’ modem to the ‘ORG’ modem of one adjacent station
and connect ‘ORG’ modem to the ‘ANS’ modem of other adjacent station.
POWER SUPPLY
Normally 24V DC (12V DC is optional) supply with battery backup is required for the system
working.
Input Voltage Range 18V…32V DC (For 24V) 9V…18V DC (For 12V)
INPUT REQUIREMENTS
Relay inputs (digital inputs) and analog inputs (voltages, currents etc.,) are required to be connected
to the system as per the requirements of RRI / PI / SSI as the case may be. Some of the inputs to be
monitored is given below:
(a) Digital inputs:
(i) Field inputs: All TPRs, NWKRs, RWKRs, ECRs, Crank Handle relays, Siding,Slot, LC gate
control relays etc.,
(ii) Control Panel inputs: All button / Knob, SM’s Key relays.

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(iii) Internal relays:
British system: All HR, DR, HHR, WNR, WRR, ASR, UCR, RR, LR, UYR,TLSR, TRSR, TSR,
JSLR, JR, etc.,
SIEMENS system: Z1UR, Z1UR1, GZR, ZDUCR, ZU(R)R, ZU(N)PR,G(R)R,G(N)R, U(R)S,
U(N)PS, UDKR, DUCR, U(R)LR, UYR1, UYR2, G(R)LR,GR1,GR2,
GR3, GR4, OVZ2U(R)R,W(R/N)R, (R/N)WLR, Z1NWR, Z1RWR,Z1WR1. WKR1, WKR2, WKR3, etc.,
(b) Analog channels
(i) 230 V AC (for power supplies in the power panel),
(ii) 110V AC (for Signal and Track transformers),
(iii) 110V DC (for Point operation),
(iv) 60V DC (Siemens relays),
(v) 24V DC (Q-series relays),
(vi) 24V DC (for Block, Axle counters),
(vii) 12V DC (for indication)
(viii) 20A (for point operation current),
(ix) 1.0V AC, 5KHz (for Axle counter channels), etc.
SOFTWARE MODULES OF DATALOGGERS
(a) Network Management of Dataloggers (NMDL).
(b) Reports.
(c) Fault Entry.
(d) Track Offline Simulation.
(e) Train Charting.
NMDL SOFTWARE FEATURES
(a) Online Relay Status
(b) Online Faults - To view information of various Online Faults, as they occur in the stations where
the Dataloggers are connected.
(c) Online Simulation - Graphical view of relay operations, train movements, etc.
(d) Remote monitoring of stations with the help of NETWORKING.
SOFTWARE OBJECTIVES
(a) Predictive Maintenance.
(b) Easy identification of failures.
(c) Crew discipline.
(d) Train charting.
REMOTE MONITORING OF STATIONS WITH NETWORKING OF
DATALOGGERS
The individual Dataloggers of various stations can be interconnected through networking technology.
The data of Remote Panel stations can be viewed in a Computer at the Central Monitoring Station. The data
of the network is collected by the FEP (Front End Processor), which in turn is transmitted to the computer
COMPONENTS OF NETWORK MANAGEMENT OF DATALOGGERS :
(a) Datalogger at stations.
(b) MODEM and Transmission medium

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(c) Front End Processor (FEP)
(d) Central Monitoring Unit (CMU) /Computer
FEP (FRONT END PROCESSOR)
FEP acts as a buffer between the Central Monitoring Unit (Computer) and the Network. It is provided
at centralised place to retrieve data continuously from station dataloggers and store in memory and transfer to
computer on request.. It stores 10 Lac telegrams. It works on 12V DC. It draws 1.6A continuous current
when all the three modems are connected. Normally it shows the number of packets pending, to be sent to the
computer, on its 7-segment LED display. It is provided with MOTOROLA 68000 microprocessor. It has 6-
nos. of RS-232 communication ports such as COM1, COM2, COM3, COM4, COM5 and COM6. COM1 is
used for Fault Analysis System (FAS) i.e. Central Monitoring Unit (Computer) connection. COM2 to COM6
are used for networking. For Bi-directional 2- nos. of ports and for Tri-directional (T-network) 3-nos. of ports
are used.
DATA TRANSMISSION
Dataloggers can be networked in Uni-directional Mode or Bi-directional Mode or T – Network Mode.
In case of loss of data, retransmission of data takes place.
(a) Uni-Directional Mode:
Each Datalogger will send data in only one direction to the FEP. Unidirectional mode network is not
preferred.
(b) Bi-Directional Mode:
Each end of Network is connected to FEP and each datalogger can now transmit data in both the directions.
Bi-directional Mode is advantageous, it enables the Data Transmission even in case of Network Failure.
(c) T - Network Mode:
If more no. of stations are in network i.e. if the network is too lengthy then T- network mode is preferred.
COMMUNICATION
The communication protocol for transmitting data and command between datalogger and CMU is
standardized by the RDSO and is given in the Specifications of Dataloggers.
(a) The type of communication used in the network is dependent on the distance between the dataloggers.
(b) For shorter distances, Opto Converter Box- Opto isolated current loop communication is used.
(c) For longer distances, Modem (Dial-up / leased) / Fiber Optic / Satellite / Microwave communication.
MODEMS
Modems are used for DATA transfer between Dataloggers and Front End Processor.These are
configured to RS 232 Serial Communication. Network is connected with two types of 4-wire modems:
(a) Internal modem card / Dual Modem card (in-built):
It is fixed in datalogger Euro rack itself. One card contains two modems. The top modem is called
ANS (answer) modem and the bottom modem is called as ORG (originate) modem.
Note: In case of networking of Dataloggers, connect ‘ANS’ modem to the ‘ORG’ modem of one adjacent
station and connect ‘ORG’ modem to the ‘ANS’ modem of other adjacent station.
(b) External modems:

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These are generally used at FEP (Front End Processor) side to connect the Dataloggers.
(i) To transfer Data from one datalogger to another datalogger / FEP Baud rate is 9600bps.
(ii) These modems are 4-wire line communication.
(iii) To transfer the data from FEP to RMU (PC) the Baud rate is 57,600 bps.
There is no difference between these modems functionally.
CENTRAL MONITORING UNIT (CMU) / COMPUTER
Central monitoring unit (Fault Analysis Unit) is a Personal Computer and its minimum configuration
shall be specified by RDSO from time to time. System Software Windows XP/Vista(OS), Norton/ Kaspersky
(Anti Virus), Interbase where Server is not available (DBMS), Oracle where Server is available (DBMS)
software are required to run Datalogger System. It is provided with Graphical User interface (GUI) based
software and retrieve data from all Networked dataloggers (up to 32) at various stations. It stores data in
standard data base files. The CMU is capable of analyzing the data and generate reports, audiovisual alarms
on defined conditions. This data can be compressed to take backup. In central monitoring unit Software, used
for analysis of data, prediction of faults etc., is written in a structured format so that purchaser can
reconfigure it, if required. It displays the status of signaling gears at any selected time in graphic form for any
selected station yard. It retrieves the stored data & simulates train movement. It sends commands to various
Dataloggers to activate audio, visual alarm or operate and electromagnetic relay.
CMU shares data available in it by other PCs through available local area network where this data can
be used for train charting / passenger information purpose. The system generates audiovisual alarm in
ASM’s/Signal Maintainer’s room in the case of power supply failure (battery voltage low) or battery charger
defective with acknowledgement facility.
(a) Each datalogger has its own identity code which will be transmitted along with data packet to central
monitoring unit.
(b) Events recorded at each station are continuously transmitted to central monitoring unit. Response time of
data transfer will not exceed 10 sec.

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OPTICAL FIBER CABLE
INTRODUCTION
The demand for bandwidth on transmission networks is increasing rapidly because video and
graphical rich contents are exchanged through the corporate network or the Internet. The Gigabit Ethernet
became ommonly used in the corporate network backbone, and 10Gbit Ethernet will be adopted in the near
future. Meanwhile in the home, the demand for high-speed network becomes popular as the wide spread of
broadband access, e.g. CATV, xDSL, and FTTH. The transmission medium with capability to transmit high
bit rate signal is necessary to satisfy these requirements.
The telecommunication transport technologies move from copper based networks to optical fiber,
from timeslot based transport to wave length based transport, from traditional circuit switching to terabit
router and all optical based networks entering into a new era of optical networking.
BASIC PHYSICS OF OFC
OPTICAL FIBER CABLE
OFC have Fibers which are long, thin strands made with pure glass about the diameter of a human
hair. OFC consists of Core, Cladding Buffers and Jacket as shown in figure
MONOCHROMATIC LIGHT, OR SINGLE COLOR LIGHT
Light or visible light is electromagnetic radiation of a wavelength that is visible to the human eye (
about 400 – 700 nm). The word light is sometimes used to refer to the entire electromagnetic spectrum. Light
is composed of elementary particles called photons. Three primary properties of light are:
Light can exhibit properties of both waves and particles. This property is referred to as wave-particle
duality. The study of light, known as optics. In free space, light (of all wavelengths) travels in a straight path
at a constant maximum speed. However, the speed of light changes when it travels in a medium, and this
change is not the same for all media or for all wavelengths. By free space it is meant space that is free from
matter (vacuum) and/or free from electromagnetic fields.
Thus, the speed of light in free space is defined by Einstein’s equation: E = mc2
Frequency, ν, speed of light in free space, c, and wavelength, λ, are interrelated by: ν = c/λ
From the energy relationships E = mc2 = hν and the last one, an interesting relationship is obtained,
the equivalent mass of a photon m = hν/c2
When light is in the vicinity of a strong electromagnetic field, it interacts with it. From this interaction
and other influences, its trajectory changes direction as shown in figure

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INCIDENT RAY, REFLECTED RAY AND REFRACTED RAY
An incident ray is a ray of light that strikes a surface. The angle between this ray and the
perpendicular or normal to the surface is the angle of incidence. Reflection is the change in direction of a
wave front at an interface between two different media so that the wave front returns into the medium from
which it originated. Common examples include the reflection of light, sound and water waves.
The reflected ray corresponding to a given incident ray, is the ray that represents the light reflected by
the surface. The angle between the surface normal and the reflected ray is known as the angle of reflection.
The Law of Reflection says that for a specular (non-scattering) surface, the angle of reflection always equals
the angle of incidence. The refracted ray or transmitted ray corresponding to a given incident ray represents
the light that is transmitted through the surface. The angle between this ray and the normal is known as the
angle of refraction, and it is given by Snell's Law.
The figure shows Incident ray, Reflected ray, Refracted ray , the angle of incidence and angle of refraction.
REFRACTIVE INDEX :-
Refractive index is the speed of light in a vacuum ( c =299,792.458km/second) divided by the speed
of light in a material ( v ). Refractive index measures how much a material refracts light. Refractive index of
a material, abbreviated as ‘ n ‘, is defined as ‘ n=c/v ‘ .Light travels slower in physical media than it does
when transmitted through the air. Refractive index (n): is a function of molecular structure of matter; optical
frequency , optical intensity; determines optical propagation properties of each wavelength ( λ ) may not be
distributed equally in all directions, is affected by external temperature, pressure, and fields.
Refractive index of a medium is a measure for how much the speed of light is reduced inside the
medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 =
0.67 times the speed in air or vacuum. Two common properties of glass and other transparent materials are
directly related to their refractive index.
First, light rays change direction when they cross the interface from air to the material , and effect that
is used in lenses and glasses.
Second, light reflects partially from surfaces that have a refractive index different from that of their
surroundings.
SNELL’S LAW:-

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When light passes from one transparent material to another, it bends according to Snell's law which
is defined as: n1sin(θ1) = n2sin(θ2)
where: n1 is the refractive index of the medium the light is leaving θ1 is the incident angle between the light
beam and the normal (normal is 90° to the interface between two materials)
n2 is the refractive index of the material the light is entering
θ2 is the refractive angle between the light ray and the normal
Snell’s law gives the relationship between angle of incidence and angle of refraction.
For the case of θ1 = 0° (i.e., a ray perpendicular to the interface) the solution is θ2 = 0° regardless of
the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent. The
above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of
Snell's law shows that the same ray paths are applicable in opposite direction.
TOTAL INTERNAL REFLECTION:-
When a light ray crosses an interface into a medium with a higher refractive index, it bends towards
the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower
refractive index medium will bend away from the normal.
This has an interesting implication: at some angle, known as the critical angle θc, light traveling from
a higher refractive index medium to a lower refractive index medium will be refracted at 90°; in other words,
refracted alon g the interface. If the light hits the interface at any angle larger than this critical angle, it will
not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a
process known as total internal reflection.
The critical angle can be calculated from Snell's law, putting in an angle of 90° for the angle of the refracted
ray θ2. This gives θ1:
Since, θ2 = 90°
So sin(θ2) = 1

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Then θc = θ1 = arcsin(n2/n1)
For example, with light trying to emerge from glass with n1=1.5 into air (n2 =1), the ritical angle θc is
arcsin(1/1.5), or 41.8°. For any angle of incidence larger than the critical angle, Snell's law will not be able to
be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1,
which is not possible. In that case all the light is totally reflected off the interface, obeying the law of
reflection.
OPTICAL FIBER MODE
An optical fiber guides light waves in distinct patterns called modes . Mode describes the distribution
of light energy across the fiber. The precise patterns depend on the wavelength of light transmitted and on the
variation in refractive index that shapes the core. In essence, the variations in refractive index create
boundary
conditions that shape how light waves travel through the fiber, like the walls of a tunnel affect how sounds
echo inside.
We can take a look at large-core step-index fibers. Light rays enter the fiber at a range of angles, and
rays at different angles can all stably travel down the length of the fiber as long as they hit the core-cladding
interface at an angle larger than critical angle. These rays are different modes. Fibers that carry more than
one mode at a specific light wavelength are called multimode fibers. Some fibers have very small diameter
core that they can carry only one mode which travels as a straight line at the center of the core. These fibers
are single mode fibers. This is illustrated in the following picture.
OPTICAL FIBER INDEX PROFILE
Index profile is the refractive index distribution across the core and the cladding of a fiber. Some
optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding
has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive
index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include
power-law index profiles and parabolic index profiles. The following figure shows some common types of
index profiles for single mode and multimode fiber.

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OPTICAL FIBER’S NUMERICAL APERTURE ( NA ):-
Multimode optical fiber will only propagate light that enters the fiber within a certain cone, known as
the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle (see figure 1.8),
θmax. For step-index multimode fiber, the acceptance angle is determined only by the indices of refraction
Where
n is the refractive index of the medium light is traveling before entering the fiber
nf is the refractive index of the fiber core
nc is the refractive index of the cladding
NUMBER OF MODES IN A FIBER :-
Modes are sometimes characterized by numbers. Single mode fibers carry only the lowest-order
mode, assigned the number 0. Multimode fibers also carry higher-order modes. The number of modes that
can propagate in a fiber depends on the fiber’s numerical aperture (or acceptance angle) as well as on its
core diameter and the wavelength of the light. For a step-index multimode fiber, the number of such modes,
Nm.
Where
D is the core diameter
λ is the operating wavelength
NA is the numerical aperture (or acceptance angle)
MODE FIELD DIAMETER :-
All light do not travels through the core of the fiber, but is distributed through both the core and the
cladding. The "mode field" is the distribution of light through the core and cladding of a particular fiber.
Mode-Field Diameter (MFD) defines the size of the power distribution. When coupling light into or out of a
fiber, MFD is important in understanding light loss.

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ADVANTAGE OF OFC COMMUNICATION :-
• More information carrying capacity Fibers can handle much higher data rates than copper. More
information can be sent in a second.
• Free from Electromagnetic and Electrostatic interference. Being insulator no electric current flows through
the fibre and due to this reason fibres neither radiate nor pick up electro - magnetic radiation. So WPC
CLEARANCE is not required.
• Low attenuation : 0.25 db/km at 1550 nm Loss in twisted pair and coaxial cable increases with frequency,
where as, loss in the optical fibre cable remains flat over a wide range of frequencies.
• Use of WDM – Switching / routing at Optical signal level
• Self healing rings under NMS control
• Small size makes fibre cable lighter in weight . So easy to handle.Optic fibre cable weight (approx)
500 kg / km Copper cable weight (approx) 1000 kg/km
The reasons are photons of light in a fibre
do not affect each other as they have no electrical charge and they are not affected by stray photons outside
the fibre. But in case of copper, electrons move through the cable and these are affected by each other.
: Optical fibre does not carry any electricity even if the cable is damaged or short circuited it does
not cause any spark or fire hazard.
: As the fibre do not radiate energy it can not be detected by any nearby antenna or any
other detector. The fibres are difficult to tap and therefore excellent for security.
: As the signal transmission is by digital modulation there is no chance of cross talk in
between channels.
: Only by adding a few additional terminal and repeater equipments the
capacity of the system can be increased, at any time once the cable is laid.
ical effects and temperature variations.
LIMITATIONS OF OFC :-
difficult.
- utilised
APPLICATION IN SIGNAL AND TELECOMMUNICATIONS
transmission circuits
-haul circuits for linking of telephone exchanges.

28 | P a g e
PROPAGATION MODES CONCEPT :-
MODE :-
Mode is an available distribution of electromagnetic field in a plane transverse to the direction of
light propagation. Each mode is characterized by frequency, polarization, electric field strength, and
magnetic field strength. Available patterns are derived from Maxwell’s equations and boundary conditions.
LINEARLY POLARIZED (LP) MODE :
A mode for which the field components in the direction of propagation are small compared to
components perpendicular to that direction. An optical fibre supports only different field patterns, called as
‘Linear Polarized’ or ‘ LP’ modes . The reasons are:
requirements.
de.
the accrual of power carried by different modes.
There are two basic types of fiber: Multimode fiber and Single-mode fiber.
Multimode fiber is best designed for short transmission distances. This is suited for used in LAN systems and
video surveillance. Single mode fibre is best designed for longer transmission distances. This is suitable for
long distance telephony and multi channel television broadcast systems.

29 | P a g e
MULTI MODE FIBER :
Multimode fiber, the first to be manufactured and commercialized, simply refers to the fact that
numerous modes or light rays are carried simultaneously through the waveguide. Modes result from the fact
that light will only propagate in the fiber core at discrete angles within the cone of acceptance. MM fiber type
has a much larger core diameter, compared to single-mode fiber, allowing for the larger number of modes
and
is easier to couple than single-mode optical fiber. Multimode fiber further categorized as Multimode step-
index and Multimode graded index fiber.
PROPAGATION THROUGH MMSI FIBER :
Figure shows the principle of total internal reflection applies to multimode step index fiber. Because
the core’s index of refraction is higher than the cladding’s index of refraction, the light that enters at less than
the critical angle is guided along the fiber.
Three different light waves travel down the fiber. One mode travels straight down the center of the
core. A second mode travels at a steep angle and bounces back and forth by total internal reflection. The third
mode exceeds the critical angle and refracts into the cladding. Naturally, it can be seen that the second mode
travels a longer distance than the first mode, causing the two modes to arrive at separate times.
PROBLEMS WITH MMSI FIBER AND SOLUTION
This disparity between arrival times of the different light rays is known as dispersion, and the result is
a muddied signal at the receiving end. It is important to note that high dispersion is an unavoidable
characteristic of multimode step-index fiber. The solutions are either use Graded index fiber or Single mode
fiber.
PROPAGATION THROUGH MMGI FIBER :
Multimode Graded-index refers to the fact that the refractive index of the core gradually decreases
farther from the center of the core. The increased refraction in the center of the core slows the speed of some
light rays, allowing all the light rays to reach the receiving end at approximately the same time, reducing
dispersion. Figure shows the Light propagation principle through multimode graded-index fiber. The core’s
central refractive index ( nA ) is greater than that of the outer core’s refractive index ( nB ).
It is very clear from the figure, the light rays no longer follow straight lines; they follow a serpentine path
being gradually bent back toward the center by the continuously declining refractive index. This reduces the
arrival time disparity because all modes arrive at about the same time. The modes traveling in a straight line

30 | P a g e
are in a higher refractive index, so they travel slower than the serpentine modes. These travel farther but
move faster in the lower refractive index of the outer core region.
PROPAGATION THROUGH SMSI FIBER :
Single mode fiber has a much smaller core that allows only one mode of light at a time to propagate
through the core. The figure shows the single mode fiber.
Single-mode fiber exhibits no dispersion caused by multiple modes. Single-mode fiber also offers lower fiber
attenuation than multimode fiber. Thus, more information can be transmitted per unit of time because it can
retain the fidelity of each light pulse over longer distances Like multimode fiber, early single-mode fiber was
generally characterized as step-index fiber meaning the refractive index of the fiber core is a step above that
of the cladding rather than graduated as it is in graded-index fiber. Modern single-mode fibers have evolved
into more complex designs such as matched clad, depressed clad and other exotic structures.
SINGLE-MODE FIBER DISADVANTAGES :
The smaller core diameter makes coupling light into the core more difficult. The tolerances for single-
mode connectors and splices are also much more demanding.
CUTOFF WAVE LENGTH :-
Cutoff wavelength is the wavelength above which a single-mode fiber supports and propagates only
one mode of light. In other words, an optical fiber that is single-mode at a particular wavelength may have
two or more modes at wavelengths lower than the cutoff wavelength. The effective cutoff wavelength of a
fiber is dependent on the length of fiber and its deployment. The longer the fiber, the lower is the effective
cutoff
wavelength. The smaller the bend radius of a loop of the fiber, the lower is the effective cutoff wavelength. If
a fiber is bent in a loop, the effective cutoff wavelength is lowered.
SIGNAL ATTENUATION IN FIBER :-
Optical fiber has a number of advantages over copper. However it also suffers from degradation
problems which can not be ignored. The first of these is loss or attenuation. Attenuation is typically the result
of two sub properties. They are scattering and absorption. Both of which have cumulative effects. The second
is
dispersion which is the spreading of the transmitted signal and is analogous to noise.
SCATTERING :
Scattering occurs because of impurities or irregularities in the physical construction of the fiber. The
well known form of scattering is Rayleigh Scattering. It is caused by metal ions in the silica matrix and
results in light rays being scattered in various directions.

31 | P a g e
Scattering limits the use of wavelengths below 800nm. The short wavelengths are much affected than longer
wavelengths. It is because of Rayleigh scattering that the sky appears to be blue ( shorter wave length ). The
shorter wavelengths ( blue ) of light are scattered more than the longer wavelengths of light.
ABSORPTION :
Absorption results from three factors. They are hydroxyl ions ( OH- , water ) in the silica, impurities
in the silica and incomplete residue from the manufacturing process. These impurities tend to absorb the
energy of the transmitted signal and convert it to heat, resulting in an overall weakening of the signal. The
Hydroxyl absorption occurs at 1.25 and 1.39 micro. The silica itself starts to absorb energy at 1.7 micro ,
because of the natural resonance of the silicon dioxide.
MACRO BENDING LOSS :
Macro-bending loss is caused by bending of the entire fiber axis. The bending radius shall not be
sharper than '30d' , where 'd' is diameter of cable. A single bend sharper than '30d' can cause loss of 0.5dB.
The fiber may break if bending is ever sharper.
MICRO BENDING LOSS :
Micro-bending loss is caused by micro deformations of fiber axis which leads to failures in achieving
total internal reflection conditions. Micro-bends are small scale perturbations along the fiber axis, the
amplitude of which are on the order of microns. These distortions can cause light to leak out of a fiber.
Micro-bending may be induced at very cold temperatures because the glass has a different coefficient of
thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable
become more rigid and may contract more than the glass. Consequently, enough load may be exerted on the
glass to cause micro bends.
Coating material is selected by manufacturers to minimize loss due to micro-bending. The linear thermal
expansion coefficient of coating material shall be compatible with that of fiber.

32 | P a g e
DISPERSION :
Dispersion is the optical term for the spreading of the transmits in the fiber . It is the bandwidth
limiting phenomenon and comes in two forms: Multimode dispersion and chromatic dispersion. Chromatic
dispersion is further subdivided into material dispersion and waveguide dispersion.
DISPERSION PHENOMENON IN OPTICAL FIBER :
Dispersion is the time distortion of an optical signal that results from the differences of time of travel
for different components of that signal, typically resulting in pulse broadening. As the distance traveled by
the signal is more, broadening of pulse is more. In digital transmission, dispersion limits on the maximum
data rate and the maximum distance i.e. the information-carrying capacity of a fiber link. The interference
from broadened pulse in the next interval shall not lead to erroneous interpretation of received signal.
OPTICAL DOMAIN :
Understanding where attenuation and dispersion problems occur helps optical design engineers
determine the better wavelengths at which information can be transmit, taking into account distance, type of
fiber and the other factors which can severely affect the integrity of the transmitted signal. The graph shown
depicts the optical transmission domain, as well as the areas where problems arise. The wavelength (nm)
is shown on X-axis and attenuation ( dB/km) is shown on Y-axis.
There are four transmission windows appear in the figure . The first one is at around 850 nm, the
second at 1310nm, third at 1550 nm and fourth at 1625 nm. The last two labeled as 'C' and 'L' band
respectively. The 850 nm wavelength at which the original LED technology operated. The second window, at
1310 nm has low dispersion. The 1550 nm called as ' C-band ' is ideal wavelength for long haul
communication systems. The network engineers can avoid transmitting signal at 1000 nm where Rayleigh
scattering , 1240 and 1390 nm where hydroxyl absorption by water occurs to avoid high degree of loss.
Optical fibers also can be manufactured to have low dispersion wavelength in the 1550nm region,
which is also the point where silica-based fibers have inherently minimal attenuation. These fibers are
referred to as dispersion-shifted fibers and are used in long-distance applications with high bit rates. For

33 | P a g e
applications utilizing multiple wavelengths, it is undesirable to have the zero dispersion point within the
operating
wavelength range.

34 | P a g e
SIGNALLING RELAYS
INTRODUCTION
A relay is an electromagnetic device, which is used to convey information from one circuit to another
circuit through a set of contact i.e. front or back contact. Constructional and electrically, relays may be
divided into DC and AC relays, because the means by which the electrical energy in the coil is converted in
to mechanical Energy in order to move the contacts are fundamentally different. In DC type, the contacts are
carried on an armature, forming part of a magnetic circuit in which a field is set up by the current flowing in
the coils. In AC types, the contacts are attached by a link mechanism to a metal sector, disc or cylinder in
which currents are induced by the alternating magnetic field produced by the currents in the coils.
Every endeavor has been made to explain the action of each type of relay in the simplest possible manner.
Relays are sophisticated switch gears used for remote control and succession control of various
electrical equipment. In present days they are widely used because they are capable of protecting the
controlled equipment from cross feeding and overloading even as they cater for speedy operations.
Most of the relays in present day signaling are electromagnetic devices, although some of the relays control
circuits through electronic components like diode/transistors/ Integrated Chips etc
Railway signaling relays are unique in that:
(a) They operate on low voltage and current
(b) They are more articulate as, according to their special features, they can work under restrictive conditions
and in any specified manner. Virtually they can cater for all situations while contributing to speed and
accuracy in operations.
CLASSIFICATION OF SIGNALLING RELAYS:
(a) According to the method of their mounting or fixture, they are classified as:
(i) Shelf type: Relays, which are loosely kept on shelves.
(ii) Plug in type: Relays, which are plugged into a pre- wired plug boards.
(b) According to their connection and usage, they are classified as:
(i) Track relays: Relay, which is directly connected to the track, to detect the presence of vehicle.
(ii) Line Relays: Other than track relay all are line relays. Relays connected to the selection circuit.
(c) According to their vitality or importance in ensuring train working safety, they are classified as:
(i) Vital Relays: All relays used for traffic control such as signal, point, controls, track detection etc.
(ii) Non-vital Relays: Relays, which operate control aids and accessories like warnings, buzzers,
Indications etc.
(d) According to their special provisions to ensure reliability of their contacts, they are classified as:
(i) Proved type: are those whose normalization after each operation shall be proved in circuit
controlled by their contacts. Contacts in which both the springs have metal surfaces on their tips. They may
get fused due to high sparking current across them during operation. These may prevent relay normalization
and causes unsafe condition in traffic control. To avoid this, proving of relay normalization after each
operation is necessary.
(ii) Non - proved type: Need not to be proved to have been normalized after each operation as their
contacts have at least one non-fusible contact (carbon contact)
(e) According to their feed source, relays are broadly classified as:
(i) DC relays: The relay, which requires DC power supply for its operations are, called DC relays. Among
the DC relays.
o DC neutral relays: This relay closes the same set of contacts on energization, with Normal polarity
or Reverse polarity supply.
o Polar Relays: This relay closes different set of contacts when energized with Reverse polarity
supply. They may or may not have contact to close when deenergized.

35 | P a g e
(ii) AC Relay: AC Induction motor track relays. Time element relays, flashing indication control etc.
(iii) Electronic Relays: DC relays with electronic components in them are called electronic relays.
DC NEUTRAL RELAY
Each Relay has usually one or two coils with a hollow center to accommodate a core.The coils are
made up of a large numbers of turns of small gauge soft drawn copper wire. The two coils can be connected
in series or parallel according to the requirement of relay resistance. The ends of the coils are terminated on
binding post to which the control wires are connected. Each coil is placed around a core of specially selected
Iron or steel having high permeability and low retentivity. The core should be susceptible to magnetism and
at the same time should have little residual magnetism. The cores are connected at the top by a yoke to
complete the magnetism coupling between two ends of coils. The bottom of each core is equipped with a
large steel or Iron block known as pole piece or face.
A flat piece of Iron or steel called armature is supported by brackets, which are securely fastened to
the pole piece. The armature, yoke, and the pole pieces are also made of specially selected iron or steel of the
same quality as the core. The armature carries the metallic spring contacts, which are insulated from it.
The circuit through the coils of the relay is closed. It sets up a magnetic flux through the core, yoke and the
armature. The flux passing between the armature and pole faces causes the armatures get attracted to the pole
faces and armature picks up and closes front contacts. When the circuit is opened the magnetic flux collapses
and the armature drops away by gravity from the pole faces, the front contacts break and back contacts close.
The front and back contacts of the relay can be utilised to make or break other circuits. Two stop pins of
nonmagnetic material are fixed either on the armature or pole faces so that the armature cannot
come in contact with the pole faces. It is essential to maintain a small air-gap between the armature and pole
faces so that low value of residual magnetism may not retain the armature in picked up position and causes
the relay to fail to drop away with a break in its control circuit.
GENERAL USAGE
DC Neutral line relays are most commonly used for Railway Signalling controls and detection.
Among them, plug-in type relays are preferred in larger installations for space considerations. Shelf type
relays are also in use, mostly in wayside stations.
There are many DC Neutral line relays in use with special features such as
(i) Delayed operation
(ii) Biased DC control
(iii) DC control unaffected by AC interference currents
(iv) Getting latched in operated condition till further feeding, and others.
Usage of DC polar relays is mostly in conjunction with block instruments that control traffic between
stations. AC line relays are almost extinct in installations of British Signalling practice. They are however,
used for time control operations, flashing indication control and such other special purposes in installations
with Siemens signalling practice widely. Track relays are used according to the type of track detection
circuits chosen for a given location and context. While most of the track circuits are still of the DC working
type requiring DC neutral track relays with them, the prospects of their being replaced with Electronic track
circuits directly feeding DC line relays, in future, are great. AC Track Circuits are used in DC Traction area,
as conventional DC Track Circuits are not suitable there. AC Track relays are used with them, almost all, of
the induction motor type. In the British practice of signalling which was first introduced on Indian Railways,
non-proved type relays with carbon to metal switching contacts are generally used for vital controls.
They facilitate simple circuit designs. But with the advent of German Practice, introduced by M/s
Siemens later, proved type relays with all 'metal to metal' contacts are widely accepted in spite of
complications in circuit design caused by them. A recent introduction is that of the same type relays made by
M/s Integra control. However, for some time now, the appreciable features of both the practices are getting

36 | P a g e
incorporated together in the indigenous designs of signalling by railwaymen. With this, the usage of all types
of relays anywhere can be found without straight jacketed segration of relay types.
CHARACTERSTIC OF ELECTRO-MAGNETIC RELAY:
The following are the characteristic of electro-magnetic relays. A brief study of them helps in
understanding the choice of their components and designs features.
1) Force of attraction
2) Effect of air gap.
3) Effect of Hysterisis
4) Transient condition.
FORCE OF ATTRACTION:
In any electro-magnetic system, the force of attraction is given by.
Where: B - is the flux density, a - is the cross sectional area of the particular part of the magnetic
circuit.
In the case of a DC neutral Relay, B is proportional to the current, that is flowing in the coil
surrounding the electro-magnet and thus the force of the attraction is directly proportional to the square of the
current. This square relationship has its own advantage especially in the case of DC track relay, in that a
small reduction in the current will have a great effect on the working of the relay. Also for a given change of
current, the make and the break will be quicker with lesser possibility of arcing.
EFFECT OF AIR GAP:
Curve ‘A’ is magnetisation curve for the iron and is all practical purpose a straight line up to the
saturation point. Curve ‘B’ is the magnetisation curve for the open-air gap, which is a straight line through
out because per magnetisation curve of the whole
magnetic circuit of the relay and for a given force is the sum of the amp-turns for the iron part and the amp-
turns for the air gap.
When the front contacts are open, the force required to pick up the armature is shown on curve ‘C’ to be F1
but after the armature has operated, it will be separated from the core by stop pins. In this position the amp-
turns required to maintain the armature is less, as indicated by the dotted line from 1 on curve C to 2 on curve

37 | P a g e
F. But actually the current in the coil is unaltered, the force on the armature is greater than required, as
indicated at 3 on curve F. Part of this extra force is used to flex the front contacts sufficiently to give good
contact pressure, when it is in energised position.
The difference between the pick-up and the drop-away current should be as small as practicable in
track relay to ensure good shunting characteristics. This is achieved firstly by the choice of good quality relay
iron and secondly by having a small air gap between armature and core. If the air gap is not available, then
the residual magnetism fluxes might cause the armature to be retained when the supply is disconnected. For
this reason, residual pins are provided to ensure a definite minimum air gap in the energised position.
EFFECT OF HYSTERISIS:
Hysterisis is the property by which the flux produced lags behind the current. In the de-energized
condition there will be small residual flux in the core. When the voltage is applied to the coils, the current in
rising to its steady value first causes the flux to rise from 1 to 2 along the curve. At this point the flux density
will be sufficient to attract the armature and reduce the air gap, the flux then raise to 3 and continue to 4
which corresponds to the steady current in the coils. When the voltage is disconnected, the current in falling
caused the flux to fall from 4 to 5 along the curve. At this point the flux density will fall below the value
required to maintain the armature, which will release, thus increasing the air gap and reducing the flux to 6.
Finally the
flux will decrease from 6 to 1 where the current will again be zero.
The relay core is made of material having high permeability and low retentivity. As mentioned in
the IRS specification, Electromagnet iron may be in the form of a
(a) Bar of silicon steel
(b) Best Yorkshire wrought iron
(c) Swedish charcoal iron
(d) Electrical steel sheets.
` This reduces the difference between pick up value and Drop away value. By selecting good quality
core material, Percentage release and sensitivity of the relay will be improved.
TRANSIENT CONDITION:

38 | P a g e
When the voltage is applied or disconnected from the coils, it takes some little time before the current
become steady. These are known as transient conditions” and are important so far as track relays are
concerned. When the voltage is first applied to the coils, the magnetic flux in rising, cuts the turns on the
coils and in so doing produces a back EMF that opposes the applied voltage and retards the growth of
current.
The growth and decay of flux are decided by the relationship between the inductance and resistance
in the circuit is known as time constant. It is not fixed quantity in the case of DC neutral relay. This value of ‘
L’ is less when the relay is in de-energised condition (L1) than when the relay is in energised condition (L2).
The magnitude of flux that is established for a given change of current is different in two cases.
When the current reaches the pick up value, the armature closes and the inductance is increased to L2,
due to reduced air gap, the flux per amp is increased. The increase in flux increased the back EMF, during the
movement of the armature after which the current continuous to raise along a new curve corresponding to the
increased inductance, until it reaches the final value (E/R). This process is indicated above in fig.2.4
When the supply is disconnected the current is obliviously reduced to zero immediately but the flux
decay comparatively slowly owing to the eddy currents, produced in the core by the rapid flux change, which
tend to maintain the flux. The drop away time on a disconnection is, however, generally negligible. See fig
below.
If the relay releases due to the reduction in current from say I 2 to I 1, caused by the application of
shunt resistance (as in the case of track relay ), the time taken is much longer than the relay is simply
disconnected. The rate of rise or fall of current during the transient conditions is also depends on exterior
circuit values because L and R apply to the whole circuit. The production of eddy current in the core, the flux
will decay at a slower rate than the current. So that the actual release time will be a little longer than it takes
the current to fall to the release.
L= Inductance

39 | P a g e
R= Resistance
It is now clear that to reduce releasing time to a minimum it is necessary that
- The relay iron should have low Hysterisis loss and low retentivity.
- The degree of over energization of the relay should be restricted
- Connecting a suitable external resistance in series with the relay to keep L/R ratio low.
In non RE area for track circuit length less than 100M 9 ohm track relay only to be used Using relay
with minimum contacts, as they require lesser current which keeps inductance value low.
Train working safety is ensured only if the track relay of shortest length track circuit is released
before a light engine running at a highest permitted speed clears it. Otherwise, the track circuit occupation
may go undetected. To avoid this, a special provision has to be made in signal control circuits, wherever
necessary.
The following methods may be adopted for reducing the time lag of track relay.
(a) Restrict the over energisation of relay since the release time depends on the initial working current.
(b) Connecting a suitable external resistance in series with the relay to keep the L/R ratio low.
(c) Using relays with minimum contacts, as they require lesser operating current, keeping the inductance
value low.

40 | P a g e
CONCLUSION
Modern signalling is vital for safe and punctual movements of trains. In the Indian railways The Signalling
and Telecommunication Department is responsible for providing modern ,effective and relaible signalling
systems as well as telecommunication systems.
The first mode of communication used in Indian Railways was the use of electric telegraph
with the help of Morose code. Morose code is the method of providing text information as a series of on-off
tones and lights, or clicks that can be directly understood by a skilled listener or observer without special
equipment.Each character (letter or numeral ) is represented by a unique sequence of dots and dashes. Later
this system becomes obselete and beggan the era of wireless communication which operated at audio level
frequency range.
In wireless communication , the significant advances took place : the transition to miniature
valves ,or filament tubes.But this was abandoned too because of excessive current consumption and over
heating of the filament tubes.
Fibre-optic communication is a method of transmitting information from one place to another by
sending pulses of light through an optcial fibre.The light forms an electromagnetic carrier wave that is
modulated to carry information. First developed in the 1970s, fibre optic communication systems has
revolutionized the telecommunications industry and have palyed a major role in the advent of the information
age. Because of its advantages, over electrical transmissions,optical fibres have largely replaced copper wire
communications in core networks in the developed world. The Indian Railways too is undergoing a transition
from Microwave communication to fibre optic communications

41 | P a g e
BIBLIOGRAPHY AND REFERENCES
1. www.wikipedia..com
2. www.britanicca.com
3. www.irfca.com
4. Motorola GP60 system manual
5. Harris FAS 7000 manual
6. TOSHIBA Manual

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