Introduction to Signalling

What is Signalling in Railways?

• Signalling is Mechanism by which the station master conveys information to the Loco driver to Stop, Go with Caution or Proceed

What are the Types of Signalling Systems in Railways

• Time Interval Method

• Space Interval Method

Time Interval Method

• Trains are Spaced Over an length of a track in such a way that , if the first train stops, the following train driver should be able to stop the train in sufficient distance without colliding with the first one.

• This type is used where traffic is less and weight of the trains are less, e.g: Trams

• This Type of System cannot be used in Passenger rails since weight and traffic is High

Space Interval Method

• In this method of “Control Over Movement”, the length of the track is divided in to sections called Blocks. The Entry of a train in to the ‘Block’ is controlled in such a way that only when it is free, a train can be allowed to enter it. This means that between two consecutive trains , there is definite space interval.

• The Space Interval Method is further divided in two types as follows:

Signals

Visual Audible

Visual Audible

FixedSignal

Flare Signal

Movable Flag

Voice WhistleDetonators

Fixed Signal

Two Aspect Multi Aspect

Colour LightSignalling

Semaphore

Colour LightSignalling

Semaphore

Semaphore Signalling

• Semaphore signals are rectangular or fish tailed arm fixed to a vertical Post.

• The arm is rotated in different angles to convey information to the Loco driver.

Stop Dead Aspect Proceed Aspect

Colour Light Signals

• In This type of signalling colour lights are used to convey information to the Loco driver. This has many advantages over semaphore signals. They may be elaborated as follows:

1. The day and Night aspects are the same, so no confusion to the driver.

2. Visibility can be available for Longer ranges, so it is easier for the driver to apply brakes in time.

3. The Signals are Placed at drivers Eye Level.

4. No Mechanical Transmission and no moving parts.

Red Aspect: Stop Dead

Yellow Aspect: Caution,

Proceed and be prepared to stop at the next signal

Green Aspect:

Proceed

Double Aspect: Attention, Proceed and be prepared to pass the next stop signal at restricted speed

Elements of a Yard

• Signals

• Track Circuits

• Points

• Slots

Track Circuits

• Track Circuits are devices that convey the presence of a train on a specified length of a track

• There are many types of track Circuits available as follows:

1. DC Track Circuits

2. High Frequency Track Circuits (HFTC)

3. Audio Frequency Track Circuits (AFTC)

4. Axle Counters ( Digital & Analog )

• What ever may the technology used to detect presence of train, the final element is a relay.

Device Electronics

Tracks

Relay Contacts Available for other Higher Level Devices such as SSIs

Points • Points also referred to as switches are

mechanical devices in the railway to change the path that trains may take through a junction. The switch positions are called normal and reverse

• These mechanical switches can be manually or Electrically Operated to Change From Normal to Reverse or Vice-Versa

Tracks

Electric DC Motor

• Point Machine Operates on 110V DC

• The Point Machine is connected to the mechanical levers to switch the position of the Point

Slots

• A slot is an element of a Yard, which as Dual Control, i.e. An Element of the Yard which can be operated by Two or More Means.

• This is generally applicable for Points, Level Crossings and Ground Frames

Example: Normally a Point is operated by means of Electric Motor but whenever the motor is failed, a permission is granted by the station Master of that yard to the signalling department, so that the field staff

Can go and manually operate the point and lock it. This is done by the means of a crank handle. So that there is no detention of traffic.

What is a Railway Interlocking system

A railway interlocking system controls the traffic in a railway station, and between adjacent stations. The control includes train routes, shunting moves and the movements of all other railway vehicles in accordance with railway rules, regulations and technological processes required for the operation of the railway station

What is meant by term Interlocking In Railways

A term used for the logical relationships between physical entities in the railway yard such as points, signals, track circuits, and so on. In SSI, this is programmed in the Software; in relay-based interlocking this is hardwired into the relay circuitry, and in ground-frame interlocking it is manifest in the mechanical linkages between physical components

What is RRI

RRI Stands for Route Relay Interlocking.

An Interlocking System When built

completely using Electro mechanical relays is

called as Route Relay Interlocking System .

Example RRI Relay Circuits

MAIN/SH. SIGNAL ROUTE SELECTION RELAY

RWKR

UNR

LR

UUYNR

GNR

Concerned CH/GF/LX_KLCR

Own

ASR

NWKR COGGNRGNR

EGRNR

MN/SH_LR

WNR

EUYNR

&

WFR

&

WFR

UYR2Concerned CH/GF/LX_YR

CNF_

LRs

What is SSI

SSI Stands for Solid State Interlocking. An Interlocking System When built using Electronics replacing traditional Mechanical Levers and Electro mechanical relays is called as Solid state Interlocking System.

Why SSIs are Required

SSIs are required to replace the existing RRI and PI Systems Since the traditional systems are very expensive and difficult to maintain because of the huge number of relays and mechanical levers used. SSIs are a better solution to the older systems since they are costing only ¼ the cost of RRI or PI and the maintenance cost is negligible and are easy to maintain.

SSI Rack

Point Machine

S2S4

S12

S13

S11SH5

Data Logger

S14

MaintenanceTerminal

Control cum Indication Panel

Relay Rack

Track Circuit

Serial Communication Links

Why SSI is cheaper than RRI

SSI is cheaper than RRI because of the following factors:

1. Number of relays Used in SSI are reduced to ¼ of Relays used in RRI

2. Cabling cost is much lesser than compared to RRI

3. Regular Maintenance is not required for SSIs

How many Types of SSIs are AvailableBased on the Deployment view SSIs are

mainly of Two Types:1. Centralized Systems: As the Name

Suggests all Controls of the system are at one place. Cables from the system are taken to the field gears of the Yard.

2. Distributed Systems: In Distributed System the Controls are Distributed across the yard and are kept near to the field gears keeping the cable length to a bear minimum

Based on the Architectural View SSIs can be classified in to following Types:

1. Single Processor

2. 2 out of 2 Systems

3. 2 out of 3 Systems

4. Hot standby Systems

These types are generally chosen by the customer based on the type of requirement such as the size of the yard, amount of traffic in the yard and budget allotted

RAMS Engineering

• Reliability: The reliability can be defined as the ability of an item to perform a required function under stated conditions for a stated period of time.

• Redundancy: The existence of more than one means of accomplishing a given function. Each means of accomplishing the function need not be necessarily identical.

• Hardware (Software Diversity): Two or more different Versions of Hardware (Software) working in a system to achieve a same result.

• Failure: The termination of the ability of an item to perform a required function.

• Maintainability: The ability of an item, under stated conditions of use, to be retained in, or restore to, a state in which it can perform its required function, when maintenance is performed under stated conditions and using prescribed procedure and resources.

• Availability: The ability of an item (Under combined aspects of its reliability, maintainability, and maintenance support) to perform its required function over a stated period of time.

What is fail safety

• Fail Safety is the concept in which even when a system fails, it fails on the safer side.

Example: A Relay when power is cut off the

Output drops resulting in a safe state

Methods to Achieve fail safety in Electronic Systems

Inputs

Processor Reading at Time 1

Processor Reading at Time 2

Fig No: 1 Time Redundancy

System Outputs

Inputs

Inputs

Processor 1Identical Software and Hardware

Processor nIdentical Software and Hardware

VOTER

Processor 1 Outputs

Processor n Outputs

System Outputs

Fig No: 2 Hardware Redundancy

Inputs

Inputs

Processor 1Hardware 1Identical Software

Processor nHardware nIdentical Software

VOTER

Processor 1 Outputs

Processor n Outputs

System Outputs

Fig No: 3 Hardware Diversity

Inputs Software 1

Software 2

System Outputs

Fig No: 4 Software Diversity

Inputs

Inputs

Processor 1Identical HardwareSoftware 1

Processor nIdentical HardwareSoftware n

VOTER

Processor 1 Outputs

Processor n Outputs

System Outputs

Fig No: 5 Diverse software on redundant hardware

Inputs

Inputs

Processor 1Hardware 1Software 1

Processor nHardware nSoftware n

VOTER

Processor 1 Outputs

Processor n Outputs

Fig No: 6 Diverse software on Diverse hardware

System Outputs

Selection Table

• Selection Table is representation of the Interlocking between Signals, Tracks, Points and Slots of particular Railway Yard.

• It gives the conditions required for setting a route i.e. Reception and dispatch of trains

Route No Button Press Approach Locked By Tracks

Back Locked By Tracks

Controlled By Tracks

Detects Points Conflicting Routes

Slots Involved

GN UN Normal Reverse

s1_TDMA S1 TDMA 1AT 1BT,1CT,11BT,12T

1BT,1CT,11BT,12T,DMT

11,12,18 S30_TDMA CH1,CH2

S30_TDMA s30 TDMA T6 T6,T30,T40,T50

T6,T30,T40,T50,DMT

11,15,18,12

S1_TDMA,SH10_TUM

CH1,CH2,CH3

Software Flow in SSI systems

Start

Post Routines and diagnostic Functions

Establish Communication with Subsystems

Scan for Yard Inputs

A

Interlocking Logic

Set Field Outputs

Outputs Read Back and inform supervisory

Log Data in Data Logger

Start

How errors are detected in SSI Systems

Post: Power on Self test• After power ON, each processor would start

its operation from a predefined vector location irrespective of its previous state. In this state each processor first defines all control registers of internal and external peripheral devices. It then performs a series of self-checking functions to ensure the healthiness of all its internal components.

• Within POST, each processor performs following checks.

• RAM test• ROM test• I/O Bus Test• Processor Identity Check• Address Check• I/O Configuration Check• Relay Input Integrity Check• Shutdown Control Voltage Check

Diagnostics

• Diagnostics are a series of tests conducted on the hardware by the processor to check their Integrity.

• The Tests performed in diagnostics are listed below:

• RAM test

• ROM test

• I/O Bus Test

• Processor Identity Check

• Address Check

• I/O Configuration Check

• Relay Input Integrity Check

• Shutdown Control Voltage Check

Operational Modes of SSI

POST

Degraded Mode

Normal Mode

SafeShutdown

mode

Power ON

Or UserReset

T1

T4 T3

T5

T6T7

T2

Need for Independent Verification and Validation

♦ Complexity of computer based interlockings demands rigid procedures and strict requirements for verification and validation

♦ Computer based technologies allowed for a new approach towards signalling rules

♦ Computer technology allows much more functional flexibility through the software

♦ CENELEC standards have been elaborated and introduced♦ Reorganization of the railway companies, which among other issues

caused that V&V activities have been split up and assigned to independent organizations

All these changes offer chances as well as threats for a professional verification and validation of interlockings.

What are Fail safe Tests

• Fail safe tests are one of tests carried out after system Integration.

• In these tests deliberately faults are injected in to the system and the outputs of the system are measured and results should be on the safer side

What is FMEA

• FMEA Stands for Failure Modes and Effect Analysis

• FMEA is a part of the fail safety tests that are conducted on the system. This the vital part of Validation and for obtaining safety Certification.

• FMEA can be done on card Level and At a System Level

Example:

Card1 Card2

Card3 Card4

Component

System

System Outputs

• In the above Example if the component fails in the Card1, Card1 may fail or may not fail and if the component failure is detected Card1 will fail. If Card1 fails and if it is a non-Vital Card, the system will still function, but if the card is Vital Card, the system will go to shut down.

• In all the above process the system outputs should be noted and in none of the cases the output should be un safe.

VCCP

0

VCCP

0

BD0BD1BD2BD3BD4BD5BD6BD7

DL_LENDL_OE#

VCCP

BD8BD9BD10BD11BD12BD13BD14BD15 BD7

BD1

BD6BD5

BD2

BD4BD3

BD0BD8BD9BD10BD11BD12BD13BD14BD15

DL_LENDL_OE#

VCCP

U16

74HC573

111

20

1918171615141312

23456789

OELE

VCC

1Q2Q3Q4Q5Q6Q7Q8Q

1D2D3D4D5D6D7D8D

U17

74HC573

111

20

1918171615141312

23456789

OELE

VCC

1Q2Q3Q4Q5Q6Q7Q8Q

1D2D3D4D5D6D7D8D

C150.1uf

C160.1uf

Data Loop Test

Sample Circuit

FMEA Sample Sheet

Sno IC Reference Number Pin No Type of Fault Results

Card Level System Level

1 U17 1 Struck at 1Card shall shut down since the

data loop test will failSystem will function in

2 out of two mode

2 U17 1 Struck at 0Card shall shut down since the

data loop test will failSystem will function in

2 out of two mode

3 U17 1 OpenCard shall shut down since the

data loop test will failSystem will function in

2 out of two mode

4 U17 2 Struck at 1Card shall shut down since the

data loop test will failSystem will function in

2 out of two mode

5 U17 2 Struck at 0Card shall shut down since the

data loop test will failSystem will function in

2 out of two mode

6 U17 2 OpenCard shall shut down since the

data loop test will failSystem will function in

2 out of two mode

How Safety Integrity Levels are calculated

• A fundamental problem in estimating reliability is whether a system will function in a prescribed manner in a given environment for a given period of time. This depends on many factors such as the design of the system, the parts and components used, and the environment. Hence it is necessary to consider the reliability of a system as an unknown parameter that is defined to be the probability that the given system will perform its required function under specified conditions for a specified period of time.

• Let S(t) be the number of surviving components still operating at time t after the beginning of the ageing experiment, let F(t) be the number of components that have failed up to time t.. then the probability of survival of the components, also known as the reliability R(t), is

• R(t) = S(t) / N --- (1)• The probability of failure of the components, also known as the

unreliability Q(t), is• Q(t) = F(t) / N --- (2)• Since total number of components, (N) = S(t) + F(t) --- (3)• By adding (1) and (2) equations and substituting equation (3) in the

result, we get • R(t) + Q(t) = 1• The failure rate, also known as the hazard rate, Z(t) is defined to be the

number of failures per unit time compared with the number of surviving components;

• Z(t) = [ dF(t) / dt ] / S(t) --- (4)

• i.e. Z(t) = λ --- (5)• To get the reliability in terms of failure rate, combine

equations (1) (4) &(5). After combining and integrating the final expression we get the reliability in terms of failure rate is

• R(t) = -ℯ λt --- (6)• It is understood that the reliability of a system falls down

as the system hazard rate rises exponentially as shown in Fig. (2)

• The above relationship is known as the exponential failure law; λ is typically expressed as percentage failures per 1000 hours or as failures per hour. When the product λt is small, equation (6) becomes

• R(t) = 1 – λt --- (7)• This gives the system reliability for system failures

occurring during the useful life period that are entirely due to component failures.

• THRS FRA/SDRA X FRB/SDRB X (SDRA + SDRB) ------------ (17)

• SDRS SDRA + SDRB • Here FRs stands for potential hazardous failure rates.• If periodic testing times are used as detection times then

above eqn 17 may be used with mean test times• i.e T/2 +negation time = SDT = 1/SDR

---------------------- (18)• For practical conditions the negation time is practically 0

(micro seconds) in our calculations SDT = 1/SDR = T/2 • In our calculations we have still considered the actual

time as T SDT = 1/SDR = T -------------------- (19)

• The time T as explained above is the detection and negation time.• The reports generated by the software are enclosed in the annexure 1.• In the report under the heading environment quality different levels of

quality can be specified as described below:• Level 0: Components procured on commercial considerations only,

with no evidence of reliability. • Level 1: Components are procured on commercial considerations, but

with evidence (usually from the component vendor) of reliability. • Level 2: Components are procured on the basis of sufficient quality

and Reliability demonstration. Procurement specifications require that the components have suitable reliability for the purpose

• Level 3: Fully assessed reliability.

Table for SIL Allocation

Tolerable Hazard Rate THR per hourAnd per function

Safety Integrity Level

10-9 <= THR < 10-8 4

10-8 <= THR < 10-7 3

10-7 <= THR < 10-6 2

10-6 <= THR < 10-5 1

Presented By

Sandeep Patalay

Email: sandeep.patalay@gmail.com