2. Signalling and Safety Systems
Point
machine
Point machine
Axle counting
Intermittent
train control
system
Intermittent
automatic train control
system EUROBALISE
Continuous
automatic train
control system
Computer room
Control room
Axle counting
Continuous
automatic train
control system
Interlocking
S- bond
3. Signalling Overview
A T O
A T P I S
A T S
Safety Layer
Automatic Tra
Supervision
Interlocking
Automatic Train
Operation
Automatic Train
Protection
4. What is Fail Safety?
Failures- whether Equipment or Human
- can be minimized
-but can not be eliminated
Therefore, steps are required to be taken to ensure that there
is no unsafe effect of failure
Signalling Systems are designed in such a way that every
Failure has a safe Reaction
This is called Fail – Safe Principle
5. Fail – Safe Principle
Fundamental principle of design of Signalling
system is:
--- safe state corresponds to the lowest energy level
--- to keep the system in a permissive state, constant
energy/effort should be applied
This ensures that due to any inadvertent situation or
failure,the system comes back to the state of lowest
energy—ie. Safe Sate
7. Fail - safety
Fail – safe Principle is adopted in the design of all
signalling systems- mechanical, relay based as well
as software based systems
Example- Semaphore Signal
-Mechaniical design is such that”stop” aspect is the stable state
-Constant Force required to keep required to keep the signaling “
proceed” aspect.
Signal returns to “stop” aspect in case of breakage of transmission wire
or any other failure.
8. Fail – Safety-Examples
Signalling Relays:
-Stable state- Dropped (Maintained by gravity/spring
action)- safe state
- Red signal aspect controlled by Relay-” dropped”- which
is lowest energy state.
- permissive aspect controlled by Relay –”picked up”
- Constant current required to maintain the relay in “picked
Up”
9. Software Based Systems
Software based Signalling systems require
repeated positive action to be taken to be
taken by- both,software as well as hardware
to keep it in permissive state.
Disruption of this positive action due to any
failure results into reversion of the system to
safe state.
10. Microprocessor and other
component
Disadvantage
Are not fail safe
Don’t have well
defined failure modes
Are not reliable enough
to meet 10-9
unsafe
failures/our. They are
approx. 10-5 to
10-6
Advantage
Speed
ability to perform
complex task
Miniature size
Low price
11. Then How is Safety Achieved?
Employ more resources than required
(redundancy)(both hardware & software)
Self check procedures to detect a fault within
given time period dt such that prb. Of
occurance of a fault within dt is <10-9
watchdog timers
12. What is Redundancy?
Redundancy:
Is the use of additional resources(whether hardware or
software) than required for the normal functioning of the
system
The additional resources should be configured judiciously to
obtain max. advantage in terms of safety and reliability
The amount and type of additional resources and its
configuration will depend on the safety and reliability
requirements.
13. OR
UNIT 1
UNIT 2
PF
=P2
, PWSF
=2P
AND
UNIT 1
UNIT 2
PF
= 2P , PWSF
=P2
PF
=Probability of failure
PWSF
=Prob.of wrong
side failure
Safety
Availability
This Will not increase safety
19. Self Check & Watchdog timers
Periodical check of microprocessor,
buses,memory, peripheral especially input
circuits
Watchdog timers-within specified time
window if command is not received then
system goes to safe state.
20. Essentials of Interlocking
(as per indian railway SEM)
It shall not be possible to take ‘OFF’ a running signal, unless all points
including isolation are correctly set, all facing points are locked and all
interlocked level crossing are closed and locked against public road for the
line on which the train will travel including overlap.
After the signal has been taken ‘OFF’ it shall not be possible to move any
points or lock on the route, including overlap and isolation, nor to release
any interlocked gates until the signal is replaced the ‘ON’ position.
It shall not be Possible to take ‘OFF’ at the same time, any two fixed
signals which can lead to any conflicting movements.
Where feasible, points shall be so interlocked as to avoid any conflicting
movement.
