Signalling Systems
The entire signalling system can be classified into two main categories.
(a) Mechanical signalling system
(b) Electrical signalling system
In addition to these two main categories of signalling systems, a solid-state signalling system is also in use. Each system of signalling comprises four main components.
(a) Operated units such as signals and points
(b) A transmission system such as single- or double-wire transmission or electrical transmission
(c) Operating units such as levers and press buttons
(d) Monitoring units such as detectors, treadle bars, and track circuiting
The comparison between mechanical and electrical signalling based on these four broad components is given in Table 31.8.
Table 31.8 Comparison of signalling systems
Component |
Mechanical |
Electrical |
Operated units Signals |
Mechanically operated signals |
Coloured light signals with two- |
|
as per lower quadrant or upper |
aspect, three-aspect or four-aspect |
|
quadrant signalling |
signalling |
Points |
Mechanically operated points; |
Electrically operated points (by |
|
locking with the help of point |
converting the rotary movement of |
|
locks, stretcher bars, and |
electric motors into linear push or |
|
detectors |
pull); locking with the help of slides and solid rods. |
Level crossing |
Manually operated swing leaf |
Electrically operated lifting |
gates |
gate or mechanically operated lifting barriers |
barriers |
Transmission |
Single- or double-wire |
Electrical transmission through |
systems |
transmission to the requisite |
overhead wires or underground |
|
points by means of rods or double wires |
cables |
Operating units |
Hand levers with a range of |
Push buttons, rotary switches, or |
|
500 to 2000 m used in collaboration with single-wire or double-wire lever frames |
electrical signalling equipment |
|
Mechanical interlocking with |
Interlocking through |
|
tappets, etc. |
electromagnetic switches known as relays or solid-state switching devices |
Monitoring |
Monitoring of points with the |
Monitoring with the help of direct |
units |
help of detectors; monitoring |
current track circuits, alternating |
|
of the passage of trains using a |
current track circuits, electronic |
|
treadle, which is an electromechanical device |
track circuits, axle counters, etc. |
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