The electrical signalling system is progressively replacing the mechanical signalling system on Indian Railways, especially with the coming up of railway electrification projects. The main reasons behind this are as follows.
(a) There are a number of movable parts in the mechanical signalling system such as rods, wires, and cranks, which cause heavy wear and tear, frictional losses, and many of these parts can be sabotaged by unauthorized persons.
(b) The arms of the semaphore signals used in mechanical signalling afford poor visibility during the day. The night indications of these signals are also not satisfactory.
(c) The operational time of the mechanical signalling system is much greater than that of the electrical signalling system.
In the electrical signalling system, electrical energy is used for displaying signals. The transmission of power is done electrically and the units are operated by electrical push buttons while system is monitored by electrical systems.
31.7.1 Operated Units
The operated units consist of signals and points. The electrical signalling system is either coloured light signals or signals with semaphore arms operated by electric motors.
A point is operated by converting the rotary movement of the electrical point machines fastened on the sleepers near the point into a linear push or pull force. There are low-voltage point machines operated with a 24-V dc supply and high-voltage point machines operated using a 110-V dc supply. The operation of a point machine involves first unlocking the lock, bringing the point from normal to reverse or reverse to normal as the case may be, and then locking the point once again. The operating time of these point machines varies between 3 and 5 sec.
31.7.2 Transmission Medium
The medium of transmission for operating electrical equipment is either an overhead alignment or an underground cable. The overhead alignment is used when the number of conductors is limited. In areas provided with 25-kV ac traction, it is not possible to use overhead alignments due to the induced electromotive force (EMF) generated as a result of electrostatic and electromagnetic induction. In big yards, cables are used as a medium of transmission for the operation of point machines. The cables are either hung on hooks and run by the side of the track or laid underground. In areas provided with ac traction, under ground screened cables are used.
31.7.3 Operating System
Normally push buttons and rotary switches are used for operating signalling equipment that work on electricity. The complete yard layout is represented on the face of a console. Signals, tracks, points, and the gates of level crossings are depicted in their geographical positions on this console and the positions of these switches are then marked at the foot of signals and on various tracks.
Complete interlocking is achieved through electromagnetic switches known as relays. The two methods of interlocking available are panel interlocking or route relay interlocking. The details of these two systems of interlocking are given in Section 31.9.
31.7.4 Monitoring System
It mainly consists of point detectors, track circuits, and axle counters, all of which are discussed here in detail.
Electrical point detector
The electrical point detector detects and ensures that points are properly set. It also works on a ‘slide system’ as used in the mechanical system. These slides are so adjusted that a gap of 3 mm is left between the switch rail and the stock rail so that the two do not come is contact and, therefore, it is not possible to turn the signal off at any time.
The track circuit is an electric circuit formed along with the running rails and connected to the signal and cabin. Its function is to indicate the presence of a train (or vehicle) on the track. In order to set up a track circuit, the ends of the rails forming the circuit are isolated by insulating the rail joints. The rails are laid on wooden sleepers so that they are electrically insulated from each other. The ends of the rail on one side of the track are connected to a battery through resistances, etc., while on the other side of the track, the ends of the rails are connected to a relay. When the track is free, energy from the battery reaches the relay and energizes it. As soon as the track is occupied, the two rails are short-circuited because of the wheels and axle of the train and the relay does not get any feed from the battery. It,
therefore, gets de-energized, thereby breaking the circuit connected with the signals, thus ensuring that the necessary signals are set to indicate danger. The various types of track circuits used on the Railways are as follows.
(a) Direct current track circuit
(b) Alternating current track circuit
(c) Electronic track circuit
As already mentioned, two consecutive rails need to be insulated from each other for setting up a track circuit. The most essential requirement for track circuiting is the use of wooden sleepers. Due to the shortage of wooden sleepers on Indian Railways, an attempt is being made to progressively use a device known as an axle counter, which can be used as a substitute for track circuiting to detect the presence or absence of a vehicle on a track. A pair of rail inductors are installed at either end of the track for counting the axles. As soon as a train enters the track section from one end, the number of axles entering the section are counted automatically. Similarly, when the train leaves the track section at the other end, the axles are counted once again at the other end. If the same number of axles are counted at both the ends, it indicates that the section is free or unoccupied. If the number of axles counted at the exit end are less than the axles counted at the entrance to the section, it means that the section is still occupied.