1)In railway signaling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings.
2)An interlocking is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.
It is defined as the technique achieved through mechanical or electrical devices or agencies by which it can be ensured that before a signal is taken to off position for the route, which the signal controls, is properly set and held, and the same time all the signals and points, the operation of which may be lead to conflicting the moments are locked against the feasibility of such conflicting moments.
Configuration and Use:-
1)A minimal interlocking consists of signals, but usually includes additional appliances such as switches and derails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking inroute.
2)Signals may not be operated to permit conflicting train movements to take place at the same time
3)Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that affected.
4)Once a route is set and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either
5)The train passes out of the portion of the route affected
6)The signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal liline.
*Points and signals at junctions must operate in harmony to ensure that no unsafe moves are set up. The process in known as "interlocking“
*In the example above (Figure), Points C4 and C5 must be set to allow Train 1 to proceed to the main line.
*Points C4 are used to provide "flank protection" to ensure that Signal C2 cannot be cleared.
*Points are normally set in this way to provide such protection even if there is no signal to block.
*Train 1 will proceed to main line without any conflictiline.
*Signal c2 will be so cleared to allow the train 2 to enter the main lline.
*Therefore points c4 & c5 must be set to allow Train 2 to proceed to main line from branch line.
Types of interlocking:-
4)Entrance-Exit Interlocking (NX)
In mechanical interlocking plants, a locking bed is constructed, consisting of steel bars forming a grid
* Power interlocking may also use mechanical locking to ensure the proper sequencing of levers, but the levers are considerably smaller as they themselves do not directly control the field devices.
Interlocking effected purely electrically (sometimes referred to as "all-electric") consist of complex circuitry made up of relays in an arrangement of relay logic that ascertain the state or position of each signal appliance.
Modern interlockings (those installed since the late 1980s) are generally solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware.
Entrance-Exit Interlocking (NX):-
was the original brand name of the first generation relay-based centralized traffic control (CTC) interlocking system introduced in 1936.
Solid state interlocking:-
(SSI) is the brand name of first generation microprocessor. Based on interlocking development in 1980 by British rail.
Derailing:- improper interlocking