Interlocking : Means an arrangement of signals , points and other appliances, operated from a panel or from lever frame, so interconnected by mechanical locking or electrical locking or both that their operation must take place in proper sequence to ensure safety.
Essentials of Interlocking
* 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 gates are closed and locked against road traffic for the line on which the train will travel, including the 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 LC gate until the signal is replaced to 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 and
* Where feasible points shall be so interlocked as to avoid any conflicting movement.
Railway Signalling Principles of Interlocking and Standards