Essentials of Interlocking

Before laying out the Essentials of Interlocking, lets just understand or refresh our knowledge  by the Definition of Interlocking:

"An arrangement of signals, points and appliances, operated from a panel or lever frame, so interconnected by Mechanical locking or electrical locking or both that their operation must take place in proper sequence to ensure safety".




Again, to clear any confusion, ON aspect means the most restrictive aspect of a signal, whereas, OFF aspect means any aspect other than the ON aspect of the signal.
ON   = STOP or Other RESTRICTIVE indication
OFF = PROCEED

ESSENTIALS OF INTERLOCKING

1.    Lever frames and other apparatus installed for the operation and control of signals,
points etc. must be so interlocked and arranged as to comply with the following essentials:
(a) It must not be possible to take 'OFF' a running signal unless the following are
ensured not only in the actual portion of the track on which the train has to travel, but
also in the overlap
(i) All the points are correctly set.
(ii) All the facing points are locked (at site).
(iii) All the interlocked level crossing gates are closed and locked against road traffic.
(iv) The isolation is ensured.
(b) Once the signal has been taken OFF, it must not be possible to do any of the
following unless the signal has first been put back to the 'ON' position.
(i) Alter the position of the relevant points.
(ii) Unlock the relevant facing points.
(iii) Unlock and open the relevant level crossing gate.
(iv) Disturb the relevant isolation.
(c) It must not be possible to take 'OFF' at the same time any two fixed signals, which
may lead to any conflicting movements.
(d) Where feasible, the points shall be so interlocked as to avoid any conflicting 
movement. 

2.    Interlocking will be provided between concerned levers to comply with the above. The
clause (i), (ii) and (iii) of the above Paragraphs must be rigidly complied with and no
exceptions are allowed. However, clause (iv) permits certain exemptions in locking for flexibility
of the yard and the interlocking between points leading to conflicting movements only may be
provided to the extent feasible.