Mechanical locking is effected by tappet locking - the initial movement of the lever is prevented unless all of the levers which affect the relevant function are in their correct position. This initial movement of the lever then locks these levers in their correct position and only as the lever is placed in the fully reverse position does it release other relevant levers. The locking is converse - that is if lever A locks lever B, then lever B locks lever A.
The signalling system never assumes that the equipment on the ground has operated correctly, for example a point or signal having correctly responded to the lever. Interlocking is therefore provided outside the signal box to ensure that:
The FPL is held engaged either mechanically by a fouling bar between the signal and the point or by an electric lock on the FPL lever operated by track circuits. The FPL is driven via the fouling bar and engagement of the FPL is proved by means of a mechanical slide driven sideways at the FPL engages. The ends of the point blades also drive mechanical slides and only if all three slides line up in the correct position may the signal be cleared. This is either by means of a mechanical slide in the wire run to the signal or by means of electrical switches which allow the electric lock on the signal lever to be released (and provide indication in the signal box).