What are the systems of semaphore signalling used by IR?

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What are the systems of semaphore signalling used by IR?

July 14, 2019, 3:18 PM

In IR’s lower quadrant system (Two-aspect Lower Quadrant) the semaphore arm can only be in two positions. The horizontal on position shows the most restrictive indication (requiring the train to stop or slow down or proceed with caution depending on the kind of signal), and a lowered position where the semaphore arm is at about 60 degrees or more from the horizontal shows the clear or proceed indication allowing a train to go past the signal.

The 2-aspect Lower Quadrant system suffers from a couple of disadvantages. The principal disadvantage is that the driver of a train must be prepared to bring the train to a full stop when the warner is at caution and the home signal is at danger. To address this, often warner signals are moved further back to provide sufficient distance from the home signal for braking the train to a full stop. The second disadvantage with the 2LQ system is that the indication of the warner signal is not explicit. When the warner is at caution, it may indicate that the home signal is at danger, or that the train will be received on a loop line, or that there is a speed restriction of some sort ahead. These disadvantages are addressed with the Modified Lower Quadrant system. In this, warners and distant signals (as in MAUQ, see below) are both used. The distant signals have only two aspects, Proceed and Caution. The distant signal is provided at an adequate distance to the rear of the Home signal, and a combination Home and Warner signal is provided 180m from where the block section ends. There is no difference in the placement or nature of the last stop signal. MLQ is found in the Kharagpur-Vishakhapatnam and a few other sections. It was not widely adopted as it is complex in working and provides no advantages over the competing multiple-aspect upper quadrant signalling system (see below) which also came into use and became far more commonly used on all important sections of IR.

Early versions of semaphores used in the lower quadrant system suffered from a potentially dangerous flaw, which is that in case of a mechanical failure, the semaphore arm was likely to drop by gravity into the clear position. This was guarded against in later versions by having the spectacle end of the semaphore be coniderably heavier to provide a counterweight to the arm. Generally speaking, fail-safe operation to ensure the signal shows its most restrictive aspect when the signal wire is broken is ensured by arranging counter-weights or adjusting the balance of weights between the semaphore arm and the spectacle appropriately, in both lower-quadrant and upper-quadrant signalling.

Upper Quadrant

Properly, Multiple Aspect Upper Quadrant, in this system there are three signal positions. The 12 o’clock position is clear or proceed, which gives a train permission to go past the signal without stopping. An intermediate position (at 45 degrees to the vertical) is the attention or caution indication; the meaning depends on the kind of signal. The horizontal position, where the semaphore arm is horizontal, the on position, is the most restrictive indication of the signal; it may require the train to stop, or to proceed with caution, etc., depending on the kind of signal.

More notes: In all semaphore systems, as the semaphore arm moves from one aspect to another, the end that is close to the signal mast and which has coloured glass disks (‘spectacles’) fixed to it moves in front of a lamp, changing the colour of the lamp seen at night. Today most of these lamps are electric lamps, but oil lamps were common earlier.

Semaphore signals are set up so that when viewed from the part of the track for which the signal is intended, the semaphore arm extends to the left of the mast on which it mounted. This, in addition to the colours of the semaphore arm (which are different on the front and back), provides a visual cue to distinguish between the signals meant for different directions of the track.

Assemblies of 2 or 3 or more semaphore signals on the same mast structure occur to indicate divergent routes. Usually, one of the signals is placed higher than the others, to indicate the ‘main’ line; the signals to its left or right are somewhat lower, and apply to signals to branches diverging to the left and right. Signals may be at the same height if the divergent routes are all of the same importance. Such multiple signal assemblies are seen for stop signals (home, starter, etc.) and also for distant signals (pre-warners).

Source – IFRCA.org

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