What are the systems of colour-light signalling used by IR?

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What are the systems of colour-light signalling used by IR?

July 14, 2019, 3:24 PM

There are three systems of colour-light signalling in use. (In IR terminology, the term Multiple-Aspect Colour-Light signalling includes both 3- and 4-aspect signalling, and 2-aspect signalling is usually treated as a variant of 2-aspect semaphore signalling. Hence the classification below is not the same as IR’s.)

Two-aspect colour-light signalling – In this, each signal has two lamps (one above the other). The higher of the two is a green lamp, and the lower one is a red lamp. The green lamp when lit indicates clear (the proceed indication), and when the red lamp is lit, the signal is said to be in the on position, displaying its most restrictive indication.

Three-aspect colour-light signalling – In this, each signal has three lamps arranged vertically. The top one is green, the middle one yellow, and the bottom one is red. The red and green lamps indicate indications as in the 2-aspect system, and the yellow lamp shows the caution indication.

Four-aspect colour-light signalling – This is also known just as Multiple-aspect colour-light signalling (MACL or MACLS) and adds another yellow lamp to the 3-aspect system. The additional yellow lamp can be placed above the green lamp in a 4-lamp signal. In this case, the lower yellow lamp alone is lit to show the caution indication, and both yellow lamps are lit to show the attention indication. Alternatively, a different kind of 3-lamp signal may be used (e.g., for distant signals), where the top and bottom lamps are yellow and the middle one is green. Again, both yellow lamps light up to indicate the attention indication.

Special signals such as repeaters may have other combinations, e.g., two lamps, green above yellow.

The obvious advantage of colour-light signalling over semaphore signalling is the higher reliability of electrical control over the signals compared to the mechanical means for operating semaphore signals. Colour-light signals do not suffer from distance limitations as semaphore signalling does (exception: powered semaphore signalling), allowing signal controls to be placed conveniently together even if the signals themselves are far away. In addition, the electrical circuitry naturally allows for monitoring, interlocking, and detection of failure conditions, all of which are achievable but far less reliably with mechanical means in semaphore signalling.

Source – IFRCA.org

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