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Designation of colour-light signal aspects

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Designation of colour-light signal aspects

Unread postby John Webb » Fri Jul 1, 2016 5:44 pm

The colour light aspects in the IRSE books and in the actual signals are designated 'R' for the red aspect, which seems logical; 'H' for the single yellow, 'HH' for the second yellow, and 'D' for the green aspect. These latter designations seem rather illogical. Can anyone enlighten me as to how these designations arose, please?
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Re: Designation of colour-light signal aspects

Unread postby Mike Hodgson » Fri Jul 1, 2016 7:30 pm

These are from BS376 and the standard is associated more with relays than with the signals. There are only 26 letters available to designated everything in relay interlocking circuitry, so the choice is very limited and you couldn't necessarily use the most obvious letter.

I think of H for Home signal relay being picked (the signal it controls therefore showing a proceed aspect, which happened to be yellow) and D was for the Distant relay being picked in the original 3-aspect concept. When double yellow came got inserted before the Distant HH would then have made sense. DR does not in itself imply a green aspect - you would need the HR, HHR and DR all to pick in order to get a green light.
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Re: Designation of colour-light signal aspects

Unread postby JRB » Fri Jul 1, 2016 7:42 pm

Exactly.
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Re: Designation of colour-light signal aspects

Unread postby John Webb » Sat Jul 2, 2016 7:43 am

Thanks - I had a feeling it was along those lines that the nomenclature had arisen.
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