Re: Can HST 125s do 140 mph?

In article <a6vcl1pvj2mrfc8i8clnhmcsmk2tkeba56@xxxxxxx>, Stevie D <stevieinyork@xxxxxxxxxxx> writes 
But he wouldn't. If the signal is supposed to be showing G+Y and the
green bulb fails, he will only see a single yellow.

Unless you have clever trickery that detects a failed bulb and changes
the aspect accordingly. Which sounds quite complicated.


It's called "a relay". Or, in modern arrangements, SSI.

You have a slow-to-fall relay which is held up on its own contacts while either the lamp is lit or the aspect is not selected. When the aspect is called and the lamp fails, the relay will drop. This relay is used in the aspect selection logic to select a more restrictive aspect instead (and to raise an alarm). A push button is used to re-pick the relay after the lamp has been changed. I can't be bothered to draw a circuit diagram.

With SSI it's even easier. The trackside signal module tells the interlocking that lamp 4 (say) has failed, and the SSI logic selects the best aspect that doesn't use lamp 4, changing preceding signals if necessary. In essence it has a table like this (for a 3 aspect signal):

Working   RYG     RY      RG      YG      R       Y       G       -
Want
 R        R       R       R       *       R       *       *       *
 Y        Y       Y       R       Y       R       Y       *       *
 G        G       Y       G       G       R       Y       G       *

* = set previous signal to red

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Clive D.W. Feather                       | Home: <clive@xxxxxxxxxx>
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