1998 paper - preset shunts

[PJW]

I have attempted the 1998 aspect sequence chart.

Where some of the main signals preset a shunt signal I thought this should somehow be shown on the chart (maybe just a note against the main signal). Looking at the model answer in the study pack there is no mention of the preset shunts at all. Can anyone explain why they are not shown/mentioned?

Thanks.

I'm not an expert in this, but from what I know of preset shunts, although the calling or setting of the shunt may be proved at the route level, I have never seen any aspect level proving. If the aspect shown by the shunt is not proved in the main aspect, putting it on the aspect sequence chart would have no value. Thinking about it, given that the aspect level controls for the main signal are likely to be more onerous that for the preset shunt element, if the preset shunt route has set, any aspect level controls that would hold the shunt signal would also hold the main signal at danger.

I'd be interested to learn more if someone else has something to add.

Peter

A pre-set shunt differs slightly from the 'facing shunt' that is traditionally used on Western Region E10k panels which are a bit closer to mechanical practice and with whhich I suspect Peter is more familiar.

To NR standards, when a route is set which reads via a GPL signal part way along the route, then that GPL is preset. In SSI this is by the operation of a specific (PS) route but in some varieties of RRI than it is actually purely an aspect level control that effectively 'false feeds' the GPL when the main signal that has pre-set it is itself ready to clear. The pre-setting signal then clears once the GPL has been proved off.
Hence it is very much an ASPECT level control.

However to return to the original question, I would not expect the GPL to show on the aspect sequence chart. Although it is an aspect control, it is purely in the 'HR' level, whereas the main use of the aspect sequence chart is to define the operation of the 'better aspects'- the HHR and DR etc. As much in signalling, we do end up with a practice which had its origins in history; the HR of controlled signals were always within the interlocking relay room and could be pre-tested with a test panel for that room in isolation; the aspect sequence was often implemented trackside and from adjacent relay rooms, so generally could not be tested until the commissioning possession. I suspect this is one of the reasons why we have separate aspect sequence charts. The other reason is that they do show 'the overall picture' that relates signal spacing / braking distance / and the required warning for the driver.
Since the pre-set shunt control is not relevant to either of these, there isn't really any reason to depict; however I have seen some aspect sequence charts that make reference to pre-set shunts as a note against an aspect sequence line and perhaps this might be a practice to adopt for the IRSE exam, just to show that you are aware.