14-09-2008, 01:07 PM
alexgoei Wrote:Hello Peter,
I am not sure if I understand why 206R is entered as a condition here and would like to suggest two reasons why as a test of my understanding which perhaps you can say whether I am right or wrong.
For route 114 B(M), there are two routes leading to it: One by 122A(M). Hence when 114 is put back to Danger, 118 will be yellow and hence like you say this can be seen from track BR. The second route is via 122B(M) then 116 A(M); hence 206R a condition for this.
Is my reasoning correct?
Dinner time now. Don't let me hold you up if you do not find me on line
Thank you and Regards
No.
You need to consider the lookback in various portions.
Firstly from the signal itself to each of the signals in rear- this is the "first section". It will generally consist of a list of tracks but with point conditions if any of the signals read anywhere else than our signal. In this case we have BL, BM, BN (AN or 205N). In this case where there is no route from 116 over 205N then the condition on AN is arguable but probably worth including so that 114's locking is not held by a train on AN that has come from 109/113. Older installations did not "store" that an intrusion had occurred within the CAL area and thus the point condition less critical; nowadays then should be included as one a train has been "seen" then the CAL path is permanently inhibited for RRI sites (CAL is performed at instant entrance button is first pulled for SSI sites).
Secondly you must consider the various "section in rear"; in this case there is the behind 118 and behind 116 sections to consider.
Thirdly in a 4 aspect area only you must then consider the "second section in rear" i.e. all those sections back to the signals that would diesplay YY when our signal reverts to Red. In some cases these signals can be the same (although different routes obviously) for the various first signal in rear (i.e. there are multiple paths from the outer signal to the CT signal via various of the intermediate signals); in other cases where two entirely separate lines converge at a geographic junction then obviously the second signals in rear are entirely different.
Our case has two first sections to consider;
118 for which BP and BR are relevant. BR is only relevant though if 206N; therefore we write it: BP, (BR or 206R). i.e. just like when you have a conditionally foul track for a set of points you initially "assume the worst" and put the track in the locking, then you "get rid of it again if there is a valid "excuse reason"" by putting in the or condition. BR should lock 114 UNLESS 206R, therefore we write (BR or 206R). If we wrote the "positive "BR w 206N" it would imply that we'd actually need to have achieved Normal detection of the points to IMPOSE the locking and this could give a wrongside failure if the points lost detection; that is why we put the converse "require BR clear unless we have positive proof that the train will be deflected away from approaching 118".
For the section depedent upon 116 not being ARAFOAL we could put AP, AR (BR or 206N); however my argument before was that they'd be no need to take the lookback that far given that the aspect displayed at 122 routed to 116 would at best have been yellow and hence my expectation that 116 would be at red until AP became occupied anyway.
I think it is the concept of writing the point positions in the "unless the converse is true" rather than the "when in the correct position" which is confusing you