The original TPWS fitment approach was broadly (look at 10037 for the detail)-
1. Look at every signal in turn and dertmine whether any exclusion code applied (not a stop signal, no junction conflict, protection by virtue of trapping etc.); if no approved "get out", then fit TSS within defined position (very close to signal).
2. Look at the entries in a generic table and find one for the applicable approach speed for that signal; this dictates where the optimum position for the OSS is defined to be, and the spacing of the arm/trigger loop that iself defines the "set speed" (note that this translation is different for passenger or freight as the on board timer is deliberately different so that the speed at which intervention would occur is quite a bit lower for a freight cmpred to passenger).
This is inevitably a compromise, just the best position to maximise the average protection. Appropriate for the retrofit project needing to fit the country very quickly and using pretty inexperienced design resource. The intention initially was just to address the majority of the risk associated with SPAD; it was always known that fast trains would SPAD considerably beyond the overlap, but actually these represented only a very small percentage of all SPADs and even if one of these should occur then at least the consequences of collision would have been reduced significantly because some braking would have occurred.
HOWEVER, that didn't last for long as the "worry brigade" decided that needed to do something more since there would still significant residual risk (albeit much reduced than that with which we had previously been reasonably content). I think it was partially because the national press got the story that TPWS didn't work above 60mph (and interpreted incorrectly that this meant "not at all" rather than "wouldn't completely stop train within a 180m overlap").
3. Hence the OSS+ was invented as an outer protection; generally it was positioned as far as it reasonably could from the location at the signal- (750m) and used for situations where the approach speed such that it was needed.
That was effectively the basis on which the retrofit was initially performed in the very early 2000s.
4. Of course even the OSS+ couldn't cope with the very highest speeds and so people then started worrying about this scenario on the approach to a junction.
Hence the invention of the "double red" control so that unless the junction signal is cleared then the signal in rear would be also held to red; hence could fit this outer signal with a TSS & OSS (not to protect its own route, but to protect the junction beyond the next signal), This was a great idea in that it, via aspect sequence, effectively extended the TPWS protection beyond the technological limit for the junction signal; BUT the flip side is that it completely ruined capacity and junction reoccupation times with predictable effects on the train service.
Hence it had to be further complicated by implementing instead as "Conditional Double Red", which means that the outer signal is approach released once the approaching train has passed its OSS (i.e. it either is coming slow enough that it escaped the speed trap and thus well undercontrol, or it is comng fast but with the emergency brakes already applied). [In some way it has analogies with M / W class routes and their different lengths of overlaps].
Then we started worrying about whether it was right to be releasing the aspect of the outer signal by the route being set across the junction from the inner signal without imposing its approach locking (so this had to be changed to "once route set" so that effective even if the aspect not yet cleared) so here we added another dollop of complexity that many of us think isn't really justified by the risk it is supposedly addressing.
It was only once all this had been completed and new schemes were being developed with TPWS as an integral component, that people started thinking that if we could tolerate slightly longer overlaps then could get good protection at higher speeds without the worst of these complications; conversely if short overlaps really needed then could justify this to be acceptable by providing enough TPWS.
A few of us (including one of the current mod 3 examiners) tried for a long time to rationalise the various standards 10137, 10038 and 00028 because there was a lot of inconsistency and no single point of truth. We very nearly succeeded but fell at the last fence (one contoversial sentence in the wording of the briefing note for the new standard aboout to be published) due to what perhaps are best stated as "personalities and politics". However out of this work did emerge the current methodolgy for determining OSS positions......
A. Having decided to fit a TSS and knowing what the SOD length is beyond the signal, calculate the speed at which a train[#] will stop within that length.
B.If the speed just calculated is less that the potential approach speed then that speed is to be the "set speed" of OSS1.
Position the OSS1 such that it is placed suitably between
i) the braking cuve for a sensible braking rate to the signal (so that won't get invalid interventions on trains being suitably controlled by their drivers)
ii) the braking curve for emergency braking to the end of the SOD (so have a very good expectation that overrun will be contained safely).
C. Now calculate the speed at which this OSS1 will be effective at stopping a train within the SOD. If this is equal to the maximum approach speed then good there is protection; if not then repeat the activity with this speed as the set speed for OSS2 and position this in a similar manner.
D. Repeat working away from the signal adding further OSS with each having a set speed equal to that which is the limit of effectiveness of the one closer to the signal, until effective for the highest possibl;e approach speed.
E. Now that know the minimum number of loops that gives protection, "juggle" positions a bit if this has advantages whilst still giving that protection.
You can probably see why it is all formulae in Excel now! Hence I guess that very few actually understand the methodology utilised but just put in the entries needed and then push the "calculate" button and check that no error messages and write down what it says.
# I have glossed over a bit re "train will stop within that length". Where there are different types of trains with different emergency braking rates and brake build up time (more or less everywhere), then there can be "robust discussions" relating what rolling stock is to be considered. The whole thing is very much a compromise between giving good confidence of protection on the one hand and avoiding false activations on the other; a mix of diverse rolling stock types can make it totally impossible to satisfy all scenarios.
Bet that is more than you wanted to know!
1. Look at every signal in turn and dertmine whether any exclusion code applied (not a stop signal, no junction conflict, protection by virtue of trapping etc.); if no approved "get out", then fit TSS within defined position (very close to signal).
2. Look at the entries in a generic table and find one for the applicable approach speed for that signal; this dictates where the optimum position for the OSS is defined to be, and the spacing of the arm/trigger loop that iself defines the "set speed" (note that this translation is different for passenger or freight as the on board timer is deliberately different so that the speed at which intervention would occur is quite a bit lower for a freight cmpred to passenger).
This is inevitably a compromise, just the best position to maximise the average protection. Appropriate for the retrofit project needing to fit the country very quickly and using pretty inexperienced design resource. The intention initially was just to address the majority of the risk associated with SPAD; it was always known that fast trains would SPAD considerably beyond the overlap, but actually these represented only a very small percentage of all SPADs and even if one of these should occur then at least the consequences of collision would have been reduced significantly because some braking would have occurred.
HOWEVER, that didn't last for long as the "worry brigade" decided that needed to do something more since there would still significant residual risk (albeit much reduced than that with which we had previously been reasonably content). I think it was partially because the national press got the story that TPWS didn't work above 60mph (and interpreted incorrectly that this meant "not at all" rather than "wouldn't completely stop train within a 180m overlap").
3. Hence the OSS+ was invented as an outer protection; generally it was positioned as far as it reasonably could from the location at the signal- (750m) and used for situations where the approach speed such that it was needed.
That was effectively the basis on which the retrofit was initially performed in the very early 2000s.
4. Of course even the OSS+ couldn't cope with the very highest speeds and so people then started worrying about this scenario on the approach to a junction.
Hence the invention of the "double red" control so that unless the junction signal is cleared then the signal in rear would be also held to red; hence could fit this outer signal with a TSS & OSS (not to protect its own route, but to protect the junction beyond the next signal), This was a great idea in that it, via aspect sequence, effectively extended the TPWS protection beyond the technological limit for the junction signal; BUT the flip side is that it completely ruined capacity and junction reoccupation times with predictable effects on the train service.
Hence it had to be further complicated by implementing instead as "Conditional Double Red", which means that the outer signal is approach released once the approaching train has passed its OSS (i.e. it either is coming slow enough that it escaped the speed trap and thus well undercontrol, or it is comng fast but with the emergency brakes already applied). [In some way it has analogies with M / W class routes and their different lengths of overlaps].
Then we started worrying about whether it was right to be releasing the aspect of the outer signal by the route being set across the junction from the inner signal without imposing its approach locking (so this had to be changed to "once route set" so that effective even if the aspect not yet cleared) so here we added another dollop of complexity that many of us think isn't really justified by the risk it is supposedly addressing.
It was only once all this had been completed and new schemes were being developed with TPWS as an integral component, that people started thinking that if we could tolerate slightly longer overlaps then could get good protection at higher speeds without the worst of these complications; conversely if short overlaps really needed then could justify this to be acceptable by providing enough TPWS.
A few of us (including one of the current mod 3 examiners) tried for a long time to rationalise the various standards 10137, 10038 and 00028 because there was a lot of inconsistency and no single point of truth. We very nearly succeeded but fell at the last fence (one contoversial sentence in the wording of the briefing note for the new standard aboout to be published) due to what perhaps are best stated as "personalities and politics". However out of this work did emerge the current methodolgy for determining OSS positions......
A. Having decided to fit a TSS and knowing what the SOD length is beyond the signal, calculate the speed at which a train[#] will stop within that length.
B.If the speed just calculated is less that the potential approach speed then that speed is to be the "set speed" of OSS1.
Position the OSS1 such that it is placed suitably between
i) the braking cuve for a sensible braking rate to the signal (so that won't get invalid interventions on trains being suitably controlled by their drivers)
ii) the braking curve for emergency braking to the end of the SOD (so have a very good expectation that overrun will be contained safely).
C. Now calculate the speed at which this OSS1 will be effective at stopping a train within the SOD. If this is equal to the maximum approach speed then good there is protection; if not then repeat the activity with this speed as the set speed for OSS2 and position this in a similar manner.
D. Repeat working away from the signal adding further OSS with each having a set speed equal to that which is the limit of effectiveness of the one closer to the signal, until effective for the highest possibl;e approach speed.
E. Now that know the minimum number of loops that gives protection, "juggle" positions a bit if this has advantages whilst still giving that protection.
You can probably see why it is all formulae in Excel now! Hence I guess that very few actually understand the methodology utilised but just put in the entries needed and then push the "calculate" button and check that no error messages and write down what it says.
# I have glossed over a bit re "train will stop within that length". Where there are different types of trains with different emergency braking rates and brake build up time (more or less everywhere), then there can be "robust discussions" relating what rolling stock is to be considered. The whole thing is very much a compromise between giving good confidence of protection on the one hand and avoiding false activations on the other; a mix of diverse rolling stock types can make it totally impossible to satisfy all scenarios.
Bet that is more than you wanted to know!
(26-07-2013, 08:08 AM)dorothy.pipet Wrote: Thank-you, that is very encouraging at this stage.
You are right that my knowledge of semaphore signalling is sketchy, but also I'm not sure of how SODs are calculated and used - in my role I only a scheme plan with the TPWS already placed, and Design Logs rarely show the detail.
When I looked at the 2006 paper, I found few appealing questions and this looked like a "least worst" option, so I have surprised myself.
PJW