Differential TPWS?

[TheEdge]

As one does I've been thinking over the great mysteries of TPWS and have a question.

Does differential overspeed TPWS exist? As it different trigger speeds based on route set? Is that even possible with TPWS? Is TPWS+ the closest thing to it? Where do babies come from?

TIA

 

[Highlandspring]

I know of an installation where an OSS protecting a PSR located beyond a junction on a diverging route is suppressed depending on whether or not the diverging route is set. Unless I’m missing something really obvious - and I have a nagging feeling I am - I haven’t yet come across an instance of multiple sets of OSS equipment being provided for multiple routes. For advice on the origin of babies you’ll need to speak to your dad.

 

[GreatAuk]

I guess there's no reason you couldn't have two of the arming loops different distances before the trigger loop (therefore setting tpws off at different speeds) and only have one of them powered at the same time... No idea if that actually exists though!

 

[Sunset route]

If we are talking about tpws on signals and not PSR or for buffer stops, then I can’t see any reason to have differential triggers for diverging routes as it’s the red aspect that arms the tpws grids regardless what the route ahead is set for.

 

[142Pilot]

I guess there's no reason you couldn't have two of the arming loops different distances before the trigger loop (therefore setting tpws off at different speeds) and only have one of them powered at the same time... No idea if that actually exists though!

I'm not sure that would work.

If the trigger active then it would only work if the corresponding arm was active.

They work in pairs, so if you had one trigger and 2 arm loops then the arm loops would reset each other making the process pointless.

 

[sw1ller]

I'm not sure that would work.

If the trigger active then it would only work if the corresponding arm was active.

They work in pairs, so if you had one trigger and 2 arm loops then the arm loops would reset each other making the process pointless.

I can’t remember how they work, but could you not just switch off the arm you didn’t need depending on what route is set? You’d still have 1 trigger and 1 arm.

Do they switch on & off? Can they switch on & off??

If we are talking about tpws on signals and not PSR or for buffer stops, then I can’t see any reason to have differential triggers for diverging routes as it’s the red aspect that arms the tpws grids regardless what the route ahead is set for.

I’m thinking no, there would be no need to have different tpws grids for a red signal. I think the question is for a multiple diverging route. Let’s say the linespeed is 100 and there are two diverging routes of 50 & 25mph, a theoretical differential tpws would use less grids and equipment, therefore making them cheaper to install.

It’s a good question. I do wish there was a document somewhere giving the speeds at which they are set!

 

[TheEdge]

I can’t remember how they work, but could you not just switch off the arm you didn’t need depending on what route is set? You’d still have 1 trigger and 1 arm.

That's how I would imagine it might be possible, surely the same circuitry used to supress stop loops could be used to supress the unwanted arming loop?

Do they switch on & off? Can they switch on & off??

Signal stop loops obviously manage it

I’m thinking no, there would be no need to have different tpws grids for a red signal. I think the question is for a multiple diverging route. Let’s say the linespeed is 100 and there are two diverging routes of 50 & 25mph, a theoretical differential tpws would use less grids and equipment, therefore making them cheaper to install.

It’s a good question. I do wish there was a document somewhere giving the speeds at which they are set!

That's the sort of situation I'm thinking, so trains routed to the 50mph route triggered at 70mph and the 25mph route at 50mph.

Trigger speeds are available internally at depots, some OSS loops even have their trigger speeds signed next to them. Not sure if that is available in public however

 

[edwin_m]

TPWS is non-failsafe in the sense that the loops are de-energised when there is no requirement to control the speed of the train. This is unlike AWS where the permanent magnet ensures that a warning is given even if the electromagnet has failed. So TPWS does not need suppression and in principle multiple OSS loops could be provided for the same signal. However I suspect this is rarely of any value because the signal aspects are used to control the speed of trains approaching a diverging junction, and the TPWS follows the signal aspects.

To protect against undeteced wrong-side failures the TPWS loops are monitored via the lamp proving circuit, or I believe a dedicated circuit is used on newer installations. This results in a small risk of TPWS not operating when needed to mitigate a SPAD, but this is considered acceptable because for an accident to occur this failure would have to occur on one of the extremely rare occasions that a driver isn't obeying the signals.

 

[sw1ller]

Signal stop loops obviously manage it

Oh yeah! It was late when I replied. I have to stop replying on here as I’m dosing off!

 

[Belperpete]

If you want to enforce two different speeds, then you would need two pairs of TPWS loops, one for each speed. Whilst it might theoretically be possible to have a single arming loop with two trigger loops, I suspect it would need a significant reworking of the standard TPWS control units. The control units are designed to control and detect a pair of loops. It would be easier to just provide two standard pairs of loops, than to develop some non-standard TPWS control units for the few occasions where this non-standard arrangement might be useful.

Differential TPWS is often provided in rear of junctions with flashing aspects, where you get a speed restriction that is only applicable if the train is taking the diverging route ahead (i.e. where you get an advance warning board with an arrow). If the junction signal is off for the straight route then the TPWS is disabled, whereas if the junction signal is off for the diverging route (with flashing aspects in rear) then the TPWS will enforce the diverging speed restriction. I suspect that the same may apply at other junctions where the junction signal isn't approach controlled from red (e.g. where there are splitting distants, PRIs, etc.).