TPWS Grid Location

[PNEYorkshire]

Afternoon All

 

 

I'm trying to work out where to put TPWS grids on my layout but i've no idea where this would be. I've attached a picture of my station approach to give an indication of what i'm working with. The only signals involved are located at the ends of the platform for outbound trains. The trains leave the station and go out of view about a foot behind where the camera is sat taking the picture below, after a set of points provides access from right to left of shot. Any advice would be much appreciated and if more info is needed just let me know...many thanks in advance!

 

[phil-b259]

TPWS grids come in two main varents, An overspeed sensor (OSS) configuration and a trainstop sensor configuration (TSS).

 

For a TSS setup you have two grids located back to back adjacent to the signal. Standard practice will have the grids arnaged such that the signal post is in line with the middle of the two grids i.e. in line with where the two grids touch each other. This can be varied if there are unmoveable obstructions (i.e. AWS magnets,Impeedance bonds in 3rd rail areas or the signal is located right on the tips of a set of points, however given the whole point of a TSS setup is to stop any train SPADing the signal before it starts to foul another movement, such decisons are not taken lightly.

 

For an OSS (located such that any train approching a red signal at 75mph or less will be bought to a stop within the signal overlap), the two grids will be mounted a set distance apart. This varies depending on the linespeed as the whole point is the first grid will start a timer in the train and should the train pass over the second grid before 1 second (1.5 for freight IIRC) then the brakes will be applied. While in most cases these will be located outside the senic area on models, there are two key exceptions. Firstly where terminal platforms are concerned, TPWS OSS grids will be fitted to ensure that the train is not approching the buffer stops at excessive speed. Early problems with standard loops led to the development of the mini loop, only found in platforms, and the placing of the loops is such that any train going faster than 10mph will be 'tripped' and bought to a halt. The second reason for OSS loops being visable is they can sometimes be found to enforce permenamt speed restrictions where again the spacing will depend on the linespeed.

[Edwin_m]

Or to simplify Phil's comprehensive and correct response: On this layout you need two standard grids immediately adjacent to each other, right next to each signal. You will also need two small grids a short distance apart, about half way down each platform.

[PNEYorkshire]

Many thanks to both replies!

 

I've used your advices in the diagram below...is this representative of where they should go? The bigger grey blocks being standard size, the smaller ones down the platforms obviously being smaller....

 

[phil-b259]

Yup thats right

[PNEYorkshire]

Almost right, but not quite.

 

The OSS mini-loops should all be positioned at a consistent distance relative to the buffer stops. By current standards, the loops should be placed at 55 and 60.5 metres from the face of the buffer stops.

 

Brilliant, thanks!

 

Do you know how the measurement would convert into 00 scale?

[PNEYorkshire]

On the basis that OO scale is 1:76, that would equate to 724mm and 796mm from the buffer stop. For accuracy, the measurements are taken from the leading edge of the TPWS loop, not the centre.

 

Thank you!

 

I have the Peco TPWS grids (see pic), are these the correct things to be using for this? Are there different sized ones available as Edwin_m mentioned needing standard ones and smaller ones....

 

[PNEYorkshire]

 

Those are the correct standard-size loops - you need two of them beside each signal.

 

 

 

The mini-loops look like this:

 

Think i've managed to find some from Hurst Models...along with a 16-page guide to TPWS which might be worth investing in while i'm at it!!

[PNEYorkshire]

Finally come to fit these and realised that the OSS's should be placed (based on the measurements) almost in the same place as the TSS's because of the proximity of the signals to the buffer stops! Therefore I will only have TSS grids on my layout because i'm not fussy enough yet to go too deep into the technicalities of this situation

[Supaned]

There shouldn't be OSS grids for outbound trains anyway , just a Train Stop loop in line with the signal . However , there should be OSS loops for inbound trains , normally these are found about 5 or 6 coach lengths from the stop blocks and apply the brakes if the train is travelling above 10mph passing over them - so your trains should crawl towards to buffers to replicate the real thing.

[Dungrange]

Supaned,

 

I think the issue is that PNEYorkshire's outbound TSS grids and inbound OSS grids will, coincidently, be located at approximately the same positions - ie the platform starter signals are located around 750 mm from the buffer stops. I will actually have the same issue with my planned layout should I ever get around to building it, as it has an almost identical track plan and the platforms will only be long enough to accomodate a four / five coach train.

 

Under such circumstances, I assume that the inbound OSS grids would be moved further away from the buffers (ie the 55 metres and 60.5 metres would be the absolute minimum distances from the buffers based on braking performance) and since OSS grids will not be placed within the turnouts the inbound OSS grids would be triggered before the train negotiates the pointwork (which probably has a 10 mph speed limit anyway).

 

Based on PNEYorkshire's trackplan, I would therefore assume that the OSS grids are located under the overbridge / scenic break and can therefore be forgotten about.

 

Regards

 

David

[PNEYorkshire]

Thanks Dungrane, this is what I had hoped to hear....despite a stressful hour or two trying to get the fiddly OSS grids together I think I will discard them.

 

Many thanks for everyone's help!