Trying to affordably expand TPWS protection presumably runs into the problem that AWS, which provides control and power at most colour-light signals on the mainline railway, only has to signal green and not-green to the trains.
So, conceptually, it seems like we could provide an Over Speed System (OSS) arm-trigger loop on any signal fitted with an AWS electromagnet, but it could only be be set to trigger if the train is too fast for the least-restrictive restrictive aspect.
ie. if the train is going too fast to be appropriate for a yellow (in three aspect areas) or double-yellow (in four aspect areas) signal.
I'm not sure there are many cases where that would achieve something, and if you want to do anything better you'd need to break into the signalling system to get more control feeds rather than just using the signal that controls the AWS electromagnet.
Or at least, I think so, I may be wrong.
Discussed in some detail by
@Annetts key in
post #58 above.
Expanding on what I said earlier, the control system for the AWS also varies depending on the actual installation.
First though, let me describe the signalling nomenclature.
In the signalling nomenclature, the first part is the individual equipment name or number.
E.g. a signal number, a point number, a track circuit number or name are the most common. These may have the signal box prefix, but mostly the signal box prefix is left out unless it may cause confusion.
The second part is further divided as follows: the first letter or the middle letter(s) is/are the function, the last letter is the type of equipment (e.g. R meaning relay) and If there is a P before the last letter, it means this is a repeat relay (P meaning repeat).
As I have and will talk more about signal control relays, for the record:
H at the beginning means yellow, hence HR means the control relay for the yellow aspect (* see note).
HH at the beginning means double yellow, hence HHR means the control relay for the top yellow aspect (* see note).
D at the beginning means green, hence DR means the control relay for the green aspect (* see note).
G at the beginning means signal.
E at the beginning or in the middle means light or lamp.
C at the beginning or in the middle means proving or checking.
T at the beginning or in the middle means track as in track circuit.
Hence 57 HR is a control relay for the yellow aspect of signal 57 (* see note).
AB TPR is the repeat relay for AB TR which in turn is the track circuit relay for AB track circuit.
* note that as there are different ways of wiring up a signal, you have to consider
ALL the relevant relays that control the aspect on a signal, not just one. Just because a four aspect signal has the DR energised does not always mean that the signal will show a green aspect. If the HR is de-energised the signal should show a red regardless of the state of the other control relays.
Let me be clear what I mean by a repeat relay, as did not explain that in my earlier post. A repeat relay is provided when either the control relay it is repeating does not have enough contacts available for all the circuits that are needed or circuits are needed in a different location to where the control relay is located.
In most cases a repeat relay is wired up such that it's energised when the control relay it is repeating is energised.
Please note that there are various different repeat relay circuits and I am only describing the type that is relevant here. I'm also not including all details, otherwise the explanation will be rather long and more complicated.
There is also more than one method of using the nomenclature to show a repeat relay.
So the following are all examples of repeat relays:
HPR - repeats the HR
TPR - repeats the TR
TPPR - repeats the TPR
Sometimes a number is used rather than adding more Ps:
T2PR or - T(2)PR means TPPR, it repeats the TPR.
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Right, back to AWS controls
I will first describe the typical arrangements for a relay interlocking.
The AWS gets it's "information" from the signal control relays for that signal. What I will describe is for a signal on plain line, there is more complex circuitry if there are points between the AWS track mounted magnets and the signal.
For the electro-magnet to be energised, and hence cause a bell to sound in the cab of the train, the interlocking needs to be calling for the signal to be showing green and for it to be lit.
That means for a four aspect signal, depending on the design of how the control relays are wired up, using either the contacts of all these relays (or repeats of these relays or equivalent) wired in series: HR, HHR, DR (when all these are combined, proves the signal is suppose to be showing green) and the ECR or GECR (name varies across the regions). Or if the signal design is wired as a climbing aspect circuit, just the DR contacts and the ECR or GECR.
Here the ECR or GECR (name varies across the regions) is the signal lamp proving relay, which is energised when the signal head is drawing enough current (for tungsten filament heads/lamps, this indicates that the lamp is lit).
Note that with a climbing aspect circuit design, the DR can only be energised if the HR and HHR are also energised.
If the signal control relays (HR, HHR and DR), or repeat relays (HPR, HHPR and DPR) (or equivalent) and the lamp proving relay, the GECR, or ECR or a repeat relay are in the local location cupboard, contacts of these relays are wired up to form the control circuit for the AWS electro-magnet either in the 110V AC supply to a 24V DC power supply unit (also known as a transformer rectifier) with the electro-magnet fed from the output of this unit.
Or the 110V AC to 24V DC power supply unit is continuously powered and the relay contacts are wired between the output of this and the electro-magnet.
If the signal control relays or repeat relays (or equivalent) are not in the local location cupboard, then the relay contacts may either be wired in a 110V AC circuit, which then feeds out to the local location cupboard, which is where the 110V AC to 24V DC power supply unit will be. That then feeds the AWS electro-magnet.
There is a further variation, again, if the signal control relays or repeat relays (or equivalent) are not in the local location cupboard, a 50V DC control circuit may feed out from where the signal relays are, this then feeds a relay only for the AWS. Contacts of this then control the 110V AC supply to the 24V DC power supply unit for the AWS electro-magnet.
Now you can see that due to all the variations in design, how difficult it is to retro-fit any new train control system to existing signalling installations.
For computer based signalling, the 110V AC to 24V DC power supply unit for the AWS electro-magnet is fed from an output from a signal module, which may be the same module that controls the signal itself.
Edited to correct an "of" to an "or".
