(16-05-2011, 12:57 PM)onestrangeday Wrote: Hi Signalling professional:

Currently, I am reading the railway signalling book (by O.S.Nock) and on page 54 it explained the term ''non-reciprocal locking'' and ''delayed yellow condition''. But I still don't get what it is trying to explain.

Can anyone explain it plainly?

sorry for my poor comprehension skill on the book


thanks

(17-05-2011, 05:43 PM)PJW Wrote:

(16-05-2011, 12:57 PM)onestrangeday Wrote: Hi Signalling professional:

Currently, I am reading the railway signalling book (by O.S.Nock) and on page 54 it explained the term ''non-reciprocal locking'' and ''delayed yellow condition''. But I still don't get what it is trying to explain.

Can anyone explain it plainly?

sorry for my poor comprehension skill on the book


thanks

First I'll attempt 'non-reciprocal' ; then the actual description of the "delayed yellow".

Generally the majority of interlocking is reciprocal (i.e. symmetric converse).
In mechanical signalbox terms:
if lever 1 locks lever 2 then lever 2 locks lever 1.
i.e. we are wanting to prevent both levers being able to be reverse simultaneously, so whereas either can be pulled initially, once the first one has been pulled, then the other one cannot.

In a mechanical locking arrangement, this reciprocity arises naturally from the operation of the locking nibs in the tappet blades, however in an electrical installation (or indeed a mechanical signalbox in which the locking is provided by electric lever locks) then the reciprocal locking has to be explicitly provided (i.e. one circuit is the electric lever lock for lever 1 which includes the proving of the position of lever 2 and a totally separate circuit is the electric lever lock for lever 2 which includes the proving of the position of lever 1).

However in some less common circumstances it is desired to have "non-reciprocal locking"; i.e whereas the locking of lever 1 is dependent upon the position of lever 2, the converse is NOT desired- lever 2 is to be free from any dependency on lever 1.
In a mechanical signalbox the classic example of this would be requiring the signaller to pull the levers in a specific order, a semaphore can only be pulled if the one to which it reads is initially at danger, but then that signal can itself immediately be cleared (the reason for this is that the logic is part of the means of enforcing the overall requirement of ensuring that a new "line clear" is obtained from the signalbox in advance for each separate train).
It is possible to implement this locking entirely mechanically by using nibs with springs etc, but generally it was difficult and was easier to achieve electrically so this became the more common means even on what was otherwise a mechanically interlocked installation.
======================================================
So now coming to the specific text on page 54:
[and I remember struggling with this wording myself when studying this book some 30 years ago- I really don't think that this book is written in a good way to explain to the novice.......]
what it is trying to say is-
a) whenever practicable, signal 10 to sigal 12 should be established with a full overlap which means that sigal 8 would be Green, signal 10 Yellow and signal 10 Red
b) there are circumstances where this is too restrictive as it prevents other movements over 112 points and therefore the sigaller is given the option of selecting a "restricted overlap" [i] [this is the current nomenclature although you'll find that it is called "reduced overlap" within this book as that was the nomenclature used back then]
c) if this shorter overlap is chosen then the risk of overrun is mitigated by deliberately holding signal 10 at Red (called here a "Delayed Yellow") until a train arrives at it (and thus would have received a Yellow at signal 8), thus ensuring a slow speed approach and thus lowering the likelihood of an overrun at signal 12. Point 112 remains free and thus for example signal 52 can be used for a different train movement.
d) however should 112 then be placed normal and the restricted overlap beyond signal 10 permitted to become a full overlap (e.g. by Forward Route Setting from signal 12), then at that time points 112 must become and remain locked (because once signal 10 has been allowed to clear then "the cat is out of the bag" and the train may now be approaching faster). In more modern terminology it is the equivalent of a Warning route "stepping-up" to the Main route.
e) Hence whereas signal 10 does not always require 112 normal, 112 is locked to be locked normal once signal 10 has cleared with a full overlap.

To be honest I myself wouldn't have used the nomenclature "non-reciprocal locking" in this context; I see what he is trying to say, but doesn't seem particularly appropriate and certainly not helpful to the explanation.

Does the above help?

(18-05-2011, 02:06 PM)onestrangeday Wrote: thanks for your clear explanation. Yes, I have understood what it means. I think it is due to my lack of signalling knowledge, I found some of the explanation in the book (railway signalling) is hard to understand. So I might asking more questions in the future.

Quote:One more thing I would like to ask, how do a fouling point is marked on the layout (British Practice) ? because in the book, i cannot see any marking on the diagram that point out the fouling point. (or is there always flank protection for the crossing ?)

As the railway line i have exposed to in the past, there is always letter "F" marked on the layout to show the reader that the section is protected by the flank protection (due to the track next to each other is close enough that a train side collision might happen, as this often the case in depot.). Whereas, if it not marked by the letter ''F'', there is no flank protection.
william

(18-05-2011, 10:23 PM)PJW Wrote:

(18-05-2011, 02:06 PM)onestrangeday Wrote: thanks for your clear explanation. Yes, I have understood what it means. I think it is due to my lack of signalling knowledge, I found some of the explanation in the book (railway signalling) is hard to understand. So I might asking more questions in the future.

Indeed, happy that you ask such questions in future. Rather surprised that more people don't!

Quote:One more thing I would like to ask, how do a fouling point is marked on the layout (British Practice) ? because in the book, i cannot see any marking on the diagram that point out the fouling point. (or is there always flank protection for the crossing ?)

As the railway line i have exposed to in the past, there is always letter "F" marked on the layout to show the reader that the section is protected by the flank protection (due to the track next to each other is close enough that a train side collision might happen, as this often the case in depot.). Whereas, if it not marked by the letter ''F'', there is no flank protection.
william

Nowadays we always mark the Clearing Point as "CP" with a double headed arrow, but traditionally (as in the era of this book) it was generally only indicated for those sites which are tight and in particular where there is an IRJ which is foul. Note that the Clearing Point is further from the place of covergence than the Fouling Point which isn't marked (the FP is the place where the vehicles would actually just hit, whereas the CP is the place where the train detection limit needs to be i order to ensure a decent "passing clearance" as well as taking into account the fact that it is the corner(s) of the vehicles hich would hit whereas the train detection only knows where the wheels of the vehicle are located and there is always a degree of "overhang"- the vehicle is longer than its wheelbase.

Also be careful that "foul" isn't precisely the same as "flank"; do you understand the distinction?

We would (almost always) call and lock points as flank where they have the opportunity to deflect a train that has SPADed a protecting sgnal from collision path with the set route, irrespective of how "close" it is to the route in question.

What we mean by Foul is where the adjacent train detection section commences too close to the convergence for us to be sure that there is adequate clearance between a vehicle standing on it and one on the other route. For simplicity a train detection section would always extend sufficiently sideways from the line of the set route to prove that it would be safe to allow the passage of a train on that route, this is not always practicable and the joint between sections has to be placed within this distance. This seems to be what you are calling "Flank".