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Swinging overlaps
#1
Hello, i am new in the forum. I have some questions about overlap /swinging overlaps

1. Generally we lock and detect a point after the signal if the distance is short than the allowed for the maximum speed of the section. The position of the point (N or R) is accordingly the side that train approach the point. 
2. If we provide swinging overlaps, first we set the route after the signal and then we set the route before the signal? Is the any timer between the 2 route set ( first route after the signal, secnd route before the signl).
3. Is there any way to have an unsafe situation with swinging overall ruther than simpy overlap?
4. Is swinging overlap used in other countries expect of British railway? 

Please be free to answer!
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#2
(30-05-2017, 10:34 PM)kochronaki Wrote: Hello, i am new in the forum. I have some questions about overlap /swinging overlaps

1. Generally we lock and detect a point after the signal if the distance is short than the allowed for the maximum speed of the section. The position of the point (N or R) is accordingly the side that train approach the point. 
2. If we provide swinging overlaps, first we set the route after the signal and then we set the route before the signal? Is the any timer between the 2 route set ( first route after the signal, secnd route before the signl).
3. Is there any way to have an unsafe situation with swinging overall ruther than simpy overlap?
4. Is swinging overlap used in other countries expect of British railway? 

Please be free to answer!

1.  Where there is a facing point beyond a signal then the general rule was that it was not locked but it was detected to be in Normal or in Reverse except when in the course of changing (i.e. 8 seconds period of non-detection permitted, after detection initially achieved) You are correct though that when the points are close to the signal (or more accurately close to the beginning of their deadlocking track) then they do become locked when the berth track of the signal is occupied after a route set up to that signal; this is known as  "Time of Operation" locking.  This means that the swinging of the overlap is temporarily inhibited when there is a risk of a SPAD.  This is not necessary where the point is further away; it is judged that if the points begun to move just prior to the SPAD occurring, then they would still have the time to complete their movement before the overrunning train could reach them.
To the most recent NR standard, facing points are not detected (other than to condition out tracks which are not in the overlap for the direction they are set).

2. No; if there is a swinging overlap the routes can be set in either order.  The route in which the points are in the overlap do not call facing points unless there is no valid available overlap beyond them in their initial position.  If the forward route is then set, this may call those points just like any other points in the route, with the exception that this is prevented by "counter-conditional" locking if the effect of moving those points would be to replace the aspect of the signal in rear.

3. Definitely possible!  The main problem is that swinging overlaps can be complicated and mistakes can be made in the design which are not found in testing.  It shouldn't happen, but it has done.  In fact due to the SSI data construct, there have been several incidents and it is possible for a false swinging overlap call to move points that one might have thought were securely locked by track occupancy or by relevant route locking.  This is very rare, but people have to be very careful to make sure that errors don't get commissioned.
The flip side is that swinging overlaps increase layout flexibility and this means that less trains need to approach red signals and since a signal showing a proceed aspect cannot suffer a SPAD, then there is an argument that provision of swinging overlaps increase safety.

4.  Don't know.  I suspect only in places whose signalling ancestry from the UK, so I wouldn't be surprised that some Australian states may feature for example.  I think that some countries have alternative overlaps but these are set first (either by point keys or setting the forward route first) and then the route needing the overlap is set once the points have been defined and thereafter locks them in that lie- this is how I read your initial description in 1 above.
PJW
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#3
Thanks al lot for the clear answers.
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#4
OP i wanted to say a big thanks for asking those questions because i was about to ask them myself and because you've already askedthem you've got amazing answers! of course, PJW, thanks a lot for the clear, informative and helpful answers! may i ask you some other questions in case you don't mind! lots of thanks! Strawberry Pasta Salad with Mozzarella & Herbs
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