(26-05-2015, 03:01 PM)SusannahW Wrote: Hi,
I’ve come across the following in Network Rail standard SSI/8500/D(1), section 4.1 “Motor Drive Outputs”. The section details the motor drive outputs of points object controllers for SSI.
“A motoring up facility is also provided in the Module which operates in the event of detection loss without any change in the points control command from the signalling.”
So I’m taking this to me that in event that detection is lost without a command from the interlocking the points machine redrives the points to regain detection with no additional command to do so from the interlocking. I have a few questions about this:
Does it do this even if there is a train in section or even on the points?
What is the history behind this, is there an incident which occurred that meant this had to be introduced?
Where the points machine is not controlled by SSI is this function still present?
Many thanks for any help you might be able to give with this issue.
Another attempt at answering, hoping that the format keeps sane this time!
Hi Susannah,
A good series of questions, demonstrating just the sort of things a signal engineer needs to be thinking of and in doing so aiding their understanding and IRSE Exam preparation.
Yes your understanding is basically correct; whereas a signal TFM has to receive a fresh message from the interlocking at least once per second or it will go into "Red Retaining mode" and show only the most restrictive aspect and cut off feed to all its other outputs, the points TFM always carries out the last command.
Hence if the points are moving at the time when it loses communication it completes the movement if it can rather than freezing things as they are and therefore potentially leaving the points in mid stroke, neither one way or the other. It drives (as in usual case) until either detection is achieved or the approx 8 second timer has expired.
The re-drive feature means that should detection become lost subsequently, then the TFM will give another 8 seconds of attempting to get the points into their last commanded position (whether or not the TFM is still receiving that commanded position on each cycle of the central interlocking or if it has lost communication and just relying on its own local memory.) The "locking" of points within the interlocking relates to the "change in the commanded position"- the equivalent of moving a mechanical point lever from Normal to Reverse or vice versa.
The TFM knows nothing of the state of the dead track or route locking etc. and works on the basis of "follow last instruction"; get the physical trackside to be into correspondence with where the interlocking last wanted those points. Hence YES it would attempt to drive the points even if a train on top of them (quite likely to be the thing that triggers the loss of detection), but note that it is driving them to where they should already be.
So in summary to attempt to answer your first question:
Does it do this even if there is a train in section or even on the points?
Yes it does; the TFM just trying to get points on the ground with the last instruction it received from the interlocking.
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Question 2:
What is the history behind this, is there an incident which occurred that meant this had to be introduced?
Quick answer is I don't know.
The history I believe (and I was only a very young graduate back then) was that when SSI was developed British Rail had decided not to perpetuate the use of point machines but the Mk1 Clamp Lock was the best thing since sliced bread and was the point operating equipment of choice for the future. Re-drive really makes no sense with point machines such as the 63 or HW since even if voltage is applied to the terminals of the machine then will never reach the motor if the point is fundamentally in the desired position because the cut-off contacts won't be making at that end of the stroke. The internal FPL will still be locking the points.
Conversely to keep a clamp lock locked, it is essential that the lock arm cannot drop and therefore that the Drive Lock Slide cannot, by vibration etc, start edging away from the extent of its travel. Should this start happening in service then the fixed cam will mean that the Left Hand micro-switch on the closed side will become depressed and lose detection, and by causing the point contractor to re-energise then the hydraulic pressure in the extended ram would push the DLS back where it should be and thus oppose any tendency for the points to become unlocked. Whether or not there was an incident in the early days of clamplocks I do not know (I think I was still at school- may just have gone to university- when introduced). It may well have been developed as a requirement frm an academic consideration; today we might call a HAZID but I doubt anyone in 1980s British Rail would have recognised the term. Actually since SSI was developed by BR Research Derby and this organisation was recruited from engineers of other industries, notably aerospace, then it is perhaps quite possible that they would have brought more of a "systems engineer way of thinking" to the industry and considered such interfaces more from first principles.
London Underground has long experience of air operated points, where electric valves control pressure from the eir main to direct to relevant side of cylinder to push piston and indeed hold it in that position until there is a need to drive in the opposite direction. They certainly are aware of the potential for stored energy and counter that by providing a separately operated "ground lock" that is a bit like a bolt that has to be withdrawn slightly before the air pressure is applied for the new lie and then relocked once that move has been made; hence it would take "2 faults" to get an invalid movement.
There certainly was an incident in the early 1990s involving re-drive. A set of reasonaly long switches at Marylebone were clamp lock operated with hydraulic backdrive and there was a derailement of a train. From my somewhat hazy memory there were a combination of faults, including a stuck clamp lock solenoid valve and a loss of hydraulic fluid within the system. A tin traversed the points which were not as completely over as they should have been at the heel end and the passage of that train pushed further over the correct way. Unfortunately the lack of hydraulic oil meant that the fluid had to come from somewhere and in thoise days the main drive and backdrives were just from the same outlet with a T piece. Therefore the fluid was stolen from the ram at the toe end of the points causing it to begin to unlock (after the incident there was the introduction of 4 port hydraulic power packs and non-return valves to prevent this type of thing happening). This caused the DLS to bwithdrawn enough that detection was lost and re-drive therefore came into action. I don't quite recall all the circumstances, but the solenoid valve for the opposite direction had never closed properly and when the pump was started the side then had a "head start" compared with the solenoid for the correct direction that was just about to be opened, so the result was the points went into mid-stroke and then, with both valves now open, although the pump was pumping the two rams were in opposition and the points stayed half-and-half. It would be wrong to say that re-drive actually caused the derailment (as there were at least 2 faults with the clamplock- the stuck solenoid and low fluid), but if it hadn't been provided then actually it may well not have happened as the amount of initial fluid transfer probably would not have been enough actually to fully unlock the points if it had not been for the pump's assistance once the DLS had been displaced enough to cause loss of detection.
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Question 3:
Where the points machine is not controlled by SSI is this function still present?
Generally yes. Haven't checcked most recent circuits but certainly in the 1980s then losing detection on a clamplock (e.g. by a tester giving detection breaks during point correspondence) then they clamplocks did remotor. There was a squeel as te pressure relief valve in the powerpack released the fluid since obviously the point switches already hard against the stock rail and so were going nowhere. Actually useful since the SWTH demand one "long break" so that can be certain that panel tester is seeing actual detection rather than the down-proving of the contactors; there was absolute certainty when had held long enough as the site tester (and the panel tester via the radio - no suucings as mobile phones in those days!) could hear the sudden silence.
There was a slight difference on the Western Region (and possibly a few other places but not sure). Here the E10k interlocking circuitry had always double cut the points LR (Lock Relay) contacts in the coil circuit of the BR943 contactors. Hence if there was any locking (a route set through the points, a dead track occupied or the Individual Point Switch not in the centre "route setting" position, then the LR would be down and the contactors would not repick. So there was redrive, but NOT when trains were about. In fact the WR felt so strongly that on the few installations where they were required to use the BR standard circuits (derived from the Southern Region "Yellow Book", now broadly the NR "T series" drawings) that they introduced a special extra WLR relay to effectively get the functionality of the LR in the contactor feed- this actually about 5 years PRIOR to the Marylebone incident. When the region adopted SSI it had no option to create "a lower quadrant points TFM" so had to follow the national standard!
So in summary:
A. SSI redrives even when train on the track
B. RRI generally redrives when train on the track, but WR specifically made sure that it didn't in those circumstances
C. Redrive is irrelevant for most traditional point machines as always ineffective; redrive is for Clamp locks (including IBCL). Not sure what an HPSS or a T72/VCC and some other mechanisms would do.
D. Uncertain of the actual history, but have told you whatI know.
Hope that helps; feel free to ask more on this or other subjects.
PJW
PJW