(27-07-2010, 01:59 PM)Hort Wrote: Hi,
I attempted question 5 from the 2009 paper about level crossing controls. Would anyone care tell me whether I am speaking from the right orifice?
H
Certainly on the right lines.
I thought your first part re track circuit initiation was spot on. You gave perhaps more detail than you needed to re the length of warning (for an AHBC) which was not really relevant to the question.
You'd have been better off having "compared and contrasted" the different kinds of automatic crossing (AOCL, ABCL, staff warning light/ barrow light crossing / MSLs for footpaths and bridleways) because the RISKS are different and hence pertinent to the question. This is an example of needing to
read between the lines- the question said
automatic and should have sparked the thought- "there are lots of different types I can bring into this: they have left it deliberately vague for me to exploit". Indeed 4-boom automatic crossings exist elsewhere and are coming to the UK, so here would have been another diverse example. Therefore it would definitely have been worth a paragraph at the beginning to define the context of your answer and make it clear that you were interpreting as wide as possible; it would have been useful later on when answering the i)-iv).
The paragraph re using train detection elements in combination was certainly good but would have been better as summary at the end; it doesn't strictly answer the question
For each of the above, describe ways in which the hazards identified could be eliminated through either system design or operational rules. As a final conclusion though it would round off the piece very nicely.
You could have mentioned using a different variety of track circuit or splitting a single functional track circuit into several individual ones to overcome the low ballast resistance problem.
Also you could have mentioned AHBCs do tend to get used on lines with moderately high speed traffic and that the more lightly used lines where ballast conditions are poorer would often be those of lower speed and therefore have shorter strike-in tracks; further it is often on those lines that AOCL/ABCL are used instead and these strike-ins are short due to the crossing speed. Similarly if a strike-in track remains down too long, the DCI times out and then after another time out the barriers can be raised and crossing open thus saving delay to road users. Can't do that safely with AHBC, so we provide monitoring and an alarm in the signalbox so that someone can be dispatched to operate the barriers in manual operation.
The hazards recorded for the axle counter seem reasonable, although you made more of transmission delays than I would have done (but have no direct experience so perhaps you know something I don't). I would be more concerned what was going to happen to level crossings within any reset-restore process and indeed the level of reliability which to all accounts has not been as good in the UK as some were predicting it would be. I preferred the way where you put a sentence explaining a mitigation directly after each hazard and, although there was only one for each, at least there was a variety.
You could have made more re the crossing predictor giving a constant warning time
if only you had included the hazard (present for all the other methods) which this initiation method mitigates- the hazard of the road user abusing as they feel the crossing is often down far too early and that they can zig-zag the barriers and get away with it before the train comes.
Indeed I think that I would have made a table in which I tackled the question much as you did but having four additional narrow columns for the different methodolgy of train detection: i), ii). iii), iv). Some hazards may have been applicable to only one but some would have been pertinent to two or more and you could have covered this with additional ticks in the appropriate columns. Others may have given rise to the same general hazard but by a slightly different mechanism: e.g. there could be a failure to strike in by TC due to a lightweight vehicle and therefore needing some operational procedure to mitigate, whereas a failure to strike-in by treadle may be the treadle has become dislodged from the rails and could be mitigated by having a treadle on each rail opposite each other, striking in if one operated but using some cross proving / cyclic proving to force the crossing into a failed state if only one of the pair were actually operated on any occasion.
The issue re excessive warning times because of a station within the strike-in is something that you should have made more generic; indeed it was a bit ironic that you included in the Lx predictor section which actually may be the least appropriate (although I agree that in many cases the station is immediately prior to the crossing and therefore within the "positive strike-in" area defined.
Section on treadles was the weakest, but I have given some ideas above. Also with a treadle i t is only momentry so you need some form of memory to know that a train has passed the strike-in whereas other forms of train detection are continuously detecting (ok the axle counter has a memory just the same but is "internal" and tends to be supported by UPS etc so isn't too great a concern to the signalling application engineer who can treat outputs lmost as if track circuits); with treadles the railway tends to be far more basic and memories are stick relays and therefore prone to power dips etc and are also "invisible" to the signaller and technician alike. Can be designed to operate in a failsafe manner but that also means a fail frequently manner; if anything untoward happens then tend to need to caution trains through to operate all the treadles and get everything back in kilter again- the crssing may remain failed for ages or need to be put into local control, so all the hazards of human operation, disrupted train working etc. Note that the past few years have featured several bad or extremely near-miss incidents involving human operators and level crossings......
So I think it was a reasonable attempt overall; I am sure that there would have been far worse answers. It could have been a bit longer and certainly for its length did dwell too long on non-core material; precisely how a LX predictor works re measuring rates of change of voltage and phase only becomes directly relevant to the question asked if you identify a hazard (so why didn't you say what the result would be of the passive termination shunts becoming disconnected.....)
The good things that came across were that you know your material and your answer was easily matched against the question. The less good things were you were considering more the fault modes of the train detection rather than thinking of the system as the level crossing, also that a little more thought at the very beginning regarding where overall the answer was going rather than treating it as 4 separate questions would have given a better overall impression.
A comfortable pass but I think very much the smae material presented slightly differently because of a bit of prior thought before launching in might have got a high Credit.
I say this even though by posting a LX question the day I organised the module 5 postings into topics you effectively pointed out that I had missed an important category so I sheepishly have gone backl to create!
