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| AHB Crossing Failures 11/05/2013 at 07:52 #44581 | |
Jezalenko
25 posts |
I've recently been playing both Glouster and Westbury, and there is the occasional AHB crossing failure (as indicated by the red FAI button at the crossing). Almost all failures have occurred with trains approaching. In this situation, would you cancel the protecting signal, causing an ACoA? Or can the train proceed through the crossing? Thanks Log in to reply |
| AHB Crossing Failures 11/05/2013 at 08:10 #44582 | |
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Stephen Fulcher
2084 posts |
In reality there can be any number of reasons why an AHB will show up failed. Theoretically they are designed to fail safe, and many failure modes will cause them to stay down. The most common cause of a crossing showing failed in SimSig (possibly the only one to be a "genuine" fault) is down to track circuit failures. In the case of track circuit failures in reality, the barriers will either stay down after a train, or time out after (usually) two minutes and then raise. Sometimes you would get an alarm which would latch, sometimes you would get an alarm which would cancel itself again as soon as you acknowledge it in the box, but this is dependent on exactly which failure mode has occurred, and is not replicated in SimSig. There are also a few other issues with the crossings which in reality would generate a failed alarm, such as issues with the directional controls in the interlocking, and someone taking the barriers under local control. There are also some scenarios whereby the barriers will fail to raise at all, which generates an alarm in the box after a specified passage of time. Reference Westbury, I believe that there is an outstanding issue with Greenland Mill AHB which it may be worth searching the forum for. Log in to reply |
| AHB Crossing Failures 11/05/2013 at 09:00 #44585 | |
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Noisynoel
989 posts |
" said:AHB's are not "usually" interlocked with the signalling and therefore would not give a COA under normal circumstances. " said: Yes, the driver will be stopped at the last controlled signal and advised to stop at the crossing and proceed over if the driver deems it safe to do so (ie the crossing is clear). An interesting aside is that some AHB's actually start raising before the train is completly clear of them, often causing them to be reported, especially by the public, as not working correctly. This is not the case, they are able to raise before the train is clear, after all, you'd probably notice the massive great train more than the tiny thin barrier!!! Noisynoel Log in to reply |
| AHB Crossing Failures 11/05/2013 at 09:01 #44586 | |
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Late Turn
699 posts |
It's an interesting one. The signalling regs say that you should stop and caution all trains over the crossing - because of the increased risk of a motorist weaving around the barriers. It's not really clear whether it's necessary to replace a signal to Danger in the face of a train to achieve this - I'd suggest not, as the resulting heavy brake application and probable SPAD (sorry, 'operating incident' probably carries more risk.Having thought long and hard about it and discussed it in a few different circles, I've struggled to think of a situation where an AHB could fail with the barriers raised and no red road lights and cause a failed indication in the box (e.g. a failure - unlikely - to detect the approach of a train wouldn't cause such an indication - how does the equipment know that it's supposed to have detected a train?!), so it's a fair bet that the barriers are down (or red road lights illuminated at the very least) if a failed indication is received. Log in to reply |
| AHB Crossing Failures 11/05/2013 at 10:42 #44589 | |
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Stephen Fulcher
2084 posts |
" said:" said:Generally you would caution trains over a crossing which is showing a failed state. As I mentioned above there is a coding issue with Greenland Mill which causes it to show failed when a train is sat on the approach track circuit under certain circumstances. This is discussed in detail in several threads on this forum.AHB's are not "usually" interlocked with the signalling and therefore would not give a COA under normal circumstances. There are some circumstances where AHB crossings are interlocked with the signalling system. For instance, if a signal is positioned between the strike in point for the crossing, and the crossing itself then the crossing will not activate with that signal at red, even if a train has hit the strike-in treadle. In these cases once the signalman sets a route from the signal, the crossing sequence will initiate and the barriers will have to be proved down BEFORE that signal will clear. If the signal is "off" before a train strikes in then the crossing will behave in the normal manner. There are also sometimes more than one strike-in point for a crossing, for instance the stopping and non-stopping controls on Northway Crossing in the Gloucester sim. The other main way in which crossings "share" interlocking functions with the rest of the signalling system is that they share track circuits with the rest of the system - even if not all of them are indicated back to the signalman. Log in to reply |
| AHB Crossing Failures 11/05/2013 at 14:40 #44592 | |
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Firefly
521 posts |
Quote:Almost all failures have occurred with trains approaching. In this situation, would you cancel the protecting signal, causing an ACoA? As late turn and Steve have said it's a difficult judgement call and would probably depend on the circumstances. The crossing failed alarm will come up if the barriers have been working for more than 220 seconds, therefore if a train is known to be travelling very slowly you'd be expecting the alarm and would take no action. However the alarm could come up because a motorists just lost control of their car wiped out a load of barrier equipment, it could be that the wigwags aren't working because the local yobbo's just shot them with an air gun so in both cases you'd want to stop the train. I suspect most signallers would replace the signal to red in front of an approaching train if the alarm comes up straight away with no obvious explanation. If it's observed that the barriers have been working for a long time therefore the train is going slowly or a track circuit has failed then they wouldn't be inclined to cause a SPAD by replacing the signal. The problem is in this world of litigation you have to be able to justify your actions, could you imagine having to stand in court and explain why you didn't prevent the death of Mr and Mrs smith by replacing a signal to red following their car accident on the crossing. @Steve Quote: the crossing sequence will initiateYou only need to prove the crossing initiated for a set amount of time (CON SR down and HER down for XX seconds). The amount of time depends how far away from the crossing the signal is but it ranges between 5 and 29 seconds. FF Last edited: 11/05/2013 at 14:40 by Firefly Log in to reply |
| AHB Crossing Failures 11/05/2013 at 14:45 #44593 | |
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Hpotter
205 posts |
Ok, lets get this sorted then.... :whistle: Most of the cases of AHB's showing failed in SimSig is that the exit track circuit from the AHB is not broken down into a few different parts and also if the crossing has not had 'Roll back' protection disabled. Also is cause by the strike-in sequence being started, but a train not going over the AHB within 120 seconds, also the case if a train has passed the AHB, raised, but continued to be within the strike-in area (if rollback protection not disabled), will cause the AHB to resume working, thus again, not having a train going over it within 120 seconds, cause it to fail. As for the example of Greenland Mills AHB (Westbury simulation), the AHB has the strike-in started on approach to Bradford-on-Avon station, so with a train stopped at W186 for a while or doing station duties, will cause the AHB to fail. In real life, Greenland Mills AHB has stopping/non-stopping controls on it for the timing of the AHB. *Stopping control will only start when the TRTS plunger has been operated by the train crew for W186, normally taking about 50 seconds from when signal clears to train being on the crossing. Log in to reply The following users said thank you: BarryM, Jezalenko |