AHB Level Crossing minimum times?

[OxtedL]

Without wishing to look like a complete idiot, I didn't think that AHB crossings had protecting signals. Indeed, I thought that was the point.

 

[Old Timer]

Firstly the LC is monitored in a Controlling signalbox. The term Controlling only specifies the signal box which is responsible for monitoring the crossing and taking the necessary action to protect the crossing.

There are no dedicated protecting signals because the whole point is that the crossing is automatic.

The signalman will be warned if there is a power failure to the crossing or if the barriers are down for an extended period. Obviously there are emergency as well as roadside telephones at the crossing as well.

Track circuits and treadles have a time release which is based upon the longest occupation time, following which the equipment will auto-raise the barriers even though the crossing cycle has not been completed. This is to get around the old problem of people operating the strike-in treadle, and any failure that may occur to the track circuit.

The power supply is backed up by a battery - It used to be 10 hours or thereabouts but I am sure that has altered now.

The nature of the controls are such that a complete failure (power/battery) will cause the barriers to drop, as they are held off by hydraulic rams. When a barrier motor is close to failing for example you will hear the motor start as the barrier arm droops and then shut down as the arm is driven back upright.

If there is a complete power failure, then the hydraulic system loses pressure, and the barrier arms drop. This causes the red lights to flash, and they will continue to flash until the battery finally gives out. Generally what happens however is that the barriers fail to rise after a train because there is insufficient power to run the motors. There will still be power however to illuminate the lights for a period.

There are supposed to be regular inspections of the barriers, for example when I was involved with them we used to visit and time at least twice per week, and we also used to pass by everytime we were on the route just to make sure that all was in order.

If I remember rightly we also had to do a formal recorded 3-monthly inspection which included observing trains passing in each direction.

Generally AHBs are pretty reliable. In my time it was generally people messing with the treadles that was the problem.

Because of the presence of the treadles, backed up by track circuits, there is little likelihood of a train failing to start the lowering sequence, although one can never say hand on heart that a wrong-side failure will not occur, but I was never involved with one, and my area was infested with worked and automatic crossings of every type known to man.

 

[Tomnick]

Track circuits and treadles have a time release which is based upon the longest occupation time, following which the equipment will auto-raise the barriers even though the crossing cycle has not been completed. This is to get around the old problem of people operating the strike-in treadle, and any failure that may occur to the track circuit.

If you'd be able to explain that further, I'd be grateful! I'd always understood that the barriers would remain down in that case, as opposed to a locally monitored crossing where the white light would cease flashing after a certain period, and shortly after the barriers would raise and the road traffic signal extinguish. In the case of an AHB where a treadle or TC on the approach has failed (without a train being present), what would stop the cycle timing out and the barriers raising in front of an approaching train? I appreciate that the failure alarm would sound in such a case, but with a train already in a long-ish section, that might not be much help.

For what it's worth, my direct experience of AHBs is limited to encountering them as a motorist on numerous occasions (do you speak of the same Lincolnshire level crossings?) and occasionally observing the indications in various supervising signal boxes, so any further enlightenment would be much appreciated!

 

[GB]

It is my experience on the GEML region that any track circuit that shows occupied within the crossing controls will keep the barriers down untill such time the fault is fixed or the barriers are taken on local control.

 

[Old Timer]

If you'd be able to explain that further, I'd be grateful! I'd always understood that the barriers would remain down in that case, as opposed to a locally monitored crossing where the white light would cease flashing after a certain period, and shortly after the barriers would raise and the road traffic signal extinguish. In the case of an AHB where a treadle or TC on the approach has failed (without a train being present), what would stop the cycle timing out and the barriers raising in front of an approaching train? I appreciate that the failure alarm would sound in such a case, but with a train already in a long-ish section, that might not be much help.

Each Controlling (maybe Supervising would be a better term) signalbox has as I remember six failure/fault indications
(a) Power failure ;
(b) Barriers down for longer than normal - normally a max of 2 mins ;
(c) A failure of both the red light bulbs in one of the wigwag units ;
(d) Occupation of a controlling track circuit for longer than normal ;
(e) The Bi-Directional controls failing to reset, and ;
(f) A local control unit door being opened.

As regards level crossing failure and reset modes, the barriers are required to fall in the event of a power supply failure.

The barriers will remain lowered if :
(a) There is a total red light failure in any of the wig wag units ;
(b) Any barrier is not detected as having lowered ;

The crossing controls are permitted to time out and reset if :-
(a) there has been an intermittent track circuit failure, or a treadle activation :
(b) there has been a power suppply failure that interrupted the track circuits/treadles ;
(c) the strike-out controls have failed to operate ;
(d) there is an intermittent track circuit or tredale failure at the overrun treadles if these are provided.

What I should have made clear but didn't through bad wording is that to reset, the track circuit has to be clear, although the treadle may be disarranged. You can therefore get crossings cycling through the sequence in the worst case.

Operationally the supervising signalbox has Special Instructions relating to the level crossing which tell him what requires to be done. In a nutshell however these require him to caution trains and send for a local attendant.

I seem to recall we had a couple of level crossings where the auto raise was modified to raise with a track circuit failure because of the high incidence of vandalism. I seem to recall a figure of either four or ten minutes - which covered the likelihood of a train proceding towards the crossing. Obviously the LC would be in a failed state at this stage anyway.

Again as I recall trains were allowed to proceed provided that the red lights were alight at the time of passing over the crossing because the red lights are the statutory stop signal not the barriers.

There was also a requirement in the Rule Book for any slow moving train, or a train that had stopped within the barrier contro,s and then restarted, to check the lights were still operating before passing over, as the barriers could have risen in that period.

A quick check of the latest Standard on the subject does not appear to cover this anymore so I guess it must have been removed or exists elsewhere.

 

[boing_uk]

I must admit, I was under the impression that the audible signal sounded on half barriers all the while that the barriers are down as has been my usual experience. But while looking actually for videos of LED traffic signals, I came across this video where they silence after the barriers come down.

Is there a standard or is it a site by site assessment?

(PS on the first video, would you beleive that the track heading toward Barnetby has been down for ooh 20 years or so - the track on the right towards Grimsby has been down maybe 7 years... look which one is the more level - amplified of course by the zoom on the camera, but it is noticeable to the eye as well on that crossing)

 

[Tomnick]

Thanks chap, all very interesting. I only recall seeing 'raised', 'working' and 'failed' (or similar) indications relating to AHBs, with no indication of the specific fault - is/was this common?

 

[Flamingo]

I saw a pager message a few weeks ago along the lines of a car being stuck on a level crossing somewhere in deepest Cornwall. Idiot driver had jumped one light, struck the barrier and was now stuck on crossing. The Signaller was awaiting police attendance before raising barrier to release the car!

 

[eMeS]

... I came across this video where they silence after the barriers come down.
...

What a short delay! (assuming that the video hasn't been edited.)
The full width automatic LC barrier at Bow Brickhill on the Bedford-Bletchley line has a 2 min 20 secs. timing from barriers fully closed to front of train arriving at the crossing.

 

[plymothian]

I must admit, I was under the impression that the audible signal sounded on half barriers all the while that the barriers are down as has been my usual experience. But while looking actually for videos of LED traffic signals, I came across this video where they silence after the barriers come down.

Is there a standard or is it a site by site assessment?

(PS on the first video, would you beleive that the track heading toward Barnetby has been down for ooh 20 years or so - the track on the right towards Grimsby has been down maybe 7 years... look which one is the more level - amplified of course by the zoom on the camera, but it is noticeable to the eye as well on that crossing)

I would guess that some silence near residential areas to stop the locals being disturbed at all hours. It does tend to be rural LCs that the klaxon can be heard when you pass over.

 

[Tomnick]

What a short delay! (assuming that the video hasn't been edited.)
The full width automatic LC barrier at Bow Brickhill on the Bedford-Bletchley line has a 2 min 20 secs. timing from barriers fully closed to front of train arriving at the crossing.

Bow Brickhill is a manually controlled (CCTV) crossing (supervised from Marston Vale SCC??), not automatic - as such, the crossing needs to be proved clear before the protecting signal can clear. To avoid delaying the approaching train, that needs to be done before the train reaches the first adverse signal aspect in rear of the crossing - hence the much longer delay.

 

[boing_uk]

Ahh if only MKDC had built the V10 all the way to Kellys Kitchen; you can see the propoded route it would have taken, to the right of the buildings by the LC.