As you say, a signal really does not "protect" an AHBC level crossing; the crossing fundamentally protects the road by closing it a sufficient time before a train can arrive.
The signal, when ARAFOAL, prevents the AHBC from seeing the approaching train and in that circumstance has to take over the duty of protection. It does this by imposing a delay between when the AHBC can "see" a train and when the driver is authorised to proceed, thus ensuring that a full warning period is always given. The time is calculated on the basis of
1. determining how the fastest accelerating train (typically a powerful locomotive running "light engine" without a train to pull) could reach the crossing if it had been decelerating to a stop at the red signal which at the last minute then changes to green (generally the worst case is around 100m on the approach if I recall correctly),
2. comparing this to the total warning which the crossing needs (the minimum standard road user warning, addition for skew crossing, addition for equipment operation time etc, addition for Minimum Road Open Time if applicable),
3. if 2 >1 then the delay timer is the difference between time 2 and time 1.
Hence you are correct:
a) on the AHBC Control Table there will be a reference to the signal inhibiting strike-in,
b) on the signal Control Table there will be a reference only to the delayed clearance.
The signal, when ARAFOAL, prevents the AHBC from seeing the approaching train and in that circumstance has to take over the duty of protection. It does this by imposing a delay between when the AHBC can "see" a train and when the driver is authorised to proceed, thus ensuring that a full warning period is always given. The time is calculated on the basis of
1. determining how the fastest accelerating train (typically a powerful locomotive running "light engine" without a train to pull) could reach the crossing if it had been decelerating to a stop at the red signal which at the last minute then changes to green (generally the worst case is around 100m on the approach if I recall correctly),
2. comparing this to the total warning which the crossing needs (the minimum standard road user warning, addition for skew crossing, addition for equipment operation time etc, addition for Minimum Road Open Time if applicable),
3. if 2 >1 then the delay timer is the difference between time 2 and time 1.
Hence you are correct:
a) on the AHBC Control Table there will be a reference to the signal inhibiting strike-in,
b) on the signal Control Table there will be a reference only to the delayed clearance.
(20-11-2012, 06:13 AM)jenni.joseph9 Wrote: Hi,
I have a query related to AHBC controls.
There is a protecting Signal near to AHBC and it is less than 800m from the crossing. There is a regulation on that signal clearance. Do we include any AHBC Normal conditon in the Signal Control Table other than the note related to Signal regulation.
As there will be AHBC control Table separately, I dont think that the AHBC normal condition is required in the Signal Control Table.
Please correct me if am wrong.
Thanks & Regards
PJW