28-01-2013, 03:43 PM
Some allownce for contingency is usually sensible unless it is evident that the specified headway is very much less than would ever be needed to make some allowance for perturbation in a timetable.
The more the train service approaches that of a metro to be used to maximum capacity for an extended period of time, the greater the allowance which should be made; where there are a few spasmodic trains within the hour then no significant margin between the "timetabled need" and the "signalling design" is needed- even so it is sensible to hae a small allowance to give a small margin. f you were signalling contractor and designed signalling precisely to deliver say 180second headway, yet because of slight rounding errors and slightly false assumptions re how trains are driven etc it only delivers 182secs, then the railway company could state you had failed to meet an essential criteria and to modify would be extremely expensive and take a lot of time and effort, If only the design had been for 175sec, then if it turned out that it really only delivered 178sec then still within the threshold.
The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Undderstand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the absolute value in any particular circumstance!
The more the train service approaches that of a metro to be used to maximum capacity for an extended period of time, the greater the allowance which should be made; where there are a few spasmodic trains within the hour then no significant margin between the "timetabled need" and the "signalling design" is needed- even so it is sensible to hae a small allowance to give a small margin. f you were signalling contractor and designed signalling precisely to deliver say 180second headway, yet because of slight rounding errors and slightly false assumptions re how trains are driven etc it only delivers 182secs, then the railway company could state you had failed to meet an essential criteria and to modify would be extremely expensive and take a lot of time and effort, If only the design had been for 175sec, then if it turned out that it really only delivered 178sec then still within the threshold.
The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Undderstand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the absolute value in any particular circumstance!
(28-01-2013, 11:33 AM)kiran218 Wrote: Hi NJK,
10% of contigency is required for headway time. Under what conditions we have to fallow this?
With Regards,
KiranKumar
(27-08-2012, 05:26 AM)NJK Wrote: Dear Members,
I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.
Thanks in advance.
Regards,
NJK
PJW