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Service braking distance & Sighting allowance - mangeshwakankar - 14-11-2008
HI,
Can any body help me in sending the formulae for Service braking distance and the sighting allowance and also the overlap. Most of the times overlap is seems to be 200m so , is it standard.
the follow information is given .....
Speed 145kmph
Headway-90s
Length of train-250m
RE: Service braking distance & Sighting allowance - Peter - 14-11-2008
mangeshwakankar Wrote:HI,
Can any body help me in sending the formulae for Service braking distance and the sighting allowance and also the overlap. Most of the times overlap is seems to be 200m so , is it standard.
the follow information is given .....
Speed 145kmph
Headway-90s
Length of train-250m
In the UK, there are standard braking tables for different categories of rolling stock type. These are given in GK/RT0034. However, in the exam, you are usually told a braking rate quoted as a fraction of g (the acceleration due to gravity) and then expected to work out the braking distance from there. This requires some basic school maths and there is an attachement in this post which will give you an explanation of this calculation if you are not familiar with them. Pay particular attention to the units (ie metres, not km and seconds not hours).
Sighting distance is dependent on the practice you are signalling it to and the era. The UK used to use the position of the AWS (normally 200 yards) but since this is a variable feast now, a more normal measure is a given time which is then translated into a distance according to the line speed (eg 10 seconds at a line speed of 100kph). The key is to state what you are using and stick to it.
Overlaps again used to be fairly fixed, but subject to risk assessment can be reduced. Other railways calculate lengths of overlap. Find out what is the practice for the railway principles you are going to signal the layout to.
So in summary
O/L and sighting - no formula, find out your principles
Braking - may be tables, but in exam you'll need to calculate
That gives you some of the informaiton that you need to put into the headway calcualtion formuale. If you are not familiar with these, have a look at Doug's post which has a calculation attachment.
Hope this helps. Let us know how you get on.
RE: Service braking distance & Sighting allowance - PJW - 14-11-2008
There is also quite a bit in the Study Pack including worked examples for several Past Papers; you ought to get it issued to you when you apply to sit the exam.
I posted a draft version of the headway / braking portion on this Forum much earlier in the year: http://www.irseexam.co.uk/attachment.php?aid=1
Also a word of caution that "Sighting Disance" does actually mean something quite a bit different for Metros using 2-aspect Red/Green signalling (rather than multiple aspect signalling in which there is aspect sequence giving the driver prior notice of the need to stop). Since this means that the driver must be able (from whatever is the approach speed) to stop the train within the distance for which it is visible, there are fomulae to determine what this should be in various circumstances.
RE: Service braking distance & Sighting allowance - mangeshwakankar - 15-11-2008
Thanks for the information it is very useful.
PJW Wrote:There is also quite a bit in the Study Pack including worked examples for several Past Papers; you ought to get it issued to you when you apply to sit the exam.
I posted a draft version of the headway / braking portion on this Forum much earlier in the year: http://www.irseexam.co.uk/attachment.php?aid=1