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2009 Track Circuit Question
#11
(06-10-2009, 10:37 AM)nthomso3 Wrote: Out of curiousity...what would be the procedure if it was the case that an error has been made? Will there be any communication as such prior to the results being distributed?

I doubt if there is anything laid down but I would be staggered if there was any communication. For one thing until the results are imminently to be published a candidate number is not associated with a candidate name.

The examiners have the problem how to mark those who attempted the question but then seemed to get stuck without explaining why. I believe that it would be easy enough to tell who demonstrated competency on that style of question from what they produced. I guess the bigger difficulty would be in assessing:
a) how much time a candidate may have wasted on that question and therefore may excuse mediocre performance on another question
b) whether a candidate was actually pretty cute and avoided the question completely and perhaps chose another one instead which may not have been so far up their street. However there were quite a few othrs to choose from.

I think if I were marking my initial approach would be fairly hard line:
1. The paper was the same for all candidates
2. There are no guarantees of such a TC calculation question so it should not be relied upon and could have been avoided,
3. If the candidate really knew what they were doing and were confident then they would have detected and reported error / stated suitable assumption.
4. Part of exam technique is not to spend excessive time on any one question

Having marked the paper I may then have been feeling a little guilty and therefore would be looking for signs that the candidate really had been put off / had wasted excessive time. I would therefore be minded to give more "benefit of the doubt" if the particular candidate were close to a grade boundary. Realistically I could probably not tell if someone had avoided a favourite question, but there would be hints for those who had worked unproductively on that question.

To be frank I suspect the people that were particularly put off would be those who were always going to be marginal, not those who really understood what they were at. Anyone in exam conditions that finds the question is not going their way will no doubt be distracted by that; however in life all is not plain sailing and it is how you cope with those set backs is what counts.

Remember that has always been made clear that the marks are as much for how you tackle and explain the question and calculations- not just getting the numbers to come out.
PJW
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#12
(05-10-2009, 01:00 PM)MJB Wrote: I raised the issue with the invigilator during the exam, he came back to me and said that there was a worked solution to the question, and said he couldn't comment as he hadn't done the maths himself, which is fair enough, but does suggest a typo rather than a dirty trick.

If the feed resistance was 10 ohms (not 100 ohms), and you did assume 9 ohms for coil resistance, and that rail resistance refered to the loop resistance of the rail and that ballast resistance sits entirely in parallel with the coil (which on relflection it must do as the conductivity between rails will vary along the length of the track circuit and be dominated by the specific point along the track ciruit with the lowest resistance between rails, so the concept of a "ballast resistor" is a bit abstract, yet alone specifying a dimensioned value per KM) then the question was very simple, hence only 10 marks. (or was it 15).

Anyhow, I'm sure it should be Mod 1 I'm more worried about than Mod 5

I also tried this question assuming feed resistance as 10 ohms and 9 ohm for relay coil but I am not sure how to consider rail resistance in the track ckt calculation.
could anyone help me to figure it out?
Thanks

(05-10-2009, 01:00 PM)MJB Wrote: I raised the issue with the invigilator during the exam, he came back to me and said that there was a worked solution to the question, and said he couldn't comment as he hadn't done the maths himself, which is fair enough, but does suggest a typo rather than a dirty trick.

If the feed resistance was 10 ohms (not 100 ohms), and you did assume 9 ohms for coil resistance, and that rail resistance refered to the loop resistance of the rail and that ballast resistance sits entirely in parallel with the coil (which on relflection it must do as the conductivity between rails will vary along the length of the track circuit and be dominated by the specific point along the track ciruit with the lowest resistance between rails, so the concept of a "ballast resistor" is a bit abstract, yet alone specifying a dimensioned value per KM) then the question was very simple, hence only 10 marks. (or was it 15).

Anyhow, I'm sure it should be Mod 1 I'm more worried about than Mod 5

I also tried this question assuming feed resistance as 10 ohms and 9 ohm for relay coil but I am not sure how to consider rail resistance in the track ckt calculation.
could anyone help me to figure it out?
Thanks
Reply
#13
In the type of simple calculations asked in the IRSE exam, the whole point is that the rail series resistance must be considered negligible- indeed at dc and low frequency ac then the rail resistance is indeed so in practice (large cross sectional area of steel which is reasonably good conductor), but it is the intermediate bonding, rail jumpers and track leads that can become significant.
If the rail resistance (impedance for ac of course) is not negligible then the voltage on the rail will vary along its length and the whole simplicity of lumping all the leakage currents distributed along the entire length as if it due to a simple "ballast resistance" between those rails isn't applicable.
One is then faced with a calculation involving differential calculus which I forgot how to do more than 30 years ago and becomes an exercise in mathematics not railway signalling.

(14-09-2014, 09:37 AM)Prm Wrote: [quote='MJB' pid='1065' dateline='1254744007']
I raised the issue with the invigilator during the exam, he came back to me and said that there was a worked solution to the question, and said he couldn't comment as he hadn't done the maths himself, which is fair enough, but does suggest a typo rather than a dirty trick.

If the feed resistance was 10 ohms (not 100 ohms), and you did assume 9 ohms for coil resistance, and that rail resistance refered to the loop resistance of the rail and that ballast resistance sits entirely in parallel with the coil (which on relflection it must do as the conductivity between rails will vary along the length of the track circuit and be dominated by the specific point along the track ciruit with the lowest resistance between rails, so the concept of a "ballast resistor" is a bit abstract, yet alone specifying a dimensioned value per KM) then the question was very simple, hence only 10 marks. (or was it 15).

Anyhow, I'm sure it should be Mod 1 I'm more worried about than Mod 5

I also tried this question assuming feed resistance as 10 ohms and 9 ohm for relay coil but I am not sure how to consider rail resistance in the track ckt calculation.
could anyone help me to figure it out?
Thanks
PJW
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#14
FYI
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