29-05-2012, 01:48 PM
30-05-2012, 11:01 PM
This question differs from the more usual TC calculation questions so I am not quite sure what the examiners expected. How much this was due to the desire to "ring the changes" and how much it is the examiners not risking putting dodgy numbers into the question, we may never know!
There has been a pretty broad hint that any TC calc question in 2012 can be expected to be unlike the traditional ones.
I think I'd have interpreted the question a little differently from you-
1. I'd have made the initial diagram more PHYSICALLY representative. Your feed and relay ends are arguably ok (but I'd have depicted rail leads closer to the IRJs, track cable from loc links). Certainly I'd have shown the train shunt as an axle with contact resistance and the ballast resistance as being a result of a large number of sleepers holding the two rails with some imperfect insulation from rail to rail and also each rail to earth. This would have depicted more of the physical reality and a greater contrast to the electrical representation (that was how I' d have shown) and the place to record some assumptions such as negligible resistance in the track connections or along the length of plain line rail.
2. I also think the question was asking you to propose some specific values for relay resistance, operating current, percentage drop-away, ballast resistivity, feed voltage etc. This is what I think the question means by "including the electrical parameters" and certainly reinforced by wording at beginning of the 2nd part of the question.
Hence although you show ability with Ohms Law, Kirchoff Law and manipulating equations, when you get to your answer as case 1 and case 2, I am left wondering what you have actually demonstrated to me; does it really show how the relay reliably picks and drops?
Perhaps some words here would have been beneficial in supporting your claim, but I'd have made things a bit simpler and more obvious by using some typical numbers that I think was the examiners' intention.
In contrast the 3 lines at the end of page 4 were very good start for the last part of the question (although I feel that some reference should have been made to why the wet condition of the ballast affects resistance presented). You have produced an equation for the current through the relay as a function of the ballast resistance and identified the other terms as constants, but to me without giving an indication of the relative magnitudes then your answer fell short of actually "showing the effect".
Overall I just felt that you gave too much in the way of mathematical expressions (it was quite a long answer to produce in the time), but not enough signal engineering in terms of making it "real" by putting in some typical numbers.
Despite that I thought that overall it was a reasonable interpretation of the question, you certainly showed ability to do the calculations required so I think a definite Pass or even perhaps a Credit. However I guess that the examiners were actually looking for something a little different and they may think you made a meal of certain elements but skimped on the explanation and showing an intuitive understanding.
I may be biased as I have never really liked maths; it'd be interesting to hear Peter's view who may differ.
There has been a pretty broad hint that any TC calc question in 2012 can be expected to be unlike the traditional ones.
I think I'd have interpreted the question a little differently from you-
1. I'd have made the initial diagram more PHYSICALLY representative. Your feed and relay ends are arguably ok (but I'd have depicted rail leads closer to the IRJs, track cable from loc links). Certainly I'd have shown the train shunt as an axle with contact resistance and the ballast resistance as being a result of a large number of sleepers holding the two rails with some imperfect insulation from rail to rail and also each rail to earth. This would have depicted more of the physical reality and a greater contrast to the electrical representation (that was how I' d have shown) and the place to record some assumptions such as negligible resistance in the track connections or along the length of plain line rail.
2. I also think the question was asking you to propose some specific values for relay resistance, operating current, percentage drop-away, ballast resistivity, feed voltage etc. This is what I think the question means by "including the electrical parameters" and certainly reinforced by wording at beginning of the 2nd part of the question.
Hence although you show ability with Ohms Law, Kirchoff Law and manipulating equations, when you get to your answer as case 1 and case 2, I am left wondering what you have actually demonstrated to me; does it really show how the relay reliably picks and drops?
Perhaps some words here would have been beneficial in supporting your claim, but I'd have made things a bit simpler and more obvious by using some typical numbers that I think was the examiners' intention.
In contrast the 3 lines at the end of page 4 were very good start for the last part of the question (although I feel that some reference should have been made to why the wet condition of the ballast affects resistance presented). You have produced an equation for the current through the relay as a function of the ballast resistance and identified the other terms as constants, but to me without giving an indication of the relative magnitudes then your answer fell short of actually "showing the effect".
Overall I just felt that you gave too much in the way of mathematical expressions (it was quite a long answer to produce in the time), but not enough signal engineering in terms of making it "real" by putting in some typical numbers.
Despite that I thought that overall it was a reasonable interpretation of the question, you certainly showed ability to do the calculations required so I think a definite Pass or even perhaps a Credit. However I guess that the examiners were actually looking for something a little different and they may think you made a meal of certain elements but skimped on the explanation and showing an intuitive understanding.
I may be biased as I have never really liked maths; it'd be interesting to hear Peter's view who may differ.
(29-05-2012, 01:48 PM)hopkin Wrote: [ -> ]Here attach my attempt to 2011 Qn5. Any comment or suggestion.
31-05-2012, 12:37 AM
I do not think that Peter's comments are far from the mark. I certainly think that one thrust of the question is to allow you to demonstrate typical parameters for the track circuit (eg what value would you expect for Vf?)
You have undoubtedly shown that you can model the inter-relation between the parameters.
I think that what you have shown is a mathematical proof for the general abstract case rather than some calualtions to show the examples asked for. It is difficult to say how the examiner would have interpreted this - they have often said that even if you do not quite interpret the question as they intend, provided that you demonstrate signal engineering knowledge, you may get credit.
The biggest thing that I would say is a problem is the interpretation (or lack of it) on the "reliable" operation. You have shown the algebraic condition for relative values when the relay will pick and drop, but have made no real comment on margins of operation and have assumed the ideal operation of the shunt. I think that you ahve written that Rs will be 0 for drop, although you have used the term R(infinity). This is not a normal operating parameter.
Don't be disheartened - you obviously understand the maths behind it, just make sure you put down the context and the typical expected paramenters and describe the operation in some words. Maths can describe most things, but don't be afraid to use words to relate it to the real world.
Peter
You have undoubtedly shown that you can model the inter-relation between the parameters.
I think that what you have shown is a mathematical proof for the general abstract case rather than some calualtions to show the examples asked for. It is difficult to say how the examiner would have interpreted this - they have often said that even if you do not quite interpret the question as they intend, provided that you demonstrate signal engineering knowledge, you may get credit.
The biggest thing that I would say is a problem is the interpretation (or lack of it) on the "reliable" operation. You have shown the algebraic condition for relative values when the relay will pick and drop, but have made no real comment on margins of operation and have assumed the ideal operation of the shunt. I think that you ahve written that Rs will be 0 for drop, although you have used the term R(infinity). This is not a normal operating parameter.
Don't be disheartened - you obviously understand the maths behind it, just make sure you put down the context and the typical expected paramenters and describe the operation in some words. Maths can describe most things, but don't be afraid to use words to relate it to the real world.
Peter
31-05-2012, 01:43 PM
Thanks for PJW and Peter. I'll keep on my study on M5.
01-06-2012, 02:31 PM
This was one of the questions I done last year and didn't like it one bit.
In the months before the 2011 exam we were warned that a DC tc calculation question might well not come up due to the problems encountered in the few years previous. That said, I couldn't help look on it as anything other than a real 'banker' if it did come up because I found the calculation questions easier than the essay ones. Therefore I spent more time practicing this type of question than any other before the exam. When I opened the paper on the day and read the question I wasn't that surprised though, in a way was still relieved there was a track circuit question in there at all.
Remember being about 25 minutes into the question and I had to keep reading the question over and over again, just couldn't figure out what the examiner was after. Ended up making up my own track circuit for the last part, doing the calculation and then writing a small essay on why the figures turned out they way they did. Didn't feel right as i was doing it and ended up taking about 40 mins. Just had time to leave a note to the examiner telling him/her that I didn't like their question - not recommended (was the last question of a long hot day in my defence).
After the exam review in 2012 and going on what has been said in the last few years I would steer well clear of the dc track circuit questions this time around. Think this will be the year they finally get dropped and your revision time might be better spent elsewhere. If they do drop it would be nice to see another calculation type question in its place though (give the writing hand a slight break for 30mins, constant essay questions are a real killer....including extra one in Mod 3).
Not offering any advice on your answer (will leave that to those who know something) just wanted to let you/others know what I thought of this particular question.
Q5 Mod 5 - 2011....still bugging me, but I'm on the road to recovery.
In the months before the 2011 exam we were warned that a DC tc calculation question might well not come up due to the problems encountered in the few years previous. That said, I couldn't help look on it as anything other than a real 'banker' if it did come up because I found the calculation questions easier than the essay ones. Therefore I spent more time practicing this type of question than any other before the exam. When I opened the paper on the day and read the question I wasn't that surprised though, in a way was still relieved there was a track circuit question in there at all.
Remember being about 25 minutes into the question and I had to keep reading the question over and over again, just couldn't figure out what the examiner was after. Ended up making up my own track circuit for the last part, doing the calculation and then writing a small essay on why the figures turned out they way they did. Didn't feel right as i was doing it and ended up taking about 40 mins. Just had time to leave a note to the examiner telling him/her that I didn't like their question - not recommended (was the last question of a long hot day in my defence).
After the exam review in 2012 and going on what has been said in the last few years I would steer well clear of the dc track circuit questions this time around. Think this will be the year they finally get dropped and your revision time might be better spent elsewhere. If they do drop it would be nice to see another calculation type question in its place though (give the writing hand a slight break for 30mins, constant essay questions are a real killer....including extra one in Mod 3).
Not offering any advice on your answer (will leave that to those who know something) just wanted to let you/others know what I thought of this particular question.
Q5 Mod 5 - 2011....still bugging me, but I'm on the road to recovery.
01-06-2012, 09:33 PM
Despite that, did you get through M5, or was it traumatic because you have to go through it again?