2008 Question 9

[Peter]

This year's TC question read:

A double rail d.c. track circuit has a fixed feed end resistor and a track circuit relay at the relay end. The parameters are:
Feed voltage:5v
Feed resistance: 12ohm
Relay resistance: 9ohm
Relay pick up current: 40mA
Minimum ballast resistance: 2.5ohm km
Neglecting tail cable resistance, calculate the maximum length of the track circuit for reliable operation. Explain any assumptions you have made.
For the maximum length track circuit you have calculated, calculate the minimum drop shunt of the track circuit.

For a detailed work through of the numbers for the first part, use this sheet (note there is a typo in the diagram which gives the drop shunt voltage as "relay voltage", but that discrepancy is irrelevant in this case).
If you want to see the drop shunt calculation, then view [attachment=177].

These sheets assume virtually ideal conditions such as no loss in the rails; ideal voltage source; no loss at the rail connections; distributed ballast resistance; that "reliable" operation means the limit at which the track will just pick after a train leaves it (in reality, you would want a margin above this).

I've included the hand written basis of the original calculation for information of a quick layout for the answers. Note that there is a bit circled in red on the first page followed by lots of crossings out. When I checked through it, I realised that in the hurry of doing it against the clock, I had written a 7 badly and taken it as a 1 when I put it in the calculator - only a of minor arithmetic significance so the answer was not orders of magnitude wrong and so slipped by, but it does illustrate the need to write clearly so not only can the examiner read it, you can read it too. [attachment=178]

One final point I would like candidates feedback on is the wording of the last part of the question. I think the answer I have given here is what the examiner wanted, but not what the wording (in my opinion) implies. I have calculated the value above which the track circuit will not shunt. The question asks for the minimum value of drop shunt. Surely the simplistic answer to this is 0 ohms since the tc described will drop perfectly well with a short circuit applied. Did anyone struggle with the wording or what did anyone else calculate?

Peter

[MarkN]

Peter,
I am smiling because I got the same drop shunt value, But I DEFINITELY missed the English nuance at the end. interesting question. I hope they were looking for the value i supplied....
Mark

[Peter]

How did you interpret the question then?

I only ask because I asked about the exact same thing at the exam review last year and was really surprised to see the same wording this year. Perhaps it is me missing something, but what else could they mean?

[MarkN]

How did you interpret the question then?

I only ask because I asked about the exact same thing at the exam review last year and was really surprised to see the same wording this year. Perhaps it is me missing something, but what else could they mean?

Peter,
I interpreted it as the highest value of drop shunt to make the relay drop at that distance. Illustrating nicely the danger of revision conditioning you to expect a certain style of question. oops.
I know my english interpretation was wrong, I'm hoping theirs was too.

Mark

[Peter]

In that case, I think you gave the right answer. The answer of zero ohms (the minimum possible) is not worthy of several marks for a second part of the question. Given that deduction, the only meaningful thing left for them to be asking for is, as you say, the highest value of shunt that will operate the relay, ie what is the worst value of train shunt that will occupy the track.

Perhaps they deliberatly word it like that to see that you are trying to give sensible though to what they are asking, but I will ask the question again at the exam review.

[Hort]

Hello everyone, this is my first post on the forum!

PJW,
I attempted question 9 of Module 5 2008 without any prior revision in an attempt to gauge how difficulty the exams are. I compared my answers to yours and I realised that where I stumbled was not knowing that the ratio of release current to operating current for a track relay is 68%, I later found this in BR938. Could you please tell me how many marks do you think I would have lost if I stated in my answer that I assumed a certain ratio (not 68%) and used the same methodology to calculate my answer? I would just like an idea of how much weight, in terms of marks, is given in the exams to remembering such details.

Regards
Gethin

[PJW]

Hello everyone, this is my first post on the forum!

PJW,
I attempted question 9 of Module 5 2008 without any prior revision in an attempt to gauge how difficulty the exams are. I compared my answers to yours and I realised that where I stumbled was not knowing that the ratio of release current to operating current for a track relay is 68%, I later found this in BR938. Could you please tell me how many marks do you think I would have lost if I stated in my answer that I assumed a certain ratio (not 68%) and used the same methodology to calculate my answer? I would just like an idea of how much weight, in terms of marks, is given in the exams to remembering such details.

Regards
Gethin

My guess noe at all for not quoting that value; doesn't have to be any particular relay. Yoy would have to quote something reasonable; i.e. recognise that there would be a significant difference between DA and PU currents. IRSE are not interested at all in the detailed recall of values as the exam is not about MEMORISING FACTS. It is very keen that people have INHERENT UNDERSTANDING so "order of magnitude / indicative values" are absolutely fine.

[shivi]

For the second part of the question, which is for calculating Minimum Drop Shunt Value, why should we consider Minimum Ballast Resistance?
I think if we consider minimum ballast resistance, then there is chance that under DRY conditions the train may not shunt the track circuit because of more current passing through relay.

I was always thinking that we should consider Maximum ballast resistance (Infinity if not given) for calculating Drop Shunt Value.

Can anybody clarify this doubt?

Shivani Singh.

[Peter]

For the second part of the question, which is for calculating Minimum Drop Shunt Value, why should we consider Minimum Ballast Resistance?
I think if we consider minimum ballast resistance, then there is chance that under DRY conditions the train may not shunt the track circuit because of more current passing through relay.

I was always thinking that we should consider Maximum ballast resistance (Infinity if not given) for calculating Drop Shunt Value.

Can anybody clarify this doubt?

Shivani Singh.

Your point is a very interesting one and adds to the discussion about what is being asked for when the term is used "calculate the minimum drop shunt". Taking your interpretation, if you use "good" ballast conditions (ie infinity) then you will come up with a value of DS which is lower than the one shown in the example answer, hence coming up with the value of drop shunt that is the lowest value that MAY be neeeded to drop the track in all the foreseeable conditions.

Perhaps there are other comments that people have.

Peter

[wo345743452]

I''ll see it carefully before starting my calculation