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Did anyone else attempt thetrak circuit question this year and discover that the track relay could never pick as:
Vsupply = 50V
Rfeed = 100 Ohm
Ipickup = 500mA,
Given Vsupply and Rfeed, the maximum circuit current could only ever be 500mA, meaning the relay could never pick with ANY additional resistance in the circuit.
I think there must have been a typo, given that the question was asking for the calculation of ballast resistance, did anyone else get stuck doing this question?
I was too far in to attempt a different question, so had to make the most of it. If it was a typo, what is the process now?
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Job Role: System Architect
04-10-2009, 06:52 PM
(This post was last modified: 06-10-2009, 10:40 AM by Peter.)
I have not had the chance to look at the mod 5 papers yet as no on took it at our centre so the "spare" papers that the exam office send to us are still in their sealed envelope.
If your statement about the numbers is correct, you are right that with the given Rfeed, you are not going to get enough to pick the relay with anything else affecting the circuit.
I suspect the view of the examiners will be that 100ohm is fairly obviously a problematic value given the other numbers, and hence would expect candidates bursting with TC knowledge to make some comment.
On the positive note, the examiners look at the cases that are borderline and then start taking other factors into account. I suspect that the term borderline may be a bit more liberal if there is a mistake that may have deprived people of marks. So, if you were borderline credit / distinction with only the few makes that you were genuinely able to get from this question and lots from the other two, the view could well be "excellent candidate, the only thing stopping them was the typo in that question, slot into the higher grade".
I'll try to look at the paper tomorrow and give you some other comments on the numbers.
Peter
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The whole question looked funny to me, I skipped it! I had prepared for AC and DC questions - banking on the TC question!
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The question also gave a rail resistance, and once this is considered the effect of ballast resistence is non-trivial (see attached diagram) as current is limited in the series circuit by the rail resistance (simple to calculate), but at each and every point along the rail, some current will "leak" out of the series circuit due to ballast resistance, giving the arrangement in my diagram.
I'm sure this arrangement does have a relativley simple equivalent circuit (like the cube of resistors problem), but I wasn't about to try and derive this in the 15 minutes the marks alotted!
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I looked at this question too and actually attempted it thinking that I would somehow manage to work it out.
The questions gives you -
Feed Voltage - 50V
Feed Resistance - 100ohm
Rail Resistance - 33ohm/km
Shunt Resistance - 12ohm
Pick Up Current - 500mA
Track Circuit Length - 2400m
I couldn't see how to calculate the Rail-to-Rail voltage over the relay coil without being given the coil resistance. I attempted to assume it was 9ohm but soon realised this was unworkable.
The couple of people I have asked about this have asked if I was given the realy resistance and seemed surprised when I said no.
I was also curious as to the wording of the question as to find the ballast resistance in ohms per km as usually the units for ballast resistance is ohm km which isn't exactly the same thing.
I really want to know how this is done so if anyone has any ideas...
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I have also attempted this question and came across similar concerns as others have mentioned in the thread.
I also went on assuming relay resistance as 9 ohms and came with feed end current as almost half of pick up current and left the question unanswered.
I wasn't sure whether it was possible to solve this question.
I also noticed that the question paper for this year is totally different to the pattern adopted in last five years. I was banking on few topics as power supply, fault trees, level crossing operation and track circuit. But most of the questions were asked for identifying hazards for particular system and I was quite disappointed during the first 10 min of period and could not do better.
I am not sure when one should expect the results, I was told by my invigilator that would be around Mar/Apr next year.
I now think that I can prepare better next time.
Regards,
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Results usually come out the week before Christmas mate.
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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
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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?
