BedfordBoy Wrote:Looking back at some past papers, I see that in 2000 there was a question on ac tracks rather than dc. What do we think the chances are of getting that again? I'm pretty much counting on doing well on the track calculation to get me a good way towards that magic 50% pass mark.
I say that if we get a question on ac tracks, we all walk out of the exam room. Who's with me, comrades?
There are ac tracks and ac tracks. If they are 50Hz then it is still reasonable to assume negligible volt drop along the rails and thus the treatment of all the various leakage currents as a single ballast current passing through a nominal "ballast resistance" is a valid approximation.
If you have to worry about the phase angle re ac vane track relays then you'd probably be right to avoid the question and choose another. Similarly once frequencies are in the main audio range 300-3000Hz then rail impedance cannot be ignored. Voltage does not even drop linearly along the length of the track so the simplification of "ballast resistance" breaks down and it all gets horribly complex so avoid like the plague.
No value in walking out of the exam room though as that'd be an almost definite fail. Yes do concentrate preparation on your favourite questions, but don't absolutely rely on them- they may not come up at all or may come up but tied in with something that you want to avoid. There are 10 questions of which you have to do 3; in reality you must expect that a couple of them may be completely impossible for you (e.g. a level crossing question is most unlikely to suit anyone from a Metro environment; similarly platform screen doors are likely to be unknown to Mainline candidates)- that should still leave a good choice for everyone to be able to choose the ones that they feel they can do best. So even if expecting to do the track circuit calculation do consider both it and the other options.
You have 10 minutes reading time to determine which to attempt; use it wisely. I suggest a skim read of the whole paper- there will be some questions that you might write-off after some 15 secs, but others may seem more hopeful and you may need the full minute to read and consider. With luck that leaves you 3 minutes to re-read each of those you identified as the most likely so that you can assure yourself that you really did understand what was asked, can tackle all of the question (bearing in mind the mark distribution within the question) and are interpreting it wide enough to really give a good answer to the actual quastion asked rather than just thinking that its topic is an area that you know a fair bit about.
There are certainly many that bank upon the track calcs- particularly those whose first language is not English. Conversely others just don't want to do the maths and thus would never attempt.
PJW