(05-09-2017, 12:00 PM)mattslade222 Wrote: After obtaining the 3.9Km value, what is a sensible rational for the reliable length. I divided by 2, but that was just a guess.
Well, that depends! On one hand that would seem to be reasonable as you have given yourself a good margin against the track staying down after the passage of a train, but how have you affected the other essential operating characteristics? Nothwithstanding the discussion below on the value of ballast resistance to use, the other parts of the question ask you to calculate the drop shunt. The examiner may be asking you to do this for a reason. I have, at the moment, not gone through your arithmetic steps to check that you can use a calculator, but you appear to have followed the right logical steps to give a value. You have hit on a value of 0.6 ohm. Does that look like a good value for a TC (bearing in mind that "reliable" would include picking after a train and dropping when a train is on it). Taking this to its conclusion, having a very short TC that picks after a train but requires a DS of milliohms to drop would not be good.
Quote:When calculating the drop shunt value, I used the ballast resistance based on the 2km and 2.5 Ohm/km. In other calculations I have seen in mentioned you calculate the drop shunt assuming an infinite ballast resistance (or is this only when calculating the feed end resistance?)
Q: What would a real TC do (bearing in mind that DS is something that you actually measure on site?
A: The DS is the value given the actual conditions.
[Note, soap box time here - the units of ballast resistance are Ohm.km and not ohm/km - see this explanation.]
Quote:The general clarity of the answer for the IRSE exam, I attempted to explain each step showing working. From a third party perspective how does it look?
Matt
Explanation is always good. Your layout looks reasonable and the notes are clear.
Peter