05-10-2009, 01:00 PM
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
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