General Track circuit calculations

[mattslade222]

Hi Smaddoc1 and Peter,

Whilst it is unlikely they will come up this year, I thought I would have crack at this one anyway to see if I could get my head around how to do DC track circuit calculations.

I turns out having not applied basic electronics to anything for 5+ years I appear to have forgotten the basics!! After attempting to relearn Ohm and Kirchhoff's laws, I had a crack!

Attached is my attempt at mod 5 2015 qu1 Link to file. As with your calc Smaddoc1 I got a long distance for the allowable track circuit length.
I have a few questions, if you don't mind:

1. 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.
   
2. 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?)
   
3. 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?
   

Thanks for your time,

Matt