General Track circuit calculations

[Smaddoc1]

Guys,

I am  attempting 2016 paper Q10 regarding TC calculations and it is driving me round the bend  (it is the first time I have attempted this type of question). I have tried to use the thread and associated spreadsheets to work my way through the a) and b) sections but I am struggling with calculating the drop shunt values for c) and d). Could anyone please point me in the direction of a simple formula I can follow to help me answer it? I also think I may have done something wrong on the first sections as well as I have ended up with 3.9km as an answer for maximum length.

b)

·        V relay PU = Relay resistance (9) x relay pickup current (0.05) = 0.45v
 
·        Assume voltage on the rail to be same as v relay PU = 0.45v
 
·        V Feed – V Rail = 5-0.45 = 4.55v
 
·        I Feed = (V Feed – V Rail 4.55v)/R Feed (6 ohms) = 0.758A
 
·        I R Bal (current lost in ballast) = I Feed (0.758A)-I Relay (0.05)=0.708A
 
·        R Bal (at each meter) = V Rail (0.45)/ IR Bal (0.708) = 0.636 ohms
 
·        Ballast resistance = R Bal x length or Length  = Ballast resistance(2.5)/R Bal (0.636) = 3.93km


Any help would be appreciated!

Thanks in advance

Stu

Been a bit busy working nights recently, so will add this to the pile if not responded to by others. 
At first sight, calcs don't look unreasonable.

However for the 2017 exam, the chances of a TC calculation being included in the exam is virtually zero given the examiner's comments at last year's exam review, so probably not a priority

Thanks Peter, yes I do remember this being said at the IMechE but wasn't sure how definitive a decision it was. In that case as you say it is not a priority and I will concentrate my efforts elsewhere.

Cheers