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