02-09-2017, 11:20 AM

(25-08-2017, 09:42 PM)Smaddoc1 Wrote: 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

There is nothing wrong with your logic or arithmetic. The only thing to point out is that the value you have worked out is the max length at which the TC will just pick following the passage of a train. Strictly, the question asks for you to work out the max length for reliable operation so you would have to add in whatever margin you judge to be needed for reliability.

Having worked through the first part as you have, apply a margin, have a look at what value you get for the DS and take a view on whether you would consider that to be a reliable TC (for both having a reasonable shunt and picking after the train).

Peter