General Track circuit calculations

[Peter]

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

[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

[Peter]

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.

Well, that depends! On one hand that would seem to be reasonable as you have given yourself a good margin against the track staying down after the passage of a train, but how have you affected the other essential operating characteristics? Nothwithstanding the discussion below on the value of ballast resistance to use, the other parts of the question ask you to calculate the drop shunt. The examiner may be asking you to do this for a reason. I have, at the moment, not gone through your arithmetic steps to check that you can use a calculator, but you appear to have followed the right logical steps to give a value. You have hit on a value of 0.6 ohm. Does that look like a good value for a TC (bearing in mind that "reliable" would include picking after a train and dropping when a train is on it). Taking this to its conclusion, having a very short TC that picks after a train but requires a DS of milliohms to drop would not be good.

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?)

Q: What would a real TC do (bearing in mind that DS is something that you actually measure on site?
A: The DS is the value given the actual conditions.

[Note, soap box time here - the units of ballast resistance are Ohm.km and not ohm/km - see this explanation.]

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?

Matt

Explanation is always good. Your layout looks reasonable and the notes are clear.

Peter