2010 Track Circuit Question

[Peter]

Hi Peter et al,

Any comments on my attached attempt on the 2010 question appreciated. I fell into the trap with the Resistivity being quoted with wrong units (annoying - and I should really know better!). My final part with the chnage in resistivity ends up with an unworkable track circuit due to Ballast Current draw.

Sitting Mod5 in couple of weeks so any feedback welcome.

Thanks,

Andy M

I have not been through with a calculator, but all of your steps look perfectly reasonable by inspection and I can seen no obvious errors.

Well done for spotting your own mistake on the ballast resistance part. One of the most useful things I was taught at school was the technique of "Dimensional Analysis" which is where you take the units of the things you are manipulating and check that the sort of units you would get out of the sum would be right. What you first wrote was Length = RB / Resisitivity which dimentionally would be kilometres = ohms / ohm km which gives you a very quick check that this is wrong, wheres, after a page of writing, you conclude (correctly) that it should have been km = ohm km / ohm.

My only criticism is that you are asked to calculate the max length for reliable operation. You defined this as the point at which you will get the relay to pick. As you then found out in the last part, when the ballast resistance drops, your TC does not work. I think therefore, you would have been better advised to build in the reliability margin by stating a higher rail voltage to achieve this. You would then have had a shorter track, but one which still works when, in the last part, you had the poorer site conditions, would have still worked.

Hope you get on OK in few weeks.

Peter