2002 Track Circuit Calculation

[PJW]

Hi Peter,
Please see the attached attempted Track Cirucit calculation question for 2002.

For Part B, the comments are below:-

Wet weather has caused the increase in the ballast resistance from 5 Ohms to 40 Ohms, therefore, the current through the ballast resistance has reduced from 52mA to 6.5mA. The current through the relay resistance and feed resistance is the same, hence the additional current flows through the drop shunt resistance (i.e. the train wheels), hence the new drop shunt resistance value is lower than the original. The prsence of the train causes the track relay to de-energise and drop away its current.

Please provide your feedback.

Thanks
Regards
Aditi

Hukk, hkkrrr. I think you meant that DRY weather has increased the ballast resistance.
I am not sure what drop shunt you use on LU, but on NR the minimum is 0.5 ohm (well actually there are special cases on dc 3rd rail lines with impedance bonds that we accept lower because it isn't practicable to do otherwise and risks are mitigated by various factors- but that is another story). So I think what the examiners were expecting is for the candidate to point out that this track circuit, if set up in the poor ballast conditions initially could later fail wrongside if the ballast significantly improves. Let's hope that the trains that come along do indeed shunt away enough current actually to drop out the track relay or else we will have a track SCWO= "Shewing Clear When Occupied" which certainly isn't good.

Otherwise it looks a very good attempt to me; hope you feel happier about these calculations now. I expect that there's a very good chance that there will be a broadly similar question in 2010- and I suspect that the examiners will be double-checking that they do their own sums right this time!
I think it is likely that any TC calc question will have some codicil such as this one, which makes sure that the candidate really does appreciate the significance of what they are doing, as well as just turning the handle and getting the numbers to come out..