2010 Track Circuit Question

[PJW]

Having a TC that does not work when the ballast resistance gets worse is fine if the question is "comment on the effect of the ballast resistance changing to 1.5 ohm km" but as the question was "calculate the DS" I would expect there to be a possible answer.

Provided that you commented on the meaning of a negative number as the answer, I am sure there would be some credit, and if the arithmetic was good, you must get marks for that.

I understand that the examiners are likely to mark the question sympathetically with however the candidate interpreted the ballast resistance figure.

Obviously it should have been quoted in ohm.km but the real issue is that although we all say "ballast resistance" (as that is what it is within the equivalent circuit diagram), the propoperty of the track itself is really "resistiviity".

Hence whether the candidate used their common sense to correct the typo in the question or alternatively interpreted the words exactly literally then I believe that their answer will be assessed on that basis. Perhaps the bigger problem is the last part which is probably somewhat harder to answer if ohm/km was assumed