2010 Track Circuit Question

[PJW]

Ah, I thought I did a mistake when I got the negative resistance in exam, so I checked for many times and finally did the calculation again, but still got negative.... Because of this, I did not have enough time to answer two long questions..... But I thought although it was a mistake of examiners, it showed that I did not have enough basic knowledge for this. If I was good enough,I should see the error at first glance~
Mod 5 must fail. I will take it next year

Your mistake in retrospect was not being sufficiently exam focussed; no criticism re not recognising that there could be an examiner error earlier. but yes you should have been ruthless about the time spent and ensured that you left yourself enough time for the other two questions. Then if you had given in 2 good answers and a partially complete TC calc that was partially the examiners fault I am sure you would heve passed; because you allowed yourself to get distracted into trying to resolve the calculations its sounds as if you were unable to do adequately in the other questions.
Good that you now recognise that and I am sure you will have learned from the experience and in future years will be more focussed on time management

[AndyM]

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

[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

[AndyM]

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

[AndyM]

Thanks for the feedback Peter,

Would 10% be reasonable additional track voltage / relay pick current to factor in?

Andy

[PJW]

Thanks for the feedback Peter,

Would 10% be reasonable additional track voltage / relay pick current to factor in?

Andy

I think the fact there is a number is more important than that which it is.  Given that one can expect power supply to fluctuate by that sort of amount, I think that I might have gone a little higher at say 15% but really of little consequence for demonstrating understanding of the concept

[dorothy.pipet]

FYI