03-10-2010, 07:45 AM
A double rail d.c. track circuit has a fixed feed end resistor connected to the rails and has the following properties:
Relay resistance: 9 ohm
Track relay pick up current: 50 mA
Ballast resistance: 2.5 ohm/km
Feed end resistance: 6 ohm
Feed voltage: 5 V
a) Draw a physical representation of the track circuit and an electrical equivalent circuit;
b) Calculate the maximum length of track circuit for reliable operation (state any assumptions that you make);
c) For this maximum length calculate the drop shunt value.
[20 marks]
If the ballast resistance subsequently changes to 1.5 ohm/km what is the new drop shunt value? [5 marks]
Am I alone in wondering why the ballast resistance give in this years question is again given the wrong units like last year? The older exams (eg 2007) specify it correctly as ohm.km and I expect it would be argued that a good student should know what to expect and know that what was given in this year's paper cannot be right. Increasing the TC length decreases the effective resistance of the ballast, not as implied by the units given that each km presents 2.5 ohm, therefore 2 km would present 2 x 2.5 ohm.
I'm all for challenging students, but on an item where most people tend to struggle on the concept, giving them a duff steer in the specification is a bit off in my view. I wonder whether it was deliberate or a bad error. Throw into the mix that, as far as I can see, when you do the calcs properly, you end up with a surprisingly long TC.
What did any of the candidates find?
Relay resistance: 9 ohm
Track relay pick up current: 50 mA
Ballast resistance: 2.5 ohm/km
Feed end resistance: 6 ohm
Feed voltage: 5 V
a) Draw a physical representation of the track circuit and an electrical equivalent circuit;
b) Calculate the maximum length of track circuit for reliable operation (state any assumptions that you make);
c) For this maximum length calculate the drop shunt value.
[20 marks]
If the ballast resistance subsequently changes to 1.5 ohm/km what is the new drop shunt value? [5 marks]
Am I alone in wondering why the ballast resistance give in this years question is again given the wrong units like last year? The older exams (eg 2007) specify it correctly as ohm.km and I expect it would be argued that a good student should know what to expect and know that what was given in this year's paper cannot be right. Increasing the TC length decreases the effective resistance of the ballast, not as implied by the units given that each km presents 2.5 ohm, therefore 2 km would present 2 x 2.5 ohm.
I'm all for challenging students, but on an item where most people tend to struggle on the concept, giving them a duff steer in the specification is a bit off in my view. I wonder whether it was deliberate or a bad error. Throw into the mix that, as far as I can see, when you do the calcs properly, you end up with a surprisingly long TC.
What did any of the candidates find?