Hi all,

Find attached my attempt on the track circuit calculation question from 2013. This attempt actually took me an hour- so needs refining for exam conditions. Could anyone offer some tips for streamlining my answers for a similar question otherwise I would need to avoid on the day.

Also if anyone could offer any calculations and/or description of why the relay drop current is 68% of the relay pick current. I understand that it is to do with the electromagnetic properties of the coil, but would be helpful to understand a little more, so I don't just carry on with the old "it is because it is" method of reasoning.

Please review and comment.

Question 4(2013)

Calculate the maximum track circuit length for reliable operation based on the following

parameters:

• Feed voltage 50V

• Feed end resistance 50Ohm

• Relay pick up current 0.4A

• Relay resistance 25Ohm

• Ballast resistivity 75Ohm.km

You should state any assumptions that you make and show your calculations [15 marks]

For the length that you have calculated, calculate the drop shunt value. [5 marks]

A track circuit with the same design parameters as above has had a number of failures of

showing occupied when clear. Briefly discuss the possible causes of the failure. [5 marks]

Thanks

Tommy

Find attached my attempt on the track circuit calculation question from 2013. This attempt actually took me an hour- so needs refining for exam conditions. Could anyone offer some tips for streamlining my answers for a similar question otherwise I would need to avoid on the day.

Also if anyone could offer any calculations and/or description of why the relay drop current is 68% of the relay pick current. I understand that it is to do with the electromagnetic properties of the coil, but would be helpful to understand a little more, so I don't just carry on with the old "it is because it is" method of reasoning.

Please review and comment.

Question 4(2013)

Calculate the maximum track circuit length for reliable operation based on the following

parameters:

• Feed voltage 50V

• Feed end resistance 50Ohm

• Relay pick up current 0.4A

• Relay resistance 25Ohm

• Ballast resistivity 75Ohm.km

You should state any assumptions that you make and show your calculations [15 marks]

For the length that you have calculated, calculate the drop shunt value. [5 marks]

A track circuit with the same design parameters as above has had a number of failures of

showing occupied when clear. Briefly discuss the possible causes of the failure. [5 marks]

Thanks

Tommy