DC Track Circuit

[Archie]

Hello again, for the purposes dc track circuit, am I right in saying the lower the resistance, the higher the voltage and as a train cannot shunt all the voltage away from the relay,it would leave to much voltage on the relay coil meaning TR would not drop, causing a 'wrongside failure.'

If someone could just verify this then it would be greatly appreciated.

[Peter]

Hello again, for the purposes dc track circuit, am I right in saying the lower the resistance, the higher the voltage and as a train cannot shunt all the voltage away from the relay,it would leave to much voltage on the relay coil meaning TR would not drop, causing a 'wrongside failure.'

If someone could just verify this then it would be greatly appreciated.

If by "the resistance" you mean the drop shunt value, yes, what you have written is essentially correct. The current being supplied will go one of thee ways - through the relay, through the ballast leakage or through the shunt (shunt box or the train). A low value of shunt resistance indicates that you are having to "divert" a lot of the current away from the relay hence a train needs to emulate that low value to guarantee dropping the track.

Now that you have grasped that bit, what will be the effect on the drop shunt value (all other things being equal for a given setup of track circuit) of altering the value of the ballast resistance?

[Archie]

Hello again, for the purposes dc track circuit, am I right in saying the lower the resistance, the higher the voltage and as a train cannot shunt all the voltage away from the relay,it would leave to much voltage on the relay coil meaning TR would not drop, causing a 'wrongside failure.'

If someone could just verify this then it would be greatly appreciated.

If by "the resistance" you mean the drop shunt value, yes, what you have written is essentially correct. The current being supplied will go one of thee ways - through the relay, through the ballast leakage or through the shunt (shunt box or the train). A low value of shunt resistance indicates that you are having to "divert" a lot of the current away from the relay hence a train needs to emulate that low value to guarantee dropping the track.

Now that you have grasped that bit, what will be the effect on the drop shunt value (all other things being equal for a given setup of track circuit) of altering the value of the ballast resistance?

Many thanks for the quick reply, sorry I don't quite understand the question, maybe I have got it wrong, but if I increase the resistance, which will lower the voltage, and increase the drop shunt, or is this nothing to do with what you are asking me.

[Peter]

Many thanks for the quick reply, sorry I don't quite understand the question, maybe I have got it wrong, but if I increase the resistance, which will lower the voltage, and increase the drop shunt, or is this nothing to do with what you are asking me.

The latter half of your thought process is correct, but the first part is not.

It is true that if you have lower rail volts, you can have a higher value of drop shunt resistor to drop the relay. However, remember that the ballast resistance is in parallel with the rails so an increase in the ballast resistance will result in less leakage and hence a higher rail voltage meaning that to drop the relay you will need a LOWER value of drop shunt. An easy way to remember this to sense check any answer is that if you set up a track in wet conditions (low ballast resistance) and then it dries out, the track will be moving towards wrong side failure.

Your appear to be adopting a logical stepwise approach to thinking about the effects of a change, just make sure you are starting off in the right direction!

[Archie]

Many thanks for the quick reply, sorry I don't quite understand the question, maybe I have got it wrong, but if I increase the resistance, which will lower the voltage, and increase the drop shunt, or is this nothing to do with what you are asking me.

The latter half of your thought process is correct, but the first part is not.

It is true that if you have lower rail volts, you can have a higher value of drop shunt resistor to drop the relay. However, remember that the ballast resistance is in parallel with the rails so an increase in the ballast resistance will result in less leakage and hence a higher rail voltage meaning that to drop the relay you will need a LOWER value of drop shunt. An easy way to remember this to sense check any answer is that if you set up a track in wet conditions (low ballast resistance) and then it dries out, the track will be moving towards wrong side failure.

Your appear to be adopting a logical stepwise approach to thinking about the effects of a change, just make sure you are starting off in the right direction!

Ok brilliant many thanks for your help it is greatly appreciated.