15-04-2016, 07:45 PM
I have been asked the following question:
If you assume that the feed voltage is fixed, think about what bits of the equivalent circuit work together with the shunt resistance. The obvious element is the ballast resistance so if you think of that as a simple resistance in parallel with the drop shunt, to keep the total resistance of this pair constant, it is obvious that to have a lower value of drop shunt, you would need a higher value of ballast resistance. Hence you get the fact that when ballast resistance rises, the drop shunt falls potentially leading to the track circuit tending to wrong side failure when one set up in wet conditions starts to dry out.
Hope that helps you to understand the relationship a bit better. Try playing with the values in the sheet posted in the General Track Circuit Calculations thread.
Peter
Quote:You seem like the person to ask, what would cause the drop shunt to lower? i understand that lowering the voltage will increase the shunt and conditons of the track circuit equipment will effect the track, causing it to drop.You have an idea of how the drop shunt varies with the voltage being lowered, so the converse is true - if you up the feed voltage, you end up putting more energy into the TC and hence a shunt that allows more current to be passed will be needed to divert the energy from the track relay.
If you assume that the feed voltage is fixed, think about what bits of the equivalent circuit work together with the shunt resistance. The obvious element is the ballast resistance so if you think of that as a simple resistance in parallel with the drop shunt, to keep the total resistance of this pair constant, it is obvious that to have a lower value of drop shunt, you would need a higher value of ballast resistance. Hence you get the fact that when ballast resistance rises, the drop shunt falls potentially leading to the track circuit tending to wrong side failure when one set up in wet conditions starts to dry out.
Hope that helps you to understand the relationship a bit better. Try playing with the values in the sheet posted in the General Track Circuit Calculations thread.
Peter