General Track circuit calculations

[PJW]

Hi,

I am going through the calculations and got few doubts. Could you please clarify them.

1. The drop shunt value when the TC length of 1 km is around 2.9 Ohm.
I read that the typical drop shunt value is around 0.5 to 1 ohm where as the value here is a bit different. Is that because of the Track circuit length?
Please clarify.

2.The relay current when no train present as per the calculations is 332 mA.
Yes, this current is fair enough for the relay to pick up.
The calculation seems correct but how to find the exact value as you mentioned that it is not correct.

Please help.

Thanks in advance.

Regards,

JJ.

Yes, a drop shunt would normally be arranged to be around 1 ohm in average ballast conditions.
If the drop shunt ever becomes as low as 0.5 ohm, there is a chance that the track may not be dropped by a train (wrongside failure).
Conversely if set too high then the track would suffer rightside failure due to deteriorating ballast conditions. This is where the length of the track becomes important- the longer it is, the greater the leakage current which must be expected in such situations.

The actual drop shunt of a track circuit will depend upon the relay characteristics, the feed voltage and feed resistance and the state of the ballast resistance at that time. Generally the "on site variable" is the feed resistance and thus the trick is to set this to make sure that the track will be dropped by the specified minimum drop shunt when ballast resistance is infinite, yet still pick up reliably when the ballast resistance is as bad (low) as one can reasonably expect.

So when you say the drop shunt is 2.9ohm, the important thing is under what ballast conditions.

As Peter pointed out there was an error in the workings that were posted here; you need to be clear what values stay the same and which change between the different scenarios. It is worth re-drawing the new equivalent circuit to avoid making rash assumptions.......


Also look at this attempt