relationship between ballast resistance and tc reliability

[brenners]

Hi their I was wondering if anyone might be able to help me or point me in the right direction. Im working on a dissertation and as part of it looking at track circuit reliability. I was wanting to compare dc, TI21, and Reed tc's reliability measured against ballast resistance. Any help would be greatly appreciated.

Cheers

Brendan

[PJW]

For dc you can consider all the leakage current from rail to rail via imperfect sleeper insulations as being a single "ballast resistance" due to the fact that there is very little resistance of the rail.

Certainly not true for TI21; the audio frequency means that it is rail impedance that is significant and there is a most significant decline in voltage along the length of the TC and this is not linear particularly close to the two ends. The rail voltage is also noticeably higher and the track is particularly sensitive to deficiencies in insulation within the tuned areas. Hence need to look at the question rather more widely.

Another significant difference is the ratio of drop away to pick up shunts; for a dc track it depends upon the percentage release of the relay and there is inevitably quite a large hysterisis; it needs considerably more current flowing to establish a magnetic flux over the air gap between coil core face and the armature sufficient for the force of attraction to overcome the spring pressure, than it does to maintain the relay energised when the air gap has been reduced to the minimum permitted by the height of the residual pin. With a TI21 the current ismeasured electronically and thus if the drop shunt is adjusted to be 1.0 ohm one can expect the pick up value to be 1.1 ohm.

For any track circuit the important factors are:
a) the guarantee that the relay will drop in all conditions (i.e. maximum specified supply voltage, zero leakage through the ballast etc) when a train presents the specified maximum train shunt,
b) the ability for the track to re-pick reliably when the supply voltage is at the minimum of its spec and the leakage current is at its highest specified value due to the ballast resistance being as low as it can reasonably be.

Also bear in mind that a TI21 is rather more complex than dc and there are a lot more components and connections, so there is much more potential to go wrong than the fundamentally simple dc track circuit.

Hi their I was wondering if anyone might be able to help me or point me in the right direction. Im working on a dissertation and as part of it looking at track circuit reliability. I was wanting to compare dc, TI21, and Reed tc's reliability measured against ballast resistance. Any help would be greatly appreciated.

Cheers

Brendan

[brenners]

Hi PJW, Thanks alot for the reply thats exactly the kind of information I was looking for. Would you be aware of any books or online text I could I could use to find abit more reading on the subject?

[PJW]

You'll no doubt find some useful information in:
a) Chapter 7 of the IRSE large Green book: Railway Signalling ISBN 0713620676
b) IRSE little green book series no. 9 Track Circuits- Challis (think now re-published in a group of 4)

However don't get your hopes up too high as I doubt if either will give you as much as you want. I think your best chance may be trying a trawl of IRSE Proceedings to see what might exist from the 1980s or 1990s, although I cannot say I remember anything specific. Otherwise I think I'd try Dave Bradley who used to be known by the nickname Mr Ballaaarst Resistance".

Hi PJW, Thanks alot for the reply thats exactly the kind of information I was looking for. Would you be aware of any books or online text I could I could use to find abit more reading on the subject?

[brenners]

Thanks, I had alook in the green book and it had a little bit on ballast resistance that I can use on DC, not too much on AC but just read up online on impedance to try and get a better understanding. Cheers for your help