AC Track Circuit

[PJW]

I would like to ask if for the AC track circuits adjustment, we need to consider some safety margine as we do in DC track circuits?

in adjusting the DC track circuits regulating resistance, usually we consider a factor of 2.5 for the relay P.U.V. in its maximum value of ballast resistance and track power supply and a 1.25 in its minimum values of ballast resistance.

are these factors also valid for AC TCs?

Mirabadi

I am not 100% sure that I hvae understood what you are asking. I guess that you are thinking about undertaking track circuit calculations (module 5 subject rather than module 1) which occasional feature an ac track rather than a dc track.

Firstly be aware that the approximation of assuming zero volt drop along the track from feed to relay which is fundamentally true for dc becomes less so as the frequency of the ac increases- the rail resistance is low but the impedance does become significant. At 50Hz the effect is quite small, but certainly not when considering track circuits operating around 2000Hz. This means that the whole approach of treating the effect of imperfect insulations by a single "ballast resistance" in the middle of the track is not suitable.

You introduce the term "safety margin" without really explianing what you mean.
1. For a track to work safely we must ensure that the defined drop shunt draws enough current to reduce the current through the TR to be less than the "drop away" value (assuming the worst case of maximum feed voltage and infinite ballast resistance and the train axle just achieving the defined maximum permitted resistance).
2. For reliability we also need to ensure that the same TR will be able to pick up after a train leaves the section even when the feed voltage is at the minimum value and the ballast resistance as low as it could be in wet conditions.

There is certainly hysterisis- the current needed for pick up is rather more than is needed to keep the relay up and so the pick shunt will be a higher resistance value than the drop away shunt.
I am not sure where your 2.5 and 1.25 factors come from; in the exam you can use any reasonable approach providing you EXPLAIN. Be careful that you know why the "margin" is provided- do you mean SAFETY or is it for RELIABILITY for example? For example it sounds to me that you are saying that to ensure the relay is adequately energised rather than just on the edge of not being able to pick that you are regarding that the minimum relay current to ensure reliability is 1.25 times the stated PU.

So please review the above and challenge anything you don't understand; similarly if I have completely misunderstood what you really wanted to know then dont hesitate to restate your question and I'll attempt again!