(07-07-2010, 08:44 PM)Archie Wrote: Is there a maximum length for a track circuit what is the greatest length it can be?
Many thanks
Yes there is a maximum length. However what that length actually is depends on the track type, the ballast resistance, the drop shunt / pick up shunt that is acceptable, whether there is a need for immunity from ac overhead electrification etc.......
In general it is unusual for a track to be much longer than 1km and often can't be as long as that.
For a typical dc track fed fom an 867 feedset driving a 939 relay for which 25 kV ac traction immunity is required, then 650m is the maximum I think.
A single rail HVI over S&C can't be much more than 200m.
In the days before axle counters in particular, some single lines between mechanical signalboxes used a dc track which were usually de-energised and only "swept" when it was required to signal a train in one direction or the other along the single line; because these could be set up to detect trains due to the pick shunt rather than the drop shunt, then ballast resistance less of a problem and therefore they could be several miles long.
So not such a simple question as may at first appear and hence no simple numerical value.
PJW