08-04-2008, 06:11 PM
Yes- this is a very common error.
Think of it like this: although we call it ballast resistance in actual fact a lot of the current can run on the surface / through the sleepers (especially if thinking wet timber sleepers into which are screwed the chairs holding bullhead rail in place). Think of each of these as being say a 1kohm resistor; these are spaced approx every metre along the length of the track circuit and thus it will become blindingly obvious that the greater the length of track circuit the LOWER the reistance because there are more parallel paths available for the sneak current. Keep this picture in your mind and you should never make the mistake again!
Peter's point about impedance is very relevant. At dc (and to a very fair approximation at 50Hz ac) the rail resistance can be ignored, thus there is no volt drop along the length of the TC hence a effect of the lost current can be approximated by a theoretical equivalent ballast resistance across the rails. At higher frequency (even Reed frequencies of a few hundred Hertz) this is not true and at Aster / TI21 frequencies (circra 2000Hz) the voltage drops off very markedly along a track circuit (not linearly either- falls sharply when leaving the transmitter but then the curve flattens out). Whole concept of ballast resistance is far too simplistic to be useful; one useful by product though is that it can be easier to find the short circuit on a failed track circuit by plotting the rate of decline in voltage when walking every 10m along the track.
PJW
Think of it like this: although we call it ballast resistance in actual fact a lot of the current can run on the surface / through the sleepers (especially if thinking wet timber sleepers into which are screwed the chairs holding bullhead rail in place). Think of each of these as being say a 1kohm resistor; these are spaced approx every metre along the length of the track circuit and thus it will become blindingly obvious that the greater the length of track circuit the LOWER the reistance because there are more parallel paths available for the sneak current. Keep this picture in your mind and you should never make the mistake again!
Peter's point about impedance is very relevant. At dc (and to a very fair approximation at 50Hz ac) the rail resistance can be ignored, thus there is no volt drop along the length of the TC hence a effect of the lost current can be approximated by a theoretical equivalent ballast resistance across the rails. At higher frequency (even Reed frequencies of a few hundred Hertz) this is not true and at Aster / TI21 frequencies (circra 2000Hz) the voltage drops off very markedly along a track circuit (not linearly either- falls sharply when leaving the transmitter but then the curve flattens out). Whole concept of ballast resistance is far too simplistic to be useful; one useful by product though is that it can be easier to find the short circuit on a failed track circuit by plotting the rate of decline in voltage when walking every 10m along the track.
PJW
Peter Wrote:Ah ha, the "ballast resistance calculation" dilemma. Probably one of the most common things that people make a mistake on, and the bit that we did right at the end of tonight's study group.
Remember that the ballast resistance is effectively made up of loads of resistors in parallel, so if the TC is longer, the total will be smaller. The value for ballast resistance is quoted in the units of "ohm km" and here as a value of 2.5 ohm km. This means "the length of the tc in km times the resultant equivalent value of resistance must be 2.5", hence the formula given is correct but appears odd at first sight.
Thanks for asking the question, it shows someone is thinking about things.
You are never wrong, but there may be some things that you have not quite had the chance to learn properly yet.
P.S. For resistance read "impedance" to be fully correct and not lazy like me.