16-02-2010, 01:47 PM
(16-02-2010, 01:31 PM)Jerry1237 Wrote: There are some caveats with TIs. Freq E-H are non-preferred. There are low power and mid-fed examples. TI - Traction Immune - is not a definate. They have limitations, i.e. not in P&C.
I'll try to find the circuit diagram for the internals and get someone far more knowledgeable than I to explain what makes them TI!
J
The new digital ones will be apparently allowed in P&C; inbdeed I think there is already a trial site. Of course the French have used the predecessor UM71 in pointwork for many years and wonder why the UK refuse to do so.
Basically immunity is due to the fact that the signal oscillates between a pair of frequencies which ar 35Hz apart, both of which need to be present / absent appropriately over a suitable time period and they are slow to pick as a result. The logic is that interference of a certain freqiuency that persists for a matter of a second isn't a transient; conversely the probbaility of two frequencies which are not harmonically related to each other or the 50Hz mains supply is vanishingly improbable.
PJW