> All this talk of how EASY it is to trigger these circuits is a little
> bit counter to what little I know and what little experience I have.
> My understanding is that, for low-voltage DC circuits, the short
> circuit resistance that needs to exist to divert enough current from
> the holding relays to cause them to flip is one of very low impedance
> (less than 1 ohm?).
True. Not all our track circuits are DC, and they are not all low voltage either.
> This is why track machinery, Sprinter railcars,
> even 3801 with sand under the wheels cannot always be relied on to
> offer a path of low enough resistance to trigger the circuitry.
3801. Interesting you mention it. Measurements taken on the night of the accident show
track voltages of 5.4V and 2.41V on two sections. The resistance required to activate the
circuits were:
Feed end: 0.06 & 0.06 Ohms
Mid-Point: 0.1 & 0.06 Ohms
Relay end: 0.1 & 0.06 Ohms
All circuits were tested as functioning correctly.
--- David Johnson CityRail Guard trainman@ozemail.com.au http://www.ozemail.com.au/~trainman