In article , Clive D. W. Feather
wrote:
In article . com,
TheOneKEA writes
Is it possible for a wet track circuit to short out in such a way that
a green aspect could be obtained?

Extremely difficult.

The basic design of a track circuit is very simple:

Supply Relay
| | | |
| | | |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==
| \---/ \---/ \---/ |
/--/ \-\
| /---\ /---\ /---\ |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==

=== running rail
=:= insulated gap
= = ordinary gap between rails
=+= wire bonded to rail
|

The supply will be DC in some areas, but AC at a specific frequency on
LU. The relay will be tuned to the same frequency (so a feed from an
adjacent track circuit won't trigger it). The wheels and axles of the
train short the rails, causing the relay to de-energise. Any fault in
the wiring causes the relay to de-energise. But to get a false clear on
the relay you've got to feed significant amounts of 83 1/3 Hz (or
whatever) current into the circuit to the right of the location of the
train.


It is my understanding that 83 Hz is more complex and harder to beat than
this.

We have all seen induction motors. Sometimes they are 3 phase, but sometimes
they are single phase. A phase at right angles to the main supply is created
by passing the current through a capacitor and the rotor is rotated by
currents induced by the two magnetic fields. In railway practice, a relay
vane is lifted by the two fields, which MUST be at right angles to each
other. So, the 83 current is detected, not only because it is at the right
frequency, but ALSO because it is at right angles ("in quadrature") to a
pilot current supplied from the 83 generator. It is very difficult for a
false positive to be given.

Michael Bell
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