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.

You've stated that track circuits
require a pair of false feeds to turn green;

More that "double fault" is a general principle. In-the-field circuits
are often double cut (that is, both supply and return are switched by
the controlling relay) so that a false feed or false earth doesn't
trigger it.

in the case of East Putney
- Wimbledon, what would need to get wet to cause such a wrong-side
failure?

I'd be surprised if simply having water in the wrong place would
suffice. A wiring fault would be much more likely.

Are you talking about a specific event, or just a general enquiry?

--
Clive D.W. Feather | Home:
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