Re: [General] Signal failures

[signal_spotter@my-deja.com]

> No. The "normal" state of a track circuit is "occupied" If it is as
simple
> as a dodgy trk cct relay maybe it could be overridden, but if it is a
broken
> rail or broken trk cct bond (if they still exist between KPK and EPP)
then
> the cct will still fail and remain down.
>
> Ian may be able to correct me or elaborate here, but I wouldn't even
> consider it.

Well firstly the 'normal' state (or as Peter Dwyer pointed out, the
quiescent state) is energised when there are no trains around (or
clear). A train occupying the track will de-energise it. This
fundamental principle is the basis for all signalling (well signalling
involving track circuits) throughout the world. It's designed like that
to be 'fail safe'. So a track circuit is energised when a) there are no
trains on it, and b) all connections, terminals, fuses are made. If it
wasn't designed this way (say a train actually energised the track
circuit) then a blown fuse would mask the presence of the train and
lead to catastrophic consequences. So to the signalling system, a train
or a 'dodgy' relay/blown fuse etc are the same. They all de-energise
the track relay and put the aspect back to red. Another 'fail safe'
feature we design into relay based signalling systems is 'back
proving'. This means that before a relay can energise (say a control
relay to display a green aspect) other relays have to be proven de-
energised in it's circuitry. However the track circuit is the only
relay that cannot be backproved as it's the fundamental relay and
therefore stringent maintenance procedures and tests have to be carried
out to ensure that they are working correctly. Peter can elaborate on
that as it's probably his kettle of fish.

regards Ian
Signal Design Engineer


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