Re: No battery backup for signals?

On Sat, 29 Mar 2008 21:20:30 +0000, Clive Page wrote:

For the second time in a week I've experienced a long delay on the
Thameslink route by all the lights going out. This time it wasn't the
train, but it was a signalling fault around Harpenden. The driver kept
us informed, even mildly amused, but said that all the signal lamps had
gone out in the affected area, and that he was having to go very slowly
because he had to get permission (over his radio I assume) to pass each
non-functioning signal, and at night it was quite hard to find each
signal gantry when all the lights were out on it.

That has the ring of truth, but I wondered how this can happen. I
thought that all coloured-light signal systems were fitted with backup
batteries which set the red light on if all mains power failed for any
reason. Is that not so?

Not in modern power signalling installations. The power supply itself will
have at least one backup, possibly more. These would be obtained by
duplicate supplies from the Electricity Supply Authority (via separate
substations), such that one fault is unlikely to remove both supplies.
Backup supplies in the form of diesel generators or a connection from the
traction supply, where available, will also usually also be provided.

These measures are designed to protect against power failures external to
the railway, over which the railway has no control. Once the volts are on
the railway, their lineside distribution, and therefore the urgency of the
repair, is under the control of the railway so, generally, the distribution
is not duplicated, nor are any local backups provided at the signals
themselves.

Some earlier schemes (early 1960s) did have local battery backup, and this
was, as you say, intended to maintain a red aspect under power failure
conditions (when it would not usually be possible to give proceed aspect
anyway as the track circuits and othwer control circuits would be dead), but
this arrangement has not, as far as I am aware, been provided on any large
power schemes (of which West Hampstead is an example) in more recent years.

Smaller installations, supplied at domestic voltage, in areas where sourcing
duplicate external supplies would be difficult or prohibitively expensive,
will usually have local battery backup on everything, so the signalling will
continue to work when the external supply fails. A fringe benefit of this
arrangement is that it also protects against 'internal' failures (ie those
occurring within the railway's distribution network).


--
WZR
.

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