On Sep 30, 10:20*pm, cj wrote:

According to an apologetic poster at Clapham Junction this evening,
the racket was apparently caused by a power surge to the fire alarm
circuitry. (In my experience power surges tend to permanently fry
electrical things, not render them permanently stuck on "loud", but
then I'm not an electrician, and happy to be corrected...)

I think it's generally the case with fire alarms, for fairly obvious
reasons (so they continue to work as long as possible even if a fire
is busy burning through their cables, even though these are armoured),
that they work on the basis of "if in doubt, sound", with things like
sirens often operating semi-independently of the control unit. So if
a surge triggers an alarm and the circuitary is then fried, it may be
quite difficult to get it all to stop other than by pulling the
breakers and waiting for the batteries in the sounders to run out,
which may be some time.

It would seem that Clapham Junction has the rather annoying type of
sounder which is painful to the ear, presumably to ensure people
*actually do* evacuate rather than ignoring it. But that seems not to
have worked...

Neil