"Chris J Dixon" wrote in message
...
Mortimer wrote:

At university we had a project to develop a switched-mode power-supply in
which a thyristor is switched on and off at a constant frequency but with
a
variable mark-to-space ratio to keep the output voltage after smoothing
constant with a varying load.

We were warned that the thyristor that we'd been supplied with was rated
to
carry a certain current, but only when it was being switched and not when
it
was permanently turned on.

It was only a matter of time before one of the teams suffered a loose wire
between the thryristor and the switching circuit while the PSU was
carrying
full load...

Within a matter of about two seconds, there were five distinct sounds: a
cry
of "****!" from the person who had accidentally disconnected the
oscillator,
a yell of "duck!", a scraping of chairs on the floor as everyone took
cover,
a VERY loud bang like a shotgun, and a gasp of "F**k!" as everyone reacted
to the bang.

The end had blown off the thyristor and embedded itself in the ceiling -
thank goodness it went upwards instead of into someone.

I remember once reading a test report written by one of my
colleagues.

They had built a prototype railway rolling stock dc chopper based
on a string of thyristors. The assembly was placed on a
substantial metal bedplate in the test area. It was connected to
the power supply through a water fuse. This consisted of a
plastic bucket filled with water, having a couple of submerged
cable connectors and a thin wire between them. For safety, the
bucket had a wooden lid with a heavy weight sitting on it.

When the inevitable fault occurred, the fuse blew with some
force, creating a waterspout which lifted the lid high in the
air. Unfortunately, on the way back down, the heavy weight was
faster then the wooden lid, and it wedged itself between the
cable connectors, re-making the circuit. This would have been bad
enough on its own, but with all the water sloshing around on the
bedplate, the supply was now both shorted and earthed.

I believe it took some time to get supplies restored, and there
were no intact thyristors left.

Sounds like it was caused by the same Sods Law fairy that caused the chain
of events in Gerard Hoffnung's story about the barrel of bricks being
hoisted up the building.

The thought of water near an electrical installation sounds like an accident
waiting to happen...