Mortimer wrote:

"Chris J Dixon" wrote in message
.. .

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...

Indeed, but the characteristics of a water fuse, crude though it
sounds, actually make it very good for semiconductor protection,
and a great deal cheaper than the equivalent cartridge fuse,
which could easily cost as much as the device it protected.

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.