RE: Electric UFH &

["Keith Doxey" <ukha@xxxxxxxxxxx>]

Hi Chris

>   across the power input to deal with some faults, which result in
> over-current, some HP instrumention used to use some sort of solid
state
> device that was triggered to short blow the fuse ... they used to call
> it crow-barring ...
>

Crowbarring is a technique used to protect delicate electronic equipment
from excess voltage. This is done AFTER the power supply stage of the
equipment and is usually also done in conjuction with shutting off the
power.

I used to look after a high power amplifier that had a crowbar circuit. If
a
fault developed on the amplifier such as a large speaker destroying DC
offset on the output, the protection circuitry would shut of fthe high
voltage power supply and the crowbar would be trigered to dump the energy
stored in the resevoir capacitors.

I WOULD NEVER CONSIDER PUTTING A CROWBAR ACROSS THE MAINS.

If you short out the mains supply you get a instantaneous current of
thousands of amps which is what blows the fuse or trips the MCB. This
results in huge amounts of energy being disipated and if any part of the
circuit has a poor connection that gives rise to a bit of resistance then
you could end up with a very large current flowing that is not enough to
trip the breaker/fuse quickly enough. This power would generate a lot of
heat and would be a severe fire risk.

I once replaced a 30A rewireable fuse on a commercial dishwasher in a
nightclub that had blown for no apparent reason. The fusewire didnt look to
have died a violent death so I assumed it was just metal faigue that had
finally killed it but I did look for shorts just in case. Everything looked
OK so the fuse was replaced. I turned the isolator back on and about two
seconds later there was a loud buzzing from the busbars in the distribution
board. I immediately turned the isolator off but I was too late.

As I walked back into the club, all of the main bar was in total darkness.
Thankfully the other bar was still OK as was the sound and light system. It
transpired the heating element in the dishwasher had shorted to earth but
this wasnt apparent until a contactor kicked in for the heating cycle.
Because the newly replaced fuse wire was in excellent condition is didnt
fail before the more heavily loaded 100A main supply fuse feeding the main
distribution board.  WHOOPS !!!!