Re: Fuses

[brian whatcott <inet@INTELLISYS.NET>]

At 18:15 3/11/99 -0700, Glenn Knapp wrote:

> Gary,
>
> I do much the same thing as you - use some fibers from steel wool with a
> parallel circuit to which one can add additional light bulbs to increase
> the load.  To make it more spectacular, tape the steel wool fibers to an
> inflated balloon, then connect the ends to the circuit.  When the load
> gets large enough the steel wool will ignite and pop the balloon.  I think
> that it is a good way to drive home the idea about fuses.
>
> Glenn

I think Glenn's demo has something to tell us about breaking a circuit.
Fuses are usually made of metal - low temp fusible alloys,or wires that
won't oxidise too rapidly, or wires with low resistance like silver
and copper.

But particularly in higher voltage circuits, extinguishing the arc is
the name of the game.  There have been wire conductors surrounded by
silica sand ('silver-sand'), surrounded by boric acid powder ('deion'),
surrounded by chalk, all with the intention of deionising the gap or
taking the heat out of the molten blob.

 And certainly, some high-power contactors use an air-jet to snuff the
arc, just like Glenn's balloon! They try to widen the gap just as fast
as possible too.
There is something rather elegant about his mechanism: instead of using
the I^2.R heat to melt a wire in a blob of solder, which is then yanked
out by a spring (this is one 'slow-blow' fuse mechanism) he is using
a thermal breaker effect. And thermal contact breakers are one of the two big
classes of contact breakers ( the other being electromagnetic).
These are much nicer to have round the place than fuses - no looking
for spares. Just reset them.
..perhaps THESE are what should be featured in demos?

Brian
brian whatcott <inet@intellisys.net>
Altus OK