When the fuse blows the plasma will conduct the rest of the current straight to
ground acting
as a self sacrificial crowbar.
It's far better to avoid letting plasma loose in a circuit. You know,
that stuff is conductive! For the duration of the arc, the plasma tends
to expand, and envolve other parts of the circuit, which then arc over
by themselves and keep feeding the plasma. Once the main arc or arcs run
out of energy feeding them, the plasma blows out and expands, and in
that last stage it shorts out parts of the circuit that are further
away. After a nice big plasma arc, often there will be many components
blown, and what's worse, there may be others that still work, but are
partially damaged and will fail later, or underperform.
I think that where high voltage fuses are really required, it's by far
the best to use real high voltage fuses, that will blow as cleanly as
possible, and confine any plasma.
In many cases it's enough to fuse the primary side, where the voltage is
low enough that any normal cheap fuses can be used.
If anyone wants to see photos of the signs left by a plasma arc
expanding and envolving neighboring areas, I can shoot some and post
them somewhere. I have a carcass at hand, of a piece of equipment that
was killed in that way.
And now a question to those of you who know arcing phenomena: I have
often seen that an arc at one place starts daughter arcs at quite
distant, but electrically connected points of a circuit. What is the
phenomenon causing this? Is it a high power, high frequency signal
generatet by the arc pulsing, which is uptransformed by the inductances
and capacitances of the wiring and circuit, to reach voltage peaks high
enough to start those secondary arcs? Or what?
A very typical example: When an old style incandescent lightbulb burns
out, at least here in this 220V country it will very often go in a flash
of light, a bang, and trip the circuit breaker while doing so. Even when
the circuit breaker is rated at 10 amperes, and the lightbulb runs on
just 0.25 amperes or so! There is no sign of arcing damage inside the
bulb, only a broken filament. But separate the bulb from the socket, and
suprise, inside the socket there are signs of a BIG arc! Everything is
black, and there are little droplets of molten metal. And the wires are
missing. Clearly the small arc that forms inside the bulb, when the
filament opens, triggers a big arc inside the socket, that draws enough
current to trip the circuit breaker.
A few times I have even seen lightbulbs explosively jumping out of their
sockets, just the glass part, leaving the metal base in place, when they
burn out! These things do have character...
So, has anyone a good physical explanation of arcs triggering secondary
arcs at places where far larger voltage is required to jump the gap?
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|