When I worked for High Voltage Engineering for a short time 1969-70 after
leaving National I watched 100-250KV power supplys do their thing behind
thick steel and brick doors and thru leaded glass.
Even when operating properly the radiation could kill you and watching a
plasma event was a wonder to behold. The power was dumped but the plasma was
self feeding until the room was filled with Halon, I believe, and then
vented.
Being the Sr Engr Tech it was my job to take the system apart, write up a
report for Engineering and then rebuild with the next "fool proof" revision.
I didnt see a future in that field and then went to Sanders Associates where
I discovered the digital world.
Carl
KM1H
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: <amps@contesting.com>
Sent: Monday, October 07, 2013 5:44 PM
Subject: Re: [Amps] high voltage fuses
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
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