[Amps] high voltage fuses

[Manfred Mornhinweg]

> 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

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