[Amps] high voltage fuses

[Carl]

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 at ludens.cl>
To: <amps at 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
>
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