[Milsurplus] HV Fuses: You Were Right, I Was Wrong

[Richard Brunner]

The central problem with fuses and circuit breakers is arc interruption.
At 120 volts it's easy because it takes more voltage to sustain the arc,
and it's self-interrupting. At 240 or so volts it's iffy, and at 400 or
so volts it isn't gonna stop.  High voltage fuses and circuit breakers
often divide the arc into many series arcs, thus requiring much more
voltage to sustain them, and it interrupts.  You might try a long fuse
element in silica or borax to help cool and quench the arc.

In qualifying fuses and circuit breakers you test at many current and
voltage levels, and you may get a clean interruption at a high level one
time, and not the next.  In the end you look at your tests, draw a line,
and say it will work every time below this line.

Richard, AA1P

On Fri, 2012-06-15 at 07:15 -0700, J. Forster wrote:
> The capacitor has to be able to absorb the L*I^2 energy of the circuit,
> without charging up to a voltage big enough to sustain a metal plasma arc.
> A 0.1 is nowhere near big enough, IMO.
> 
> The original designers were not dummies. They used long fuses because they
> were needed.
> 
> -John
> 
> ================
> 
> 
> 
> 
> 
> > And therein lies a partial answer.  Put a 1 or 2 KV 0.1 MF capacitor
> > across the fuseholder to snub the reactive kick and it will likely not
> > sustain any arc.
> >
> > HTH
> > George
> > W5VPQ
> >
> > ---------- Original Message ----------
> > From: "J. Forster" <jfor at quikus.com>
> > To: "David Stinson" <arc5 at ix.netcom.com>
> > Cc: ARC-5 List <arc5 at mailman.qth.net>, milsurplus at mailman.qth.net
> > Subject: Re: [Milsurplus] HV Fuses:  You Were Right, I Was Wrong
> > Date: Fri, 15 Jun 2012 05:37:26 -0700 (PDT)
> >
> > Absolutely predictable. You could try several equal fuses in series. No
> > guarantees though.
> >
> > HV DC really arcs, especially with any inductance in the circuit.
> >
> > -John
> >
> > =============
> >
> >
> >
> >
> >> As you all probably remember, I did some testing using
> >> Picofuses (tm)  to rebuild the BC-375 high-voltage
> >> fuse links.   I "destruction tested" six or eight of them.
> >> Each opened without problems.
> >>
> >> Wellll......
> >>
> >> One opened during my 211 tests due to a gassy tube.
> >> No problem.
> >> I replaced it with another rebuilt fuse.
> >> Another gassy tube, but this time....
> >> "ZZZZZZZZZZZZZZ!!..........." from inside the
> >> dynamotor.  The Picofuse had opened, but fully carbonized
> >> and was transformed into a nice spark gap.  I'm lucky
> >> the inductive kick didn't take out any of the inductors.
> >>
> >> So.... You were Right.  I was Wrong.
> >> Back to the drawing board.
> >> Probably try 3AG pigtails mounted externally.
> >>
> >> 73 Dave AB5S