[ARC5] [Milsurplus] HV Fuses: You Were Right, I Was Wrong.. more

[Leslie Smith]

Hello Roy & Others,
Some time in the past and for a reason that is out of mind I needed some
fuses.
I remember I didn't have the rating I needed in the drawer.

I had some multi-strand hook-up wire, including 7/0076 or 7 filaments of
7.6 thou (or mil) wire.
I found the fusing curring of this wire in a copper wire table (probably
ARRL handbook).
I made a test rig from a variable PSU the wire and a low value resistor.
I put current thru the circuit and noted the current when the circuit
went "open"
I was surprised by how reliably the wire fused at the rated fusing
current.
I don't remember now, but one strand of 7/0076 wire fuses somewhere
around 6 or 7 amps - and I found this to be within about 5% of the value
given in the table.

Yes, perhaps an "amateur" trick, but knowledge is knowledge, and there
are times when a job has to be done.  A good tradesman is the fellow who
knows the limits of his trade, and how close he can sail to that limit
without putting a hole in his boat.

As for high current through glass fuses (eg 3AG) I am always suspicious
about that practice.
I have seen a fuse "blow", and then return to a conductor when the
vaporized metal re-formed on the glass case.  I would only use
sand-filled fuses for currents rated at more than (say) 7.5A


 
  Les
  vk2bcu at operamail.com


On Fri, Jun 15, 2012, at 11:58 PM, Roy Morgan wrote:
> 
> On Jun 15, 2012, at 8:42 AM, J. Forster wrote:
> 
> > If it were me, I'd try and get some 'fuse wire'. I don't know if it is
> > still available.
> 
>  From a later post, it does seem to be available, but pricey: though the
> price per roll did not say how much you get on a roll.
> 
> > ...If you can't find fuse wire, consider fine magnet wire with the
> > appropriate fusing current.
> 
> On that point, I find in my notes file on fuses the following:
> 
> >
> > fuses.txt
> > From K1LKY
> >
> > From: "Dr. Barry L. Ornitz" <ornitz at tricon.net>
> > To: "Tom Rauch" <w8ji at contesting.com>; "Old Tube Radios"
> > <boatanchors at theporch.com>
> > Sent: Thursday, December 09, 2004 1:45 PM
> > Subject: Re: Which begs the question...
> >
> > > Tom, W8JI, had an excellent discussion on protecting tubes and
> > > high voltage power supplies under fault conditions.  In this he
> > > wrote:
> >
> > Resistor content clipped out..
> >
> > > Tom also wrote:
> > >
> > > > I'm guilty of using thin enameled wire as a HV link fuse,
> > > > and I have on occasion used carbon resistors to locate an
> > > > unknown fault (like which tube is arcing?). But for reliable
> > > > protection in commercial or tell-the-world-to-do-it-this-way
> > > > articles a bit more sensible engineering would be better.
> > >
> > > I have done this too, using 32 gage and smaller wire.  Always
> > > working near chemists, I had ready access to glass tubing to
> > > run this inside.  One trick was to use the spring from a
> > > ballpoint pen to put the wire under tension.  Thus when the
> > > wire fused, the spring would quickly separate the two ends,
> > > resulting in less arcing and ionization.  Like Tom, I would not
> > > want to do this with a commercial product design for production.
> > >
> > > A good approximation for the fusing current for copper wire
> > > with the length much greater than the diameter is:
> > >
> > >         I = 10244 * d^1.5
> > >
> > > where:  I = fusing current in amps,
> > >         d = wire diameter in inches.
> > >
> > > So for 32 AWG wire, the diameter is 7.95 mils, and the fusing
> > > current is approximately 7 amps.
> > >
> > >         73,  Barry     WA4VZQ     ornitz at tricon.net
> >
> > At:
> >  http://www.railcar.co.uk/mechanical/data/fuses.htm
> > we find that no. 42 plain annealed copper wire used as fuse wire is  
> > rated at 1.5 amps, with a blowing current of 2.5 amps.  (these may  
> > be BWG sizes!)
> >
> >   Rating  Blowing current  wire size
> > 1.5          2.5                   42
> > 3             5                      38
> > 5             8                      36
> 
> 
> Roy
> 
> Roy Morgan
> k1lky at earthlink.net
> K1LKY Since 1958 - Keep 'em Glowing!
> 
> 
> 
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