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

[Roy Morgan]

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!