[ARC5] [Milsurplus] HV Fuses: You Were Right, I Was Wrong.. more
Glenn Little WB4UIV
glennmaillist at bellsouth.net
Sat Jun 16 20:43:23 EDT 2012
How can placing three fuses in series help?
Each fuse will have a tolerance.
The fuse that blows first will fail, hopefully open.
This will open the circuit.
If the fuse fails by sputtering the inside of the case, you will now
have two fuses in series with the failed circuit.
This will continue until all of the fuses fail.
The only thing that you have gained is to keep the overload condition
in place for a longer period of time.
You can hope that one of the three fuses will blow cleanly and open
the failed circuit.
This is unlikely to occur if the other fuses of this type fail by
sputtering the inside of the case.
The voltage across any fuse in the series chain is almost the same as
the voltage across one fuse.
73
Glenn
WB4UIV
At 09:46 AM 6/16/2012, Bruce Long wrote:
>FYI McMaster Carr has two types of fuses rated 1000 vdc, one is
>designed for commercial solar panel use - is somewhat expensive at
>$25 each and comes in several current ratings
>
>The other type is intended for use in multimeters and is less
>expensive Both types are spec-ed with the maximum interrupt current
>
>I have not been following this thread close enough to remember the
>voltage in question but I suppose it would be in excess of 1000
>vdc. Still these fuses would work better than conventional fuses
>and two or three in series might well do the trick
>
>
>
>________________________________
> From: Peter Gottlieb <nerd at verizon.net>
>To: jfor at quikus.com
>Cc: arc5 at mailman.qth.net
>Sent: Saturday, June 16, 2012 9:38 AM
>Subject: Re: [ARC5] [Milsurplus] HV Fuses: You Were Right, I Was
>Wrong.. more
>
>Yeah but you'll feel even worse if you spent $80 to protect that
>$500 tube and
>STILL had the $500 tube blow first!
>
>No guarantee the "proper" fuse will perform any better. I^2t
>calculations were
>rarely ever done back then, and even if they did, what could they do
>about it?
>Their choices were fast blow, normal, slow blow and no-blow (forego fuse) and
>for some components it made little difference, they were going down
>no matter what.
>
>...P
>
>
>On 6/16/2012 9:25 AM, J. Forster wrote:
> > It might be worth the $80 if you are protecting a $500 tube!
> >
> > I remember an old comment about expensive devices blowing to protect the
> > fuse....
> >
> >
> > -John
> >
> > ==========
> >
> >> I saw a spec sheet saying the 1 amp fuse wire has 510 feet on the spool.
> >> Still,
> >> for the high cost of it (and then you have a 400 year supply), the copper
> >> wire
> >> solution is looking mighty nice.
> >>
> >> Peter
> >>
> >>
> >> On 6/15/2012 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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