# Cu wire fusing amps ?

Discussion in 'Electronic Design' started by Phil Allison, Jul 18, 2013.

1. ### Phil AllisonGuest

** Been trying to test common fuse wire for ability to handle inrush surge
current.

Particularly interested in 0.5mm dia tinned copper wire and current pulses
of half cycle duration - same conditions as for non repetitive surges in
power diodes and SCRs.

I figure it is about 500 amps peak - but are there any tables that cover
this?

.... Phil

2. ### Allan HerrimanGuest

Google for "copper wire i2t". The first hit gives a PDF which contains
the formula

((I/A)^2)t = 0.0297 log ((T2 + 234)/(T1 + 234))

I = short circuit current (A)
A = conductor area (circular mils)
t = duration
T1 = max op. temp
T2 = max s/c temp

Allan

3. ### Phil AllisonGuest

"Allan Herriman"

** That formula breaks down to I being proportional to the sq.rt of 1/t -
suggesting even the largest fuse will melt in a matter of weeks or months at
tiny current. But for time intervals of up to a few seconds - it seems
plausible enough.

With 10mS and 20 thou diameter, the result is about 600A - for a half sine
wave, the peak is about double so circa 1000A.

The interesting conclusion is this: a simple 0.5mm tinned copper wire fuse
will very likely survive direct connection across a 10A, 240V domestic
supply for a full half cycle.

Cos that supply very likely cannot deliver more than about 600 amps peak
into a short.

OTOH, a C curve, 16 amp rated, thermal magnetic breaker will trip at 180
amps in under 2 milliseconds.

.... Phil

4. ### John DevereuxGuest

AIUI the concept of I^2.t ignores thermal conduction losses. I^2.R.t is
the energy you are putting into the wire, it is assumed this all goes
into raising its temperature I think. Obviously this fails for intervals
long enough to allow heat to be conducted away.

Seems to also assume constant resistance for the wire even though it
gets ~white hot?

5. ### George HeroldGuest

Yeah I assume it's for the case when the heat has no time to leave the piece of copper.
You might try some 'first principle' type calculation.

So resistance R ~ rho*L/A (rho is resistivity, L is length and A is area)
(of course rho will change with Temp (T) and that will make it harder.)

Then energy dumped into the copper I^2*R*t will be heat capacity (HC) * change in temperature (t is the time.)
HC = constant * volume = C*A*L

So delta T = E/HC = I^2 * R * t / (C*A*L) = rho/C *t*I^2/A^2

So as you say, at least the units make sense. To do better you'd have to put in some estimate for how the resistivity changes with Temp. Assuming some linear relation might be OK. (rho ~ T(in kelvin))
I'd expect the heat capacity to be roughly constant at room temp and above.
(I'll scribble some more.)

George H.

6. ### Phil AllisonGuest

"George Hairoil"
** Yeah - and most likely get a:

" solution that only works for a spherical chicken, in a vacuum "

.... Phil

7. ### JoergGuest

Pushing thickfilm far past datasheet limits is risky. They might some
day cook up another pot of conductive material, it veers a bit towards
lower specific resistance, trim cuts get deeper ... *PHUT*

Wirewound may be better here. Ayrton-Perry style if it has to be low
inductance.

My first lesson in electro-migration happened around age 16. A loud
bang, large chunks of a capacitor migrated themselves into the plaster
of a wall, with gusto. Some metallic parts rained down from there and
hissed out on the carpet. My parents were not enthused.

8. ### George HeroldGuest

Fun!
A bit off topic, (but then Phil A. doesn't want any spherical cows :^)
I was wondering last night if I put a piece of heat shrink (or maybe several layers) around a 1/4 watt through hole resistor if I could raise it's maximum DC power.. *NOT* it's pulsed power rating.

George H.

9. ### josephkkGuest

Bloody cool. You should really publish a paper about your test results.
Maybe on your company website. That would be a real contribution.

?-)

12. ### Fred AbseGuest

No problem with mplayer:

mplayer -vf flip=yes Rsense_Flir.MPG

Flips it, or mirrors it, but won't do both at the same time.

13. ### Robert BaerGuest

There may be that kind of info in the Fire Protection Handbook by the
NFPA; less informative sources is the National Electrical Code (NEC).

14. ### Jasen BettsGuest

Strange it's MPEG level one, nothing fancy.

OTOH all your players may be relying on the same codec collection.
do both like this: mplayer -vf mirror=yes,flip=yes Rsense_Flir.MPG

or fix the recoriding using avconv (ffmpeg):

avconv -i Rsense_Flir.MPG -vf hflip,vflip Rsense_rightside.mpg

or mencoder (mplayer):

mencoder -o Rsense_rightside.mpg -oac copy -ovc copy -vf mirror=yes,flip=yes Rsense_Flir.MPG

however MPEG1 video is pretty weak, and PCM audio is worse, we can do better.

mencoder -o compressed.mpg -oac mp3lame -ovc x264 -x264encopts bitrate=360 -vf mirror=yes,flip=yes Rsense_Flir.MPG

15. ### Fred AbseGuest

AHA!

I thought I'd done that, but (instinctively?), I put a space after the
comma.

Without the space (Highlighting your example and middle clicking in an xterm),
it works fine.

That's one to remember.

The man page isn't very clear. I thought it might be my mplayer, it's
horribly compiler-dependent Every new build with a different compiler
fixed something and broke something else.
Mencoder is horribly buggy,here.

Thanks.

16. ### Jasen BettsGuest

shell syntax isn't english (or c) syntax, spaces separate parameters
comma's don't.
it's very unclear. there may be some better documentation, but I've
not looked for it.
Recently I've just been pulling it and mplayer from deb-multimedia, compiling
mplayer is no small undertaking.

17. ### Fred AbseGuest

Mine I built in 2007. Took about 2 hours to compile, after I found out
that the code used pi and e, but didn't define their values anywhere. A
quick couple of #defines fixed that.

I actually have two versions, one with dvdnav, for those nasty crippled
Sony DVDs, with apparently bad tracks, which breaks a few unrelated
things, and one without dvdnav for everything else.

I have codecs for just about everything.

There's an HTML guide, that I converted to PDF, that's better than the man
page. I'll shove it up to a.b.s.e. if you like.

I now know why the Hungarians invented goulash ;-)

18. ### josephkkGuest

Go for it. I would like a really clean MM player that is reliable as
well.

?-)

VLC. For both Windows AND Linux.

Regular DVDs... Xbox media center (XBMC).

BD and HD DVDs, not so easy on ANY non-proprietary platform.

Under Windows, software decoding players usually come with the playback
device, and it is usually "Power DVD". They are on my list of folks to
sue for dropping support on hardware they were contracted to support.

Sad, my original DVD drive for a PC had its own HARDWARE based decoding
card, back before they had software decoding, and before the speed of the
PCs they were on could decode them fast enough.

I wish they would come out with a BD DVD player that had its own
decoder card. That way they could put the software for the player on the
card too.

With the computers being so fast now, the hardware decoder option will
likely never happen.

20. ### JWGuest

Great program for the most part, but it lacks a way to set a bookmark so
you can pick up where you left off watching a video.