I found this on the internet and thought that it might be of use to someone here. Dr. Leidecker works for NASA and wrote a NASA manual on fuses for spacecraft...
> From: Henning W Leidecker
Subject: Re: Tin whisker follow-up question
Date: Wed, 12 May 2004 21:41:31 -0700
Dear XXXX,
I've done a lot of experimental and modeling work with the FM08 style fuses.
Each contains a nearly straight metal wire.
For currents ranging up to about 120% of the rated current, the link opens by
"creep to rupture", but never melts. The element is held at each end under
bending moments; the ends stay cool, and so the built-in stresses there never
relax; but the middle of the link gets hot enough to creep open in tens of
seconds, even when tens of degrees under the melting temperature. I doubt
that this mechanism happens in whiskers --- they are just too flexible, and
are not fastened at both ends (only at one), so there cannot be important
bending moments acting to induce creep.
For currents from about 120% to (very roughly) 300% of rated current, the
middle of the link reaches melting, and this molten section is pulled by
surface tension back onto the cooler stubs --- the opened fuse shows a pair
of stubs with a melt ball formed onto each end.
For a whisker, one end is well-rooted in the surface from which it grows,
while the other end is "just touching" a surface at a different potential ---
the contact pressure is small since the whisker is flexible. (I am thinking
of the large-aspect ratio whiskers. I leave a discussion of the very short
aspect ratio whiskers for another time.) With gentle contact pressure, the
Hertz-disk of contact between the whisker and the other surface is small, and
the associated contact resistance is high. So there might be important
heating developed there, unlike the case for a normal fuse element. (But
this is what happens during welding --- the tip of the welding element is
usually tapered, so that it gets hotter than the rest of the rod.) So we
might initiate this adventure by having the tip of the whisker melt, and flow
into the surface --- then the contact becomes pretty good, and the whisker
tip would cool, while the middle of the whisker would get hot (this is the
part of the whisker that is most isolated now from the heat sinks at each
end). The whisker would then fuse open near its middle, with melt balls
forming on each stub.
For a fuse that is hit with (roughly) 300% to 1,500% of its rated current, the
link heats so fast that almost no heat escapes from the ends, and so most of
the length of the link heats uniformly, and reaches the melting temperature
at nearly the same time. (The middle does reach melting first.) A liquid
metal is still a conductor (the resistivity increases by 1.2 to 2.0,
depending on the metal), and so the liquid column continues to heat until
some forces act to break this liquid metal beam into different parts.
One force is gravity --- it is important for long thick wires like overloaded
transmission lines, but it is not important for thin wires or whiskers.
Another force acting on fuse links is built-in bending moments, and this may
be important for a whisker that is bent into a curve by being compressed
between two plates (the one the whisker is growing from, and the other at a
different potential) that are approaching each other.
But the usual force for fuse link (and whiskers) is surface tension --- this
excites the Rayleigh-instability once the length of the liquid column exceeds
the diameter of the liquid region --- the liquid "beads up" in to a set of
balls. (You can see this in Boys' Christmas Lecture book on soap bubbles,
reprinted by Dover --- excellent book!) Or in a lava lamp. Or in a stream
of water. We see this in x-rays of fuses: we count the number of melt balls
visible inside the fuse-tube to estimate the overcurrent.
For a fuse that is hit with more than (roughly) 1,500% of the rated current,
the continued heating of the liquid column gets the metal hot enough for
evaporation to become important before forces (gravity, built-in, surface
tension) can break the tube up and stop electrical conduction. So some
material evaporates off the hot column. This decreases the cross section
area, and concentrates the current into a smaller area (ie the current
density goes up), and so the heating rate increases --- this drives
evaporation faster, and this concentrates the current even more --- that is,
the process accelerates and the filament "explodes" in a puff of metal vapor.
(Modeling this requires attention to heat lost by radiation and also by the
heat carried off by the evaporating metal.) For overcurrents near (roughly)
1,500%, only the middle of the filament evaporates --- inspection of the fuse
envelope shows this evaporated metal present in a band near the middle ---
and this band does not extend far enough along the envelope to electrically
connect the ends. For much larger overcurrents, almost all the length of the
link can evaporate --- the metal band deposited inside the fuse envelope can
now bridge between the end connectors, and the fuse will continue to conduct
electricity. Indeed, about the same amount of metal is available for
conduction after this evaporation as before! (Just its location is
different.) Since this metal film is in intimate contact with the inside
wall of the fuse envelope, it is much better able to shed joule heat into
these walls, and so the rating of this "reborn" fuse is now substantially
larger! This is why fuse specifications limit the maximum current the fuse
is intended to interrupt. This is intended to prevent the fuse from being
used under circumstances in which it would be reborn in a more robust
condition.
A whisker that is hit with a huge overcurrent (such as few hundred
milliamperes for thin whiskers) will mostly evaporate, and the metal will
deposit onto the line-of-sight surfaces. The deposited metal could provide
conduction if the film now bridges between ANY conductors, either the
conductors that launched the evaporation of the bridging whisker, or any
other coatable conductor pairs (can you say, "Use conformal coating!" I knew
you could.) Bridging depends entirely on the locations of the surfaces
collecting the evaporated metal. The more distant a collecting surface is,
the thinner is the deposited film: films thinner than roughly 1,000 A (=0.1
um) begin to have resistivities substantially larger than bulk values, since
surface scattering of the conduction electrons begins to become important.
When thinner than about 30 A, then the deposited metal is probably present as
isolated islands, and the resistivity diverges to infinite values. So you
can stop worrying once collecting surfaces become distant.
This presumes that the available voltage and available current do not ignite
the metal vapor into an arc --- then things get exciting in other ways. But
arcing should be impossible for potential difference under 6 volts. And the
huge sustained arcs are impossible at less than 20 A or so. Your case seems
perfectly safe from arcing.
> From: Henning W Leidecker
Subject: Re: Tin whisker follow-up question
Date: Wed, 12 May 2004 21:41:31 -0700
Dear XXXX,
I've done a lot of experimental and modeling work with the FM08 style fuses.
Each contains a nearly straight metal wire.
For currents ranging up to about 120% of the rated current, the link opens by
"creep to rupture", but never melts. The element is held at each end under
bending moments; the ends stay cool, and so the built-in stresses there never
relax; but the middle of the link gets hot enough to creep open in tens of
seconds, even when tens of degrees under the melting temperature. I doubt
that this mechanism happens in whiskers --- they are just too flexible, and
are not fastened at both ends (only at one), so there cannot be important
bending moments acting to induce creep.
For currents from about 120% to (very roughly) 300% of rated current, the
middle of the link reaches melting, and this molten section is pulled by
surface tension back onto the cooler stubs --- the opened fuse shows a pair
of stubs with a melt ball formed onto each end.
For a whisker, one end is well-rooted in the surface from which it grows,
while the other end is "just touching" a surface at a different potential ---
the contact pressure is small since the whisker is flexible. (I am thinking
of the large-aspect ratio whiskers. I leave a discussion of the very short
aspect ratio whiskers for another time.) With gentle contact pressure, the
Hertz-disk of contact between the whisker and the other surface is small, and
the associated contact resistance is high. So there might be important
heating developed there, unlike the case for a normal fuse element. (But
this is what happens during welding --- the tip of the welding element is
usually tapered, so that it gets hotter than the rest of the rod.) So we
might initiate this adventure by having the tip of the whisker melt, and flow
into the surface --- then the contact becomes pretty good, and the whisker
tip would cool, while the middle of the whisker would get hot (this is the
part of the whisker that is most isolated now from the heat sinks at each
end). The whisker would then fuse open near its middle, with melt balls
forming on each stub.
For a fuse that is hit with (roughly) 300% to 1,500% of its rated current, the
link heats so fast that almost no heat escapes from the ends, and so most of
the length of the link heats uniformly, and reaches the melting temperature
at nearly the same time. (The middle does reach melting first.) A liquid
metal is still a conductor (the resistivity increases by 1.2 to 2.0,
depending on the metal), and so the liquid column continues to heat until
some forces act to break this liquid metal beam into different parts.
One force is gravity --- it is important for long thick wires like overloaded
transmission lines, but it is not important for thin wires or whiskers.
Another force acting on fuse links is built-in bending moments, and this may
be important for a whisker that is bent into a curve by being compressed
between two plates (the one the whisker is growing from, and the other at a
different potential) that are approaching each other.
But the usual force for fuse link (and whiskers) is surface tension --- this
excites the Rayleigh-instability once the length of the liquid column exceeds
the diameter of the liquid region --- the liquid "beads up" in to a set of
balls. (You can see this in Boys' Christmas Lecture book on soap bubbles,
reprinted by Dover --- excellent book!) Or in a lava lamp. Or in a stream
of water. We see this in x-rays of fuses: we count the number of melt balls
visible inside the fuse-tube to estimate the overcurrent.
For a fuse that is hit with more than (roughly) 1,500% of the rated current,
the continued heating of the liquid column gets the metal hot enough for
evaporation to become important before forces (gravity, built-in, surface
tension) can break the tube up and stop electrical conduction. So some
material evaporates off the hot column. This decreases the cross section
area, and concentrates the current into a smaller area (ie the current
density goes up), and so the heating rate increases --- this drives
evaporation faster, and this concentrates the current even more --- that is,
the process accelerates and the filament "explodes" in a puff of metal vapor.
(Modeling this requires attention to heat lost by radiation and also by the
heat carried off by the evaporating metal.) For overcurrents near (roughly)
1,500%, only the middle of the filament evaporates --- inspection of the fuse
envelope shows this evaporated metal present in a band near the middle ---
and this band does not extend far enough along the envelope to electrically
connect the ends. For much larger overcurrents, almost all the length of the
link can evaporate --- the metal band deposited inside the fuse envelope can
now bridge between the end connectors, and the fuse will continue to conduct
electricity. Indeed, about the same amount of metal is available for
conduction after this evaporation as before! (Just its location is
different.) Since this metal film is in intimate contact with the inside
wall of the fuse envelope, it is much better able to shed joule heat into
these walls, and so the rating of this "reborn" fuse is now substantially
larger! This is why fuse specifications limit the maximum current the fuse
is intended to interrupt. This is intended to prevent the fuse from being
used under circumstances in which it would be reborn in a more robust
condition.
A whisker that is hit with a huge overcurrent (such as few hundred
milliamperes for thin whiskers) will mostly evaporate, and the metal will
deposit onto the line-of-sight surfaces. The deposited metal could provide
conduction if the film now bridges between ANY conductors, either the
conductors that launched the evaporation of the bridging whisker, or any
other coatable conductor pairs (can you say, "Use conformal coating!" I knew
you could.) Bridging depends entirely on the locations of the surfaces
collecting the evaporated metal. The more distant a collecting surface is,
the thinner is the deposited film: films thinner than roughly 1,000 A (=0.1
um) begin to have resistivities substantially larger than bulk values, since
surface scattering of the conduction electrons begins to become important.
When thinner than about 30 A, then the deposited metal is probably present as
isolated islands, and the resistivity diverges to infinite values. So you
can stop worrying once collecting surfaces become distant.
This presumes that the available voltage and available current do not ignite
the metal vapor into an arc --- then things get exciting in other ways. But
arcing should be impossible for potential difference under 6 volts. And the
huge sustained arcs are impossible at less than 20 A or so. Your case seems
perfectly safe from arcing.