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roughleg(at)gmail.com
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 Posted: Fri Jun 19, 2020 6:24 pm Post subject: Minimum length of fusible link wire segment |
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Bob, in a recent post you state that there is a 9-inch rule of thumb minimum length for fusible link wire segments. Could you explain the physics behind that, please?
Pat
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bob.verwey(at)GMAIL.COM
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| Posted: Tue Jun 30, 2020 10:51 pm Post subject: Minimum length of fusible link wire segment |
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Elucidating Indeed, Mr N!
Best Regards,Bob Verwey
082 331 2727
[img]https://docs.google.com/uc?export=download&id=0B5d7rgAInTuTUUZsUjY4QmJsdVU&revid=0B5d7rgAInTuTdDJDaXRFZVh3b3lMa3FWL0s3MFdzc01YRlNvPQ[/img]
On Wed, 1 Jul 2020 at 00:06, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 07:18 PM 6/19/2020, you wrote:
| Quote: | Bob, in a recent post you state that there is a 9-inch rule of
thumb minimum length for fusible link wire segments. Could you
explain the physics behind that, please?
Pat |
Sorry to take so long on this . . . it took a few
miles of 'asphalt engineering' to figure out how best
to explain it. I THINK I've got a way . . . let's
give it a try . . .
Fusible link performance, indeed performance of any
fusible circuit protective device is tightly bound
to the fact that most electrical conductors have
a positive temperature coefficient of resistance.
This means, it's resistance rises with temperature
of the conductor.
The rate of temperature rises is a
function of power (watts) dissipated in the
conductor mass which is the product of current
(amps) times resistance (ohms). Consider a piece
of wire, any gage, hanging out in space with some
current flowing through it.
Here's a 22AWG wire that has been happily
carrying 20A for some time. Note that I
attached a thermocouple to the sample wire
out in the middle of the rather significant
free span. Suppose I conducted this experiment
with, say a 1" piece of wire? How might we
guess that center-span's rate-of-rise would
differ for the two cases?
http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
It's intuitive that heat generated in the
wire is conducted to the ends. Rate of rise
and temp equilibrium is influenced by sinking
effects of the terminals and radiation into
surrounding air. It's easy to deduce that a
short piece of wire is more resistant to
fusing than a longer one.
As a practical matter, the opening of an
electrically fusible segment tends to 'center up'
on the span. This is seen in the pictures of fuses
attached. As temperature begins to rise, heat
is generated all across the span and conducted
in both directions from any single point. The
CENTER of the span is least able to reject heat
to it's adjacent mass . . . it's getting warmed
up from both directions and also rising the fastest
due to effects of positive temperature coefficient.
Rising resistance increases electrical power dissipated
at that location more rapidly than anywhere else along
the span. Hence the temperature rise is regenerative
. . . the hotter it gets, the faster it warms up.
How long does this take? Complex question depending
on a constellation of conditions. Fuse and
breaker manufacturers work diligently to achieve
predictable performance in their products.
Here's a well written piece that explains the
thermal fiddling necessary to achieve predictable
i.e. fast versus slow blowing characteristic in fuses.
How about that little fly-spec of heat sink material
in the slow blo cartridge fuse?
https://tinyurl.com/ybkndmrc
It easy to see that control of the environment
surrounding the fusible event is critical.
A fusible link is in the 'HULK' family
of protective devices. Like its cousins,
the ANL, MANL and similar 'current limiters',
none are suited to the protection of the
more pedestrian appliance feeders. The
fusible link is SLOW and intended to clear
major faults (HUNDREDS+ AMPS) in the system
bus structure while maintaining a robust
indifference to rather severe transient overload.
Unlike our little plastic ATC friends that
can be 'hammered' into lower operating
currents by repeated excursions close to
but still short of their ratings.
Getting back to your question: Would a 6"
piece of fusible link fail to function?
How about a 1" piece? Probably not. They're
still the weakest link in the faulted pathway.
I am GUESSING that 9" figure is probably some
fusible link designer's 'happy place' for repeating
the in-service design goals he was working with.
Or perhaps it's a legacy hand-over from an automotive
industry specification.
In any case, wanting to shorten 'em up a tad is
not seriously significant to our task. My own
'happy place' would be 6". Hope this helps . . .
Bob . . .
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roughleg(at)gmail.com
Guest
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| Posted: Sat Jul 04, 2020 9:53 am Post subject: Minimum length of fusible link wire segment |
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Thanks for this, Bob. That such a little blob of solder can have such a big effect is fascinating.
Pat
On Tue, Jun 30, 2020 at 4:06 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
| Quote: | At 07:18 PM 6/19/2020, you wrote:
| Quote: | Bob, in a recent post you state that there is a 9-inch rule of
thumb minimum length for fusible link wire segments. Could you
explain the physics behind that, please?
Pat |
Sorry to take so long on this . . . it took a few
miles of 'asphalt engineering' to figure out how best
to explain it. I THINK I've got a way . . . let's
give it a try . . .
Fusible link performance, indeed performance of any
fusible circuit protective device is tightly bound
to the fact that most electrical conductors have
a positive temperature coefficient of resistance.
This means, it's resistance rises with temperature
of the conductor.
The rate of temperature rises is a
function of power (watts) dissipated in the
conductor mass which is the product of current
(amps) times resistance (ohms). Consider a piece
of wire, any gage, hanging out in space with some
current flowing through it.
Here's a 22AWG wire that has been happily
carrying 20A for some time. Note that I
attached a thermocouple to the sample wire
out in the middle of the rather significant
free span. Suppose I conducted this experiment
with, say a 1" piece of wire? How might we
guess that center-span's rate-of-rise would
differ for the two cases?
http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
It's intuitive that heat generated in the
wire is conducted to the ends. Rate of rise
and temp equilibrium is influenced by sinking
effects of the terminals and radiation into
surrounding air. It's easy to deduce that a
short piece of wire is more resistant to
fusing than a longer one.
As a practical matter, the opening of an
electrically fusible segment tends to 'center up'
on the span. This is seen in the pictures of fuses
attached. As temperature begins to rise, heat
is generated all across the span and conducted
in both directions from any single point. The
CENTER of the span is least able to reject heat
to it's adjacent mass . . . it's getting warmed
up from both directions and also rising the fastest
due to effects of positive temperature coefficient.
Rising resistance increases electrical power dissipated
at that location more rapidly than anywhere else along
the span. Hence the temperature rise is regenerative
. . . the hotter it gets, the faster it warms up.
How long does this take? Complex question depending
on a constellation of conditions. Fuse and
breaker manufacturers work diligently to achieve
predictable performance in their products.
Here's a well written piece that explains the
thermal fiddling necessary to achieve predictable
i.e. fast versus slow blowing characteristic in fuses.
How about that little fly-spec of heat sink material
in the slow blo cartridge fuse?
https://tinyurl.com/ybkndmrc
It easy to see that control of the environment
surrounding the fusible event is critical.
A fusible link is in the 'HULK' family
of protective devices. Like its cousins,
the ANL, MANL and similar 'current limiters',
none are suited to the protection of the
more pedestrian appliance feeders. The
fusible link is SLOW and intended to clear
major faults (HUNDREDS+ AMPS) in the system
bus structure while maintaining a robust
indifference to rather severe transient overload.
Unlike our little plastic ATC friends that
can be 'hammered' into lower operating
currents by repeated excursions close to
but still short of their ratings.
Getting back to your question: Would a 6"
piece of fusible link fail to function?
How about a 1" piece? Probably not. They're
still the weakest link in the faulted pathway.
I am GUESSING that 9" figure is probably some
fusible link designer's 'happy place' for repeating
the in-service design goals he was working with.
Or perhaps it's a legacy hand-over from an automotive
industry specification.
In any case, wanting to shorten 'em up a tad is
not seriously significant to our task. My own
'happy place' would be 6". Hope this helps . . .
Bob . . .
|
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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