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Minimum length of fusible link wire segment

 
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roughleg(at)gmail.com
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PostPosted: Fri Jun 19, 2020 6:24 pm    Post subject: Minimum length of fusible link wire segment Reply with 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


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PostPosted: Tue Jun 30, 2020 10:51 pm    Post subject: Minimum length of fusible link wire segment Reply with quote

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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PostPosted: Sat Jul 04, 2020 9:53 am    Post subject: Minimum length of fusible link wire segment Reply with quote

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 . . .


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