nuckolls.bob(at)aeroelect
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 Posted: Wed Nov 23, 2022 8:05 am Post subject: Fusible Links in lieu of ANL protectors |
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| Quote: | Thanks, Bob.
Do I need to protect the fusible link with a fire sleeve, or do the ready-made versions come with protection? And, for that matter, does the bulk fusible link wire come with protection?
Scott |
Check the ones in your car . . .
I wrote about his back in 2018 which I repeat here:
| Quote: | Thanks so much..as usual a real education.  I had no idea that a 22 AWG fuselink is equivalent to a 30-40AMP fuse.  Intuitively I thought it was much less.  Much appreciated.
So if a 22AWG wire is normally "rated" to 5A which means 10degC (Ch8- current rise presumably therefore needs 40A to get to melting point. |
Exactly . . . see https://goo.gl/zye61w
Circuit protection is about keeping insulation
temperatures comfortably below maximum rated
operating vales. Wire size is all about maintaining
distribution voltage drops below acceptable levels
based on system design goals. 5% is the max-drop
rule of thumb although other values may be adopted
by program managers.
Wire sizing tends to be very conservative . . . unless
you are as weight-sensitive as a moon mission or
Voyager flight, the design rules will keep wires
pretty cool.
| Quote: | | By the same token the 16AWG fuselink at the starter contactor (from the alternator) and from the battery contactor to the buss performs in a similar way. If this is correct the 16AWG fuselink rated at 12.5A for 10 degC current rise will melt at something like 100A. Whereas the 12AWG wire it's protecting will melt at something like 150. |
Fusible links in vehicles are expected to protect
major feeders of a distribution system against
hard faults . . . generally experienced only during
major disassembly of the vehicle. By hard fault,
were talking many times hundreds to thousands of
amps supplied by a battery . . . not the soft
fault precipitated by a failing appliance.
https://goo.gl/QEBeZs
Generally . . . When we use plain-vanilla wire
as a fusible link in the OBAM aircraft world, it's
a good idea to use the silicone/fiberglas sleeving
over it to limit the propagation of heat damage
during a fusing event. In the ground transportation
industries, fusible links are fabricated from wire
having friendlier outcomes during the fusing event.
Hypalon insulation (A Dupont product that targeted
electric locomotives in mines) is used on many
fusible link replacement assemblies available at your
local car parts stores . . . or in bulk wire on
places like eBay.
https://goo.gl/RH2eEY
The material selected for a fusible link has
little to do the loads presented by downstream
appliances. The wire in fusible link is EXPECTED
to burn . . . at many degrees above the insulation
rating. Selection of material is about reduction
of risk in the rare but catastrophic event.
So, fusible link wire is simply used as a short extension
of the feeder to be protected by splicing a short chunk
(6-10 inches) onto the end of the feeder.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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