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Protecting the fat wires

 
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nuckolls.bob(at)aeroelect
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PostPosted: Mon Mar 06, 2023 8:42 am    Post subject: Protecting the fat wires Reply with quote

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Hello all! This is my first time posting on this list so I'd
like to introduce myself and say hello.

Welcome aboard!

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How do you determine which fat wires need circuit protection of some sort?
All of the branch circuits off of a bus get a fuse or a breaker, but what about
the wires feeding the busses from the contactors?
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Somebody pointed out, and I do remember reading, that the fat wires will
ground to airframe and burn a hole rather than burning the wire itself.
It seems to me that the airframe is the fuse. Why not just use an ANL?

Good question. Ground faults on FAT wires (distribution
feeders at risk ONLY from energy supplied by a battery)
tend to be 'soft' faults. Insulation is compromised
by installation error, terminal falls of it's stud,
etc. Soft faults are relatively high resistance and
perhaps even intermittent. Hence they can generate
a lot of heat that is likely to melt/burn the airframe
as opposed to the wire. (Recall the elevator cable
failure on the C90 in New Mexico).

I've witnessed a boat-load of soft faults that did
not open the upstream circuit protection. Most were
INSIDE an appliance . . . much smoke came out of a
pump motor, radio, etc. without popping a breaker.

The singular purpose for ANL/FWL current limiting
is to mitigate the effects of a HARD fault where
the at-risk conductor gets firmly connected to
ground (no arcing, no heat) and suffers the abuse
possible from an energetic current source . . .
ALWAYS the battery.

Faults of this type a generally limited to
situations were the airframe is pretty badly
munched. An associate of mine at Raytheon/Beech
served many years as an accident investigator.
He used to offer the notion that many post
crash fires involved airplanes where the battery
was contained in the wreckage . . . if the
airplane did not burn, the battery was often
found 'out in the weeds'.

Anecdotal to be sure but illustrative of the
potential for large energy releases from an
airworthy battery. It's not a good idea to
mess with momma bear. It's an equally bad
idea to mess with daddy battery!


Bob . . .

////
(o o)
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< Go ahead, make my day . . . >
< show me where I'm wrong. >
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In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.


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