nuckolls.bob(at)aeroelect
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 Posted: Thu Oct 08, 2020 8:54 am Post subject: AC 43.13-1B circuit protection recommendations |
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At 07:25 AM 10/8/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Tailwind1" <timmikus38(at)gmail.com>
Looking at table 11-3 in the 43.13, the table shows lower amperage recommendations for fuses versus circuit breakers in many cases. Bob has shown us how much quicker a fuse operates versus a CB so logic seems to suggest a higher fuse rating versus a CB. What am I missing? |
I haven't a clue. I questioned this in the critical review
we submitted through EAA many moons ago but the item
was never modified or explained. If I were to WAG this
one, the writer may have been comparing performance
of fuses vs. magnetic breakers. In this case the breakers
are faster than fuses . . . but TERRIBLE choices for
general use in light aircraft power distribution.
If one simply compares the fusing dynamics of the
fuses vs. thermal breakers, the AC43-13 suggestion
doesn't make sense at all.
Actually, there's no reason to up-size the rating
of protection based on the technology. Breaker and
wire ratings are based on effects of the wire's
natural self-heating and is concerned only with
the wire's INSULATION. Long term heating effects
are a function of NORMAL current flows, voltage
drops in wire and temperature of the environment
which includes both the airframe and adjacent
wires in bundles.
That's a real snarl of snakes to unwind for
all but design goals of the most pedantic
of program managers. For our purposes, the
rule of thumb is, "if in doubt, up-size the
wire and protection to the next step."
Moving up can only reduce effects of NORMAL
heating. Consider that 99% of all
breaker/fuse events are caused by ABNORMAL
current well above the NORMAL . . . like
hard shorts or catastrophic failures in
appliances. We are virtually never victims
of a failed insulation condition . . .
The short answer is: Your common sense questioning
of the AC43-13 assertion is valid. Ignoring
it based on your own study of PROPERTIES
OF MATERIALS is not a risky thing to do.
Bob . . .
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