>
><< I recommend that fusible links for aircraft be limited to the
> 24AWG and 22 AWG sizes for supplying some of the always-hot
> feeds to thing like the essential bus alternate feed, electronic
> ignitions in any form, electronic controlled fuel injection, and
> ammeter shunts. These feeds are generally limited to 5 amps or
> so max continuous demand.
> >>
>
>
>Hi Bob,
>
>In conflict with your response to my note on the Europa net, I note that your
>drawing Z-7 shows a fusible link of #16 wire in the upper right corner. I am
>unclear on whether you have changed your philosopy since Z-7 was published or
>this is a misprint.
John, you are of course, correct. The image that came to mind when
I saw the word "alternator" was the bigger brothers to the Rotax and
LOM PM machines . . . Fusible link technology is indeed appropriate
for protection of the alternator feed to the bus. I think it's a
stretch to go after 40-100 amp b-leads on the big guys . . . a cartridge
fuse is much cleaner.
>Also, would you please publish the fuse points for the various sizes of
tefzel
>insulated wire - the only reference I found indicated that #25 bare wire
fuses
>at 30A in free air; I assume that insulated wire fuses at a lower current due
>to heat retention by the insulation but cannot find published figures. While
>knowing that a link 4 sizes smaller will protect the wire, it would also be
>useful at times to know the expected fuse point.
I'll see if I can find some data on this. You're correct that insulation
and environment can have a marked effect on the fusing constant for
any conductor . . . the -4AWG rule is intuitively valid, the weakest
link in the chain can be guaranteed to fail first, irrespective of
the conditions that drive fusing constant.
>While your analysis indicated that I have used more fusible links than
>required, I am comfortable with them since they seem extremely reliable and
>are certain to prevent damage to wire bundles which could possibly be damaged
>otherwise. I thank you heartily for this elegant concept. And I appreciate
>your taking the time to critique my electrical system design - as a first
time
>builder, I need all the help I can get.
Very good. I'm pleased that you took the time to study the data
and arrive at a comfortable conclusion for yourself based on the
physics.
Norm Doty wrote:
are we really sure we want fusable links? i would think that anything that
heats up to the point of burning thru itself isnt a good idea, mabey we can
protect the things surrounding them from the heat today but everything
ages/weakens/rots and we know no one checks everything everytime.
A good question. Keep in mind that fuses burn through every time
they're called upon to act. Fuse manufacturer's design the product
so that the burning happens in a restricted space and doesn't
propogate damage to the rest of the sytem. The fusible link does
the same thing . . .it's a bit more spectacular because there is
some smoke but contining the relatively small conductor inside a piece
of fiberglas sleeving prevents propogation of damage.
Fuselinks have one VERY attractive feature . . . unlike the
breakers and/or fuses they replace, the parts count is VERY low.
The parts are made from ordinary wiring and terminals that may
be expected to perform as well 20 years from now as they do today.
That's difficult to say for fuseholders and circuit breakers.
Fusible links are especially useful in situations where the need
to protect is very unlikely but the need to have reliable connection
is strong . . . like electronic ignition and essential bus
alternate feeds. I've pitched about $100 worth of inline
fuse holders I used to sell because my builders are having trouble
with them after only a few years.
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