Ferg, that "arc-over" problem in airliners still has FAA stumped, and
the prime focus is Kapton insulation, much thinner to save weight. I
don't think many small airplane mech's could say they've ever seen the
phenom in Tefzel. The planes just don't see the cycles and don't have
H-V wires in the bundles - 14V won't arc unless the conductors contact
adjacent wire or ground metal (completely chaffed), which they will
encounter.
Also curious how breakers are better than fuses seems gospel only in
airplane building. Even the respective product literature for fuses &
breakers contain no such hint, but rather they each have proper
application. E.g., Makita cordless drills have a breaker, as stalling
on a tight screw can ruin a ni-cad battery. There's nothing like that
on the Europa (beyond maybe alternator). It's things like big motors
or heating elements that cause "nuisance trips," and the literature
says so.
Changing fuses in flight is not easy, but like the breaker, shouldn't
be, except is dire circumstances. It popped for a real reason. On an
avionics box, reapplying the juice can cause a $300 repair to go to
$600, the way they price 50-cent components.
Best,
Fred F.
Fergus Kyle wrote:
>
> ...
> There is another reason why I won't use switch-breakers or
> whatever those "automatic" C/Bs are called - nor would i prefer to have
> handy the pullout C/Bs so beloved of airline engineers.
> I shared an article about C/Bs several years ago. It discussed
> the propensity for danger in the wire used since the 80's - the tefzel
> covered stuff. It has a habit after vibration, moisture and heat (admittedly
> not so intense in most of our milieus) of cracking. Again this occurs
> geberally after a number of years. What happens is the teeny cracks open to
> moisture, the wire carries too much current and the C/B pops open. That's
> good.
> ` What prompts the danger is pushing the offending button back in
> to see what happens, time two. That's bad. What happens is the surge bak
> into the circuit prompts a tiny but hot spark which overheats the tefzel
> locally and "turns it into fuel". In some cases it explodes enough to blow
> apart adjacent lines - not a good thing. Nor perhaps is it exactly
> applicable to our operation.
> Give me fusible links (a la Nuckolls) or give me fog. As Bob N
> says, "make a case for getting along without it because you've got a
> backup", then just get along without it till you're on the ground.
> Don't turn it on again to see what happens. It might.
> Cheers, Ferg
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