>Looking at the issue from the perspective of someone who has been in the
>electronics and avionics REPAIR business for more years than I care to admit,
>the choice is rather easy. While they are less expensive, lighter, and more
>compact, fuses and fuse holders/blocks are less reliable in the long run. I
>mentioned fuse holders and fuse blocks specifically because they tend to be
>the weakest part of the fuse system. As the contacts age, resistance
>develops, which leads to localized heating, which leads to greater
>resistance, more heat, etc. The fuse often fails as the result of the faulty
>holder/block.
Agreed, and breakers often fail for the same kinds of reasons. The
ATC/ATO fuse blocks we recommend are very new designs that have
eliminated many of the failure modes found in older, clip type
fuse holders for cylindrical glass fuses.
I had a guy walk up to me at Sun-n-Fun a few years ago and
while turning one of our fuseblocks over in his hand remarked,
"I never put a piece of $!@#$ like that in MY airplane . . .
the salt air down here would eat it up in no time."
I asked him if the salt air does NOT eat on breakers, wing
skins, and most other airplane parts in addition to fuseblocks?
Further, consider that the fuseblock with 20 protected circuits can be
REPLACED as a preventive maintenance item, say every five years
for a cost of about $35 plus a half hour's work. How many
hours/dollars is involved in replacing all your breakers?
He didn't answer . . .
>Again, I am not analyzing this problem from engineering test data, but from
>real world repair work. My experience has been that component mean time
>between failure (MTBF) ratings and what happens in the field can be
>significantly different. Bottom line is that we experience far more
>fuse/holder failures than failed circuit breakers.
Absolutly agreed . . . but are any of the failures less than
say 10 years old? Have they suffered humidity and/or salt
air damage? Are they modern blade type fuseholders with gas
tight electrical connnections between holder and fuse? Unlike
fuseholders of the past, I fully expect the majority of
these modern devices to be in place the day the airplane
is scrapped.
>Consider switch/breakers for a moment. Since you need a means to switch
>circuits as well as protect them, why not use one component that accomplishes
>both tasks. I have installed Airpax brand switch/breakers in my Cozy with
>very satisfactory results. They do not take much more room than standard
>toggle switches, so I was able to use 17 of them in the small offset area at
>the top of the Cozy panel. I found a surplus source for the 5amp units under
>$5.00 ea. Unfortunately other current ratings range from $14 to $20 ea.
>Remember though, you are replacing a switch and a breaker (or fuse).
>Like I said initially.....my 2 cents worth.
My objection to switch breakers is that you are forced to have an
extra bus on the left side were most people put their switches.
This in addition to fuses and/or breakers elsewhere. The
nice thing about fuse blocks is that you fabricate NO busses.
ALL busses are almost totally enclosed in the plastic insulation
housing of hte holder. The fuse blocks have much LESS always
hot metal exposed than any combination of switches, switch-breakers
and breaker panels . . . and you don't have to fabricate
a single bus bar.
17 switch-breakers? Then ALL of you power distribution what right
on the same panel area as pilot operating controls for thing that
did not need them . . .
Bob . . .
////
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