nuckolls.bob(at)aeroelect
Guest
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 Posted: Wed Jul 22, 2009 3:31 pm Post subject: differences in fuses versus circuit breakers |
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At 10:06 AM 7/22/2009, you wrote:
| Quote: | Hi Bob,
For the 12v/24v DC circuits typically used in our aircraft, is there
any difference in the rating one would select if using a fuse versus a
circuit breaker? In other words, a hypothetical example, if one arrived
at a value of 15 amps to protect a wire, would a 15 amp fuse be
interchangeable with a 15 amp circuit breaker for protecting the wire?
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This is a classic example of how AC43.13 offers advice that
is not explained and makes no sense based on the physics.
For example, AC43.13 says their table applies to fuses per
Mil-F-15160. Here . . .
http://www.dscc.dla.mil/Programs/MilSpec/ListDocs.asp?BasicDoc=MIL-PRF-15160
we see that the general spec covers a couple dozen types
of fast acting fuses (1 hour trip at 135%, seconds to few
minutes at 200%). The spec called out is constellation of
FORM/FIT/FUNCTION specs for a variety of fuses.
Now we go look at Mil-C-5806G . . .
http://www.aeroelectric.com/Reference_Docs/Circuit_Breaker/Mil-C-5806G.pdf
This is a QUALIFICATION TEST spec for circuit breakers
of unspecified performance. The poor sap who takes AC43.13 table
11-13, and the specifications called out (gee the talked about
Mil-W-5088 wire too!) will be pretty much in the dark as to how
a considered selection can be made. MOST readers will simply check
to see that the sales brochure for each part under consideration
simply cites these specs . . . without a clue as to how the
part performs to design goals.
| Quote: | I ask for two reasons - in a discussion on another mailing list,
someone has said that fuses and circuit breakers of the same rated value
are not interchangeable, and AC43.13-1B table 11-3 on page 11-15 in
Section 4 shows a 15 amp CB or a 10 amp fuse to protect a 16 gauge
copper wire (picture attached).
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For our purposes, breakers and fuses are size-for-size interchangeable.
The table cited from AC43.13 is but one of many examples of
poor and/or senseless advice. Please consider running section 11
through the shredder and come here to the AeroElectric List for
advice based on experience and proven recipes for success. The
article at
http://www.aeroelectric.com/articles/wiresize.pdf
speaks to the practical aspects of wire sizing and selection
of circuit protection. For the airplanes we're building,
it DOESN'T need to be complicated.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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