nuckolls.bob(at)cox.net
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 Posted: Fri Feb 15, 2008 6:45 am Post subject: No Title |
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Comments/Questions: How many terminals would you say is the max on
the hot side of a batt contactor?(4?)
Ideally, just one. There are two considerations for
stacking things on the single stud.:
(a) the more wires . . . presumably most of them
'fat' wires . . . the greater the torque-moments
that tend to unwind the nut. Obviously, if
the installer makes a considered effort to
insure integrity of the join (thread locker,
locking nut, making up the joint to the
max recommended torque value for the
materials, etc) the lower the probability
of in-service loosening.
Of course, supporting the exiting wires
soon after they depart the nut will reduce
the free-mass waving in the breeze and
reacting on the join.
(b) thread length of the exposed terminal is
an obvious mechanical limit. The books
tell us that we should drive for a minimum
headroom of 1.5 exposed threads sticking
out of the nut when we're finished. Quite
often, this is the biggest limiting factor
for how many terminals get stacked on the
stud.
Stud length can vary significantly between
brands for similar devices. See:
http://www.aeroelectric.com/Pictures/Contactors/stud_length.jpg
http://www.aeroelectric.com/Pictures/Contactors/strtrctr.jpg
http://www.aeroelectric.com/Pictures/Contactors/6041_Contactor.jpg
There is no industry-wide practice for
setting limits on how much 'stuff' you
stack on these studs. It's up to the
designer/installer to be cognizant of
the conditions that tend to undo the
design and exercise due diligence
in crafting the joint.
Where can I find the appendix for suppliers?
On the 'net. In early issues of the 'Connection
I offered a listing of handy suppliers for
tools, materials and parts useful to the
OBAM aircraft community. That was an
exceedingly difficult list to keep up to
date so, as the first paragraph of Appendix
Z points out, we deleted appendices A,
C and K in favor of offering those coverages
on the 'net.
And finally, is there a chart of recommended circuit breakers for a specific
AWG, you say 5 amp for 22awg, what are other recommendations?
Breakers protect wires . . . period. Maximum
recommended loads on any given wire are NOT
based on danger to the wire (copper) but to
the insulation. For example, in this slide
taken from my weekend seminar presentation . . .
http://www.aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
we see a 22AWG wire loaded to 20A on the
bench. The insulation temperature isn't even
close to being over-stressed. It's got another
40C to go! At the other end of the spectrum,
cross country power lines that make up the
nation's distribution grid are oft cited as
being 'dangerously overloaded'. Are they
at risk for parting the line due to melting?
No, their temperature rise is so high that
coefficient of expansion effects are causing
the wires to sag into trees. The risk is
for tripping the line off when system arcs
to ground through a tree. I'm recalling that
the great Northeast Blackout a few years ago
was triggered by a power line dropping into
a tree up in Canada.
Yes, there's a VERY conservative wire rating
table in the book and repeated here:
http://www.aeroelectric.com/Pictures/Wire/Wire_Table.jpg
Here we see that the 22AWG wire is industry
rated (or perhaps de-rated?) to 5A capacity.
This isn't a 'rule of thumb'. It's the product
of a process that says based on our experience
(airplanes with environments that range from
-55C to 100C) and bundles of perhaps hundreds
of wires, running no more than 5A through a
22AWG wire offers a 99.999% degree of confidence
for satisfactory-lifetime-of-the-airframe-performance
(SLOTAP).
Should you choose to tailor your own use
of 22AWG wire, there are innumerable tutorials
on how this is accomplished . . . but I'll
suggest your time is better spent getting
all the rivet heads smooth and the upholstery
seams straight.
Further to this, if I have a 2 amp load on a 22AWG, do I still need
5 amp CB to protect it, or is it OK if the breaker doesn't exceed
5 amps, but is lets say, 3 amps?
As discussed above, you can do about anything
you want in terms of protecting any give
chunk of wire. And based on your considered
analysis of how the wire is used in YOUR airplane,
the breaker might be anything from 1 to 10 amps!
But breakers below 5A are more expensive and
breakers above 5A start pushing out into the
corners of the performance envelope that reduce
your SLOTAP confidence.
Love the book, learned more than in 2 years at college!
I'm pleased that you're getting a good return
on your investment.
Bob . . .
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