nuckolls.bob(at)aeroelect
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 Posted: Thu Jan 01, 2009 6:31 pm Post subject: Follow-up on Carling switch failures . . . |
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A few weeks ago a discussion here on the List explored
a "rash" of failures in Carling switches sold by a number
of sources including B&C as their S700 series devices.
The earliest manifestations of failure produced obvious
effects of heating including discolored metal parts,
deformed metal parts, loose rivets, charred housings
etc.
The most convincing hypothesis generated from these
conditions centered on loss of conduction followed
by increased resistance, increased heating, etc., etc.
Sometime later in the discussions, we added the idea
that root cause for these failures might be a mechanical
loosening of the joints at locations (3) and ( in
the following sketch . . .
http://tinyurl.com/8cm52v
This loosening would be the byproduct of bending
moments on the tabs by too-tight bundling of wires
behind the switches. If these tabs are not favored
with 'slack' leadwires, effects of weight and vibration
of the whole bundle can be impressed on these joints.
A postmortem on the first such failure was conducted
back in the spring of 2006 and produced this document.
http://tinyurl.com/2a2qqp
Over the past few months, I've received about a half
dozen failed switches. Most showed effects of heating
as shown in the article cited above. However, TWO of
the recently acquired failures showed NO effects of
heating. They did have loose rivets that caused the switch
to become intermittent . . . but circumstances did
not produce subsequent heating.
Based on this new information, I'm ready to hang my hat on
the hypothesis I offered some months ago namely:
The rash of failures experienced are most likely caused
by inappropriate wire dress behind the switch that
FIRST causes loosening of riveted joints. This condition
MAY be followed by overheating events so spectacular
as to divert attention away from the loose rivet.
I can now comfortably suggest that failures we've
seen (and perceived as a sudden up-tick in frequency)
have been happening in these products for years. Further,
the smoked switches were not a failure on the part of the
manufacturer to meet electrical specifications nor
upon users for failure to respect those ratings.
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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[quote][b]
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