>I've thought of a couple things that could cause problems- is your heating
>element inductive at all?
No, but at 120V, and considering the mains are inductive (transformers,
y'know) you always get an arc when interrupting current.
>If so, if the switching happens at the current
>peak, you could draw a decent arc across the contacts, and on a similar
>note, could the you be drawing an arc across your switch contacts, by the
>act of moving a switch?
We DO draw an arc when opening the switch. the problems usually happen when
CLOSING the switch, presumably there is not much of an arc when the switch
is first closed. hmmm......
>We DO draw an arc when opening the switch. the problems usually happen when
>CLOSING the switch, presumably there is not much of an arc when the switch
>is first closed. hmmm......
My pick (PIC?) would be the arc on opening the switch slowly deposits a metal
film across the board. There may also be an impurity in the laminate that
occasionally gets you. Then when you close the switch after a number of
depositions of metal, there is enough of a path for current, and its downhill
from there once there is a carbonized path.
I think you will find this is why slots are often cut in boards around these
sorts of areas, to stop potential carbonization tracks with dirt and moisture
build up.
With the addition of time, humidity and the
steady deposition of material from arcing
(as well as any imputities latent on the
surface either through, again, time or original
manufacture) a phenonenon know as
"a creep path" can develop ...
I second the "slot" solution mentioned by Alan as we
have been successful in the past with this technique too.
+AD4-would be the arc on opening the switch slowly deposits a metal
+AD4-film across the board.
Aha+ACE- I know from doing UL shorted component tests, that an arc can
definitely spray metal everywhere. I've 120V electricity jump a gap of
nearly an inch by traveling through the plasma of an exploding PC board
trace (shorted component test is where you intentianally short components
and find out if the machine will catch fire, etc. these tests are required
for UL certification)
+AD4-There may also be an impurity in the laminate that
+AD4-occasionally gets you. Then when you close the switch after a number of
+AD4-depositions of metal, there is enough of a path for current, and its
downhill
+AD4-from there once there is a carbonized path.
You may be onto something here+ACE-
+AD4-
+AD4-I think you will find this is why slots are often cut in boards around
these
+AD4-sorts of areas, to stop potential carbonization tracks with dirt and
moisture
+AD4-build up.
I agree, a slot or two will certainly help. There are also high voltage foaming coatings which can be applied to eliminate tracking but I have not tested these coatings at very high temperatures.
( - : When used on the back of TV tubes in humid coastal areas these coatings completely eliminate any tracking or discharge. not even the normal 'crackle' on switch off. A downside is that the tube then becomes a very high voltage capacitor and must be discharged manually even hours after switching off before working on the circuit. : - )
>>> spam_OUTjvpollTakeThisOuTDALLAS.NET 06/20/00 06:10PM >>>
With the addition of time, humidity and the
steady deposition of material from arcing
(as well as any imputities latent on the
surface either through, again, time or original
manufacture) a phenonenon know as
"a creep path" can develop ...
I second the "slot" solution mentioned by Alan as we
have been successful in the past with this technique too.
If I were you, I'd stop speculating at this point and run a test.
Get a unit with 500(?) cycles on the power switch (aged but not yet
failed) and hi-pot test it. See where the arc goes? Try some fixes
based on this data. The "slot fix" may be the right answer, but you
won't really know til you run the tests.
Another avenue would be to investigate different contact material
that doesn't arc-spray as well. Power relays use special alloys, as
you probably know, which have *Higher* contact resistance, but the
tradeoff is cycle life when interrupting AC. I know you are very
cost sensitive / high volume, but its very little material...
I think you will find that electric field is the dominant effect for
where the arc plasma material ends up. There are several metal
finishing process that do exactly this on purpose.
Sounds like you're close. Good luck.
Regards,
Barry.
------------
Barry King, KA1NLH
NRG Systems "Measuring the Wind's Energy" http://www.nrgsystems.com
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