"Geo Clute W7LFD" <gclute@attbi.com> says --
>I've experienced some arcing problems with porcelain standoffs. It
>seems to start with a carbon pitting and then at some point it heats
>the porcelain and causes an explosion inside the cabinet.
I've heard of this but never seen it. Take a look at a porcelain
standoff. You'll see it's glazed on the body, but not on the ends. The
bare porcelain is slightly porous and absorbs water and other impurities
from the air. Over time -- remember your stand-off is likely WW2 surplus
-- enough stuff gets in that (1) the insulator becomes slightly
electrically conductive and (2) at some point after that, resistive heating
in the insulator is great enough to cause the impurities to dissipate
faster than the tiny pores in the porcelain will allow. And it explodes. I
suspect that mechanical and RF heating play a factor too.
The vaguely remembered reference suggested refiring the insulators to drive
off the impurities. I don't remember the particulars, but I think a small
(explosion-proof) vacuum chamber that could be heated to about 160 F at a
couple inches absolute for several hours (or days) would do the trick.
This is something that could be made with commonly available junk.
A glass or teflon (or delrin in a low-ambient temp environment) standoff
would probably avoid the problem.
Comments?
Jim N6OTQ
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|