[R-390] (by request) W9LBB on Testing the HV Capacitor: a Cautionary Tale

[Mike Andrews W5EGO]

>From the Glowbugs mailing list, a tale of a jobfh that makes being a
mere sysadmin look _good_, fun, and above all _safe_:

: I dropped out of college for a couple of years (ran out of money)
: and went to work at a capacitor factory (which will remain unnamed
: to protect the guilty) as a Quality Control Tech on the production
: line.

: The owner of the place was a MSEE, and the company specialized in
: VERY high voltage capacitors, and high voltage power supplies. We
: made a VERY good product, but the production line testing techniques
: were, to say the least, hair raising; I saw more than one tech quit
: after one day on the job.

: Typical hipot test, on an 8 MFD, 5 KV capacitor; put it on a table
: with a grounded metal top. The tech grabs a pair of home built test
: prods, connected to a high voltage supply with a variac on it, and
: applies the prods to the cap terminals.

: Specs called for charging the cap to 10 KV and holding it there for
: 1 minute.

: If the cap doesn't fail, so far so good. If it fails, it does it in
: one of a couple of modes.

: The GOOD failure mode; there is a sharp metallic click inside of the
: can as the sections arc inside of the oil and discharge.

: The BAD failure mode; the solder seals blow and oil sprays all over
: the place!

: What made it interesting every day... before starting work, we
: checked a wet bulb / dry bulb thermometer and using it's readings
: computed the humidity. If it was over 75%, we spent the day working
: on small, low voltage stuff. If you tried to charge anything big,
: sure as hell it would flash over to ground with a sound like a 12
: gauge shotgun going off in your face!

: Most of the line techs were habitually on sedatives and antacids
: because of the fear factor... and the Old Man in the corner office
: insisted that 10 KV was TOTALLY safe, and we had NOTHING to worry
: about!

: Anyway... you now have a 5 KV cap sitting there charged to 10 KV.
: What do you do with it NOW?

: VERY simple, Grasshopper.... there is a THIRD home made test probe.

: This one is tied to a bank of resistors mounted on a sheet of
: plywood. You use it to discharge this nasty little beastie.

: Not all that simple, really... it's an art that it took a couple of
: weeks to learn.

: You approach the cap like you'd approach a cobra that you're trying
: to milk for venom. SLOWLY... CAREFULLY... you move in with the
: resistor probe. You DON'T move in too fast, or you get another
: chorus of the 12 gauge Remington Pump Sonata in your face as it ALL
: lets go at once, flashing over the resistor bank.

: If you've done it RIGHT... about 6 inches from the terminal there is
: a beautiful purple streamer to the end of your probe, and a faint
: hiss as the juice goes to ground thru the resistor bank.

: Once you have the dragon stuffed back in his cage, the FIRST thing
: you do is grab one of the MANY spools of uninsulated wire lying
: everywhere and your diagonal cutters, and slap a shorting wire
: across the monster... like RIGHT NOW!

: Hairy procedure all around.

: One day a new directive came around. There was a bottleneck in the
: plant on larger capacitors; they were piling up waiting for hipot
: testing. We had only ONE high voltage testing cage (it took caps
: up to 100 KV working!), so to increase production flow the Old Man
: in the Corner Office decreed that open air testing like I just
: described would now be done on caps up to 15 KVDC working... which
: meant using hand probes about a yard long to charge to 30 KV!

: Two guys muttered obscenities along with the words "I Quit!" and
: stalked out the door. The rest of us said "No Way, Jose", and told
: the owner so in no uncertain terms!

: He came around and told us we were all cowards, and told us
: something we hadn't known about him (and which explained a lot
: really)... he'd gotten the money to start his company in London
: at the end of WW2; he'd stayed after the war & worked at DEFUSING
: UNEXPLODED BOMBS for a living!

: HE decided he'd show us how to do it, and he wanted no more bitching
: from the production line!

: The old boy grabbed the probes and a 2 MFD, 15 KV cap and had at it
: himself. EVERYBODY backed up a few yards to watch as he hoisted this
: ten pound beast onto the table top.

: You never saw such an exhibition of arcing and sparking in your
: life! Repeated shotgun blasts as that cap arced to probes, across
: insulators, and even to the steel beams that held up the building!
: Yard long test probes proved to be a good bit more awkward to handle
: than he'd thought they were.

: He FINALLY got it to 30 KV... and then went in with the resistor
: bank probe.

: Sometimes you just live right, or at least lucky. Before he got the
: probe to it the cap failed... in the BAD mode!

: There was a muffled WHOOMP! and the entire top, six inch insulator
: and all, went flying into the air as the solder seals blew out!
: That was followed by a flying spray of mineral oil, and a billion
: bits of paper and aluminum foil as the capacitor sections shredded
: themselves. It was like a cherry bomb went off inside of that
: capacitor... absolutely BEAUTIFUL, and the Old Man was COVERED with
: the flying debris as most of us dove for cover!

: Gathering up as much of his tattered dignity as he could he wiped
: off his oily glasses, put them back on, and a badly shaken MSEE
: walked away without a single word.

: Within the hour a new directive came out returning us to the old
: open air testing limit of 5 KV caps.

: Sometimes all it takes to restore sanity is for theory to meet
: reality head on...

: Mr. T., W9LBB

Happy Straight Key Night and Happy New Year, everybody! Very 73 de

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin