explosive situation?

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My hi-tech DC system for my TS consists of a shop-vac connected to a 30 gallon plastic dust bin. The dust bin is then connected to the bottom of my contractor saw with a flexible aluminum 4" dryer vent. I normally have the back of the saw closed off with 1/4" plywood and this system works fairly well.
Yesterday I needed to clean out the inside of the saw, so I unplugged the saw and turned on the shop vac. I then bent over and pushed the sawdust into the chute on the bottom of the saw. While I was doing this everytime my hand partially blocked the hole leading to the dryer vent it would compress slighty and touch the frame of the saw. That is where I saw the spark.
A tiny spark between the leg of the saw and the aluminum vent connected between the plastic dust bin and the plastice adapter on the saw. In theory, static can ignite fuel vapors or gases, but wood dust?
Both the shop-vac and TS are plugged into properly wired 3-prong outlets but this is only really grounding them in case of a motor failure. Should I somehow ground this DC system or just ignore the static?
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Wood dust in the right proportions can be explosive. You can probably find more information on it in the rec archives, but I'm sure quite a few people will respond with additional information.
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Upscale wrote:

I'd like to see that one on myth busters.
--
Thank you,



"Then said I, Wisdom [is] better than strength: nevertheless the poor
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dnoyeB wrote:

http://www.ydr.com/newsfull/ci_3604224
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wrote in message

No one says it can't happen in an industrial situation. To my knowledge, no one has ever documented a case of a dust explosion in a home workshop. If there is such documentation, I'm open to seeing it, but AFAIK, it has never been produced here when this topic has surfaced.
todd
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todd wrote:

Most of the theoretical calculations posted here indicate such an explosion is unlikely under stady-state conditions. Maybe if a clump of dust gets thrown into the air all at once, like when changing dust collector bags or filters and there is a pilot light nearby.
A few years back here in rec.woodorking someone posted that the Ann Arbor, Michigan Fire Department had documented an average one such accident per year, over the past 25 years, in home workshops, with the most common ignition source being a spark from an older open-frame electircal motor.
I've not been able to find the article in the Google Archives.
That's not at all the same as a static spark. One supposes these may have actually been vapor explosions though the topic of the thread was dust explostions.
If you throw a handful of sawdust onto a fire you can get a nice fireball. So there is no question that sawdust and air can be ixed and ignited. That's a far cry from your garage suddenly exploding while sanding down a coffee table.
--

FF


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I should have so much dust and that big of a collector!!
wrote in message

find
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wrote:

At the time the article was written, static electricity as a cause was purely speculation. They also indicated a hot ember could have started the fire.
Mythbusters would be a good venue for testing this theory. I seriously doubt that static electricity has enough energy to ignite a dust explosion. An electric arc or hot piece of metal contains significantly more energy to start the required reaction.
+--------------------------------------------------------------------------------+ If you're gonna be dumb, you better be tough +--------------------------------------------------------------------------------+
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That depends on how much static and how fine the dust. While I doubt that walking across a carpet and then touching a dust collector would do it, the kind of dusts that can be found in silos of anything (and bulk carriers being loaded etc) in conjuction with a significant static source could do it. Continuously running equipment can continuously generate static.
Mike
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On Sat, 01 Apr 2006 14:16:34 -0000, snipped-for-privacy@host122.r-bonomi.com (Robert Bonomi) wrote:

I should have been more specific. My sentence above should have read, "I seriously doubt that static electricity has enough energy to ignite a wood dust explosion from the dust that would be generated in a typical home woodshop."

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On Sat, 01 Apr 2006 09:47:58 -0700, Mark & Juanita
This might help:
http://www.chemeng.ed.ac.uk/~emju49/SP2001/webpage/deh/deh4.html

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On Sat, 01 Apr 2006 14:16:34 -0000, snipped-for-privacy@host122.r-bonomi.com (Robert Bonomi) wrote:

I (and every professional electrostatics expert I've spoken to) disagree.
Got the numbers ? What's the discharge mechanism ?
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~10^10 joules

Thunderstorm.
;-)
Ken Muldrew snipped-for-privacy@ucalgazry.ca (remove all letters after y in the alphabet)
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Andy Dingley wrote:

Ever hear of lightning?
If a lightning bolt strikes your dust collection system, duck!
--

FF


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Well, *that's* the problem. you listened to the experts. <grin> Me, I just _did_ it. reliably and repeatably. around 35 years ago.
It was a high school 'science fair' project.
Plexiglas 5-sided cube, about 8" on a side, with a circa 3" spike through the middle of one face, the outside of which was connected to a fair-sized grounding cable (heavy-duty automotive jumper cables).
Then I had a fairly hefty Van de Graff generator -- one which could pull a 'spark' somewhere around 9". Agreed, this is getting well towards 'artificial lightning', but it _is_ still just static electricity.
Throw some cake flour in the box, hold a cover piece over it and 'shake well', set it back down, slide the cover off, and promptly take the 'wand' connected to the top of the Van de Graff and bring it across the box, from the edge opposite where the spike was. ***KA-BOOOOM***!!!!
At around minimum explosive density it was merely "_really_ loud". And it got more so, as the dust density increased.
Even without any dust, the static discharge alone made a pretty fair amount of noise.
Note: then there were they guys building the *big* (as in 8-foot-plus tall) Tesla coils. _Those_ things were dangerous! While admittedly not 'static' machines, *anything* that can maintain and sustain an atmospheric arc (a "Jacob's ladder") in excess of 4 _feet_ is deserving of some serious respect. :)
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Sounds like they had a nice little toy there. In a previous career, I designed and tested high-voltage insulators. I tested using an AC set that could generate 1000kV and create an arc over 15 feet long. We also used an impulse generator which simulated lightning and switching surges that could go up to about 2500kV. All of that was very interesting.
If you want even more fun, do high *current* testing and run 20kA through some hardware. Ka-boom!
todd
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That jibes w/ my recollections -- vague memories say they were playing with circa 250KV.

Shall I mention the time I saw somebody swing a crane boom in to the _feeder_ lines to an operating sub-station, and only about 500 ft away from the sub-station. The _flame_ front was merely a couple of =hundredfeet *wide*. It went away fairly quickly -- like a second or so -- but there were also several 'booms' and small clouds of smoke from the station itself -- apparently 'protective' disconnects when the three-phase feed went *that* unbalanced.
I've heard _one_ louder noise than that -- a lightning strike on a telephone pole about 50' outside our back door. I *saw* the glass patio-doors I was sitting not 10' away from bulge (about 3 inches!!) from the pressure wave.
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Robert Bonomi wrote:

Fun. When I was a kid I was tossing a javelin (dead and desiccated hemlock plant with the root still on the tip) over the powerlines.
Was. On the last vault over, the back struck the near-side powerline on the top, and the front settled down on the far side. Smoke, and when it started flaming I turned to run inside and plead my case, when I saw the front side of the house light up in a bright blue light. I ducked and turned to see remnants of my javelin spiraling down in flame.
Couple minutes later and a PG&E truck came tooling by...
er
--
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dnoyeB wrote:

Myth Busters is pseudo-science at BEST.
They raise more questions than they answer.
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But this group is solidly science-based, aren't we? After all, we've a wealth of experience ... umm make that a large vocal group that believes they have solid factual data, with a minority that provides verifiable data, sound data analysis and (usually) a well written presentation of that analysis.
Read the FAQ. The MYTH of static-induced home dust collection system explosions is well documented.
Get to know Google ... in fact, make Google your friend. The MYTH of static-induced home dust collection system explosions is well documented. Especially important is to read the rec.woodworking archives, searching for dust + explosion.
Rick
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