explosive situation?

[RayV]

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?

[Upscale]

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.

[dnoyeB]

I'd like to see that one on myth busters.

[Dhakala]

[todd]

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

[Sweet Sawdust]

[Max Mahanke]

A while back an engineer, MIT I think, wrote an article on the subject for one of the ww mags (sorry, don't remember which one). He essensially concluded (supported by formulas & math) that you can't move enough air thru a 4" pipe with a dust collector (let alone a shopvac) to generate enough static charge to sustain a spark of sufficient duration to ignite wood dust. But urban legends die hard. Anyway, the spark you saw is the same as the one you see when you slide out of your car and touch the key to the door. And if you touch the door with your finger instead of the key you'll feel it. If you find that discharge thru your finger annoying, that's about the only reason to ground non-metalic dust collection pipe. You can ground the metal dryer pipe if it makes you feel better but you will still be building static charge in the non-metalic shop vac hose. And I'll bet you never worried about that when you were using the shopvac for other things.

[somewildmonkey]

Ray,

You might want to run a wire between the aluminum flex hose and the metal leg of your table saw.

- MB

[Woodhead]

There is a possibility that one of the two machines has a faulty ground connection and you are seeing the connection of an ungrounded machine to a grounded one. Check to ensure there is zero-5 ohms resistance between the saw chassis and the shop-vac motor frame when they are plugged in...... or....There are kits available to ground plastic 4" hose to discharge static. Guess it could happen in aluminum pipe too. Wrap a copper ground wire around the pipe and screw the ends of the wire to the saw and DC frame.

Jim

[Mark & Juanita]

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 +--------------------------------------------------------------------------------+

[Michael Daly]

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

[George]

I'd have a whole lot more confidence in your method if they didn't put (insu/iso)lation on conductors to keep them from grounding.

Testing for stray voltage or bad ground is a good idea in the conducting saw.

[Gus]

Myth Busters is pseudo-science at BEST.

They raise more questions than they answer.

[Rick M]

"Gus" wrote

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

[bridgerfafc]

you cannot ground plastic pipe. plastic is an insulator. the kits are strictly for suckers.

[Gus]

So, by your logic, if it's in the FAQ, it MUST be true.

Remind me to contact you next time I have swamp land for sale.

[Rick M]

"Gus" wrote >

Factual evidence supports the FAQ entry in question. The absence of any contrary data presented as a counterpoint by you simply illustrates your emotional denial, rather than supporting your opinion.

If you have factual evidence, kindly present it. If not, feel free to believe what you will, but don't take offense if I don't share your beliefs.

Rick

[RayV]

I've searched the wreck archives, osha site, googled the web... all I can find are anecdotal statements.

"sawdust can explode..." "any combustible material in dust form is explosive..." "PVC cannot be effectively grounded..."

What I can't find are any specs or hard facts. If I rub my feet on the carpet and touch a doorknob with a handful of sawdust will my hand explode? Without any facts I'm extremely unlikely to believe that a home shop DC system can explode. Didn't one of Adolf's boys say, "if you repeat it often enough it will become true"?

Case in point: Just about every driver's manual I have read (ok, two) states that for a car with a manual transmission...you should put the car in reverse when parked facing downhill and in first when parked facing uphill. This is totally idiotic! That advice is based on the false assumption that the engine can only rotate in one direction. But still it is part of driver's ed training.

Doesn't anyone beLIEve in facts anymore? How much dust in what concentration is required for an explosion? How much of a static charge would be required to ignite that dust?

I only found one link that supposedly had data available but the link is now dead.

[Doug Payne]

When I was younger, I worked a couple of summers on Great Lakes boats carrying wheat. That dust can definitely explode:

the right conditions and in the right amounts, sawdust is no different:

on and on...

[Phisherman]

Best person to ask is your local fire department. They might have some good stories to tell.

Fuel + Oxygen + Heat (spark) = Fire

I th>I've searched the wreck archives, osha site, googled the web... all I