Static electricity with dust collector

Unfortunately i have experience with dust explosions as part of my history in the food industry and particularly flour milling. What is critical is the concentration of dust. It is a small range but if you are in that range any spark is good enough. The fact that it doesnt happen in home workshops (that we know of) if probably due to an excess of dust. It seems wise to ground equipment but as has been stated it is not expensive or a big deal.

and in case you dont believe it can happen...

[ame="http://www.youtube.com/watch?v=zKMPC2l80NU"]YouTube - Flour Dust Explosion[/ame]

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Originally Posted by Sebastiaan56

Unfortunately i have experience with dust explosions as part of my history in the food industry and particularly flour milling. What is critical is the concentration of dust. It is a small range but if you are in that range any spark is good enough. The fact that it doesnt happen in home workshops (that we know of) if probably due to an excess of dust. It seems wise to ground equipment but as has been stated it is not expensive or a big deal.

and in case you dont believe it can happen...

YouTube - Flour Dust Explosion

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It can happen but the video shows an open flame.... I really believe you would be hard pressed to find an explosion occurring due to a spark. Open flame sure but not a spark. Also, isn't flour smaller in particle size anyway?

Interesting subject, However I would be more concerned about my lungs to be honest than an explosion. Got to be worried about all the air born particles the dust extraction doesn't pick up (shoots out)

Harlan

On the last job I was making I was using old recycled Blue Gum roof beams, the dust that came off when I was using the thicknesser was equally as fine as flower. First it came through the filter bag and then it blocked the filter bag and started to ooz out around the sides as the pressure buit up. Cleaned and emptied etc but my whole workshop was full of fine floating powder. Even if it was only a flash it wouldn't have been pretty not to mention the paint, thinners, turps, metho, chain saw fuel etc. Me I'll keep my little bit of copper wire on thank you, just in case.

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Originally Posted by Claw Hama

On the last job I was making I was using old recycled Blue Gum roof beams, the dust that came off when I was using the thicknesser was equally as fine as flower. First it came through the filter bag and then it blocked the filter bag and started to ooz out around the sides as the pressure buit up. Cleaned and emptied etc but my whole workshop was full of fine floating powder. Even if it was only a flash it wouldn't have been pretty not to mention the paint, thinners, turps, metho, chain saw fuel etc. Me I'll keep my little bit of copper wire on thank you, just in case.

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The chances of an explosion in this situation are extremely low. For a start the thicknesser motor would have been constant sparking :oo:, so why don't a lot more sheds blow up? Because the particle size distribution and concentration requirements for a wood dust explosion are very narrow. Even air-petrol explosions are more difficult to create than people think - think of how hard it is to start a flooded engine or one that has a partially blocked carby or injection system. Flour - air explosions are easier to create to create in a paper bag because purely by chance the particle size distribution and concentration of the flour dust inside a shaken bag are about right for an explosion.

A far greater personal risk from wood dust than any explosion over the longer term is constantly breathing this stuff in and is why the best place for a DC is not inside the workshop.

Explosions do occur in home workshops but they nearly always are preceded by a fire which your copper wire does nothing towards averting. So it makes far more sense to put $ towards the greatest risk reduction strategies ie the fire. One major risk in home workshops is fire. Once again I ask how many of us have a fire extinguisher and smoke detector in our shed?

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Originally Posted by Sebastiaan56

Unfortunately i have experience with dust explosions as part of my history in the food industry and particularly flour milling. What is critical is the concentration of dust. It is a small range but if you are in that range any spark is good enough. The fact that it doesnt happen in home workshops (that we know of) if probably due to an excess of dust. It seems wise to ground equipment but as has been stated it is not expensive or a big deal.

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It's not just concentration but particle size distribution. The flour example is not comparable to wood dust in workshops - see my previous post. Power tools make heaps of sparks made yet the incidence of workshop explosion is very low and most explosions are cause by fires not sparks. Instead of focussing on grounding and explosions we should be focussing on spotting fire hazards and making sure our fire alarms and extinguishers.

Somebody - please - post a link to a genuine dust explosion in a home workshop dust collector or let this go. Given the tens of thousands of woodworkers around the world using dust collectors there should be many recorded instances with pictures. Just like the cut off fingers, fires, electrocutions and hair caught in drill presses. Why can't we find them? The simple reason is that it is an exceptionally rare event - if it has ever happened at all.

I agree with BobL, spend the money on another safety issue that is much more likely to occur.

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Originally Posted by BobL

The chances of an explosion in this situation are extremely low. For a start the thicknesser motor would have been constant sparking :oo: (My thicknesser has a fully enclosed motor) , so why don't a lot more sheds blow up? Because the particle size distribution and concentration requirements for a wood dust explosion are very narrow(But imposible ?). Even air-petrol explosions are more difficult to create than people think - think of how hard it is to start a flooded engine or one that has a partially blocked carby or injection system. Flour - air explosions are easier to create to create in a paper bag because purely by chance the particle size distribution and concentration of the flour dust inside a shaken bag are about right for an explosion.( Which I believe would have been getting close to in my workspace on this project, cosistancy very similar to flower)

A far greater personal risk from wood dust than any explosion over the longer term is constantly breathing this stuff in and is why the best place for a DC is not inside the workshop. (Always were P2 masks if there is any dust around)

Explosions do occur in home workshops but they nearly always are preceded by a fire which your copper wire does nothing towards averting. So it makes far more sense to put $ towards the greatest risk reduction strategies ie the fire. One major risk in home workshops is fire. Once again I ask how many of us have a fire extinguisher and smoke detector in our shed?

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Hi Bob (I agree, I know the risk of dust explosion are very slim but why run the risk for the sake of maybe $5 and 10 min of time? I have 2 fire extinguishers one inside the workshop door (1kg)and one outside (3.5kg) and a hose within 3m. While the risk is minimal, there is a risk.)

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Originally Posted by Claw Hama

Hi Bob (I agree, I know the risk of dust explosion are very slim but why run the risk for the sake of maybe $5 and 10 min of time?

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Sure, but making any sort of deal about minimal risk factors get newbies very nervous and in the hullabaloo of the concern they lose sight of greater risk factors such as fire.

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I have 2 fire extinguishers one inside the workshop door (1kg)and one outside (3.5kg) and a hose within 3m. While the risk is minimal, there is a risk.)

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Sounds good. So you are well prepared with extinguishers, but a cold hard analysis of where best to spend the next $5 and 10 minutes would still be a fire alarm and many other things beforehand over grounding a DC. Better machinery guards, better PPE, better procedures, better tool racks, covers over blades, foot pedal safety switches on machinery, better storage of flammables etc would all rate as far more risk reducing than grounding a DC.

Mainly I spent the $5 and 10 min so I wouldn't get zapped every time I moved or brushed against the flex.

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Originally Posted by Claw Hama

Mainly I spent the $5 and 10 min so I wouldn't get zapped every time I moved or brushed against the flex.

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Yep - that is a good enough reason to do it!

OH here we go again.........this one pops its head up every few years.

No body yet has be able to produce one single example of a dust explosion in a small scale workshop due to static electricity or plastic ducts......none ... not one.....and this same issue has been beaten to death in every single woodwork forum on the net.

NOT one single case.

And there are very good proven reasons why not..and they are detailed in the links posted.

Even mythbusters had a go at a similar issue.

there are however plenty of people ( including manufacturers of steel duct systems) who are prepared to beat this one up for all they are worth.

metalic down spout is entirely unsuitable for dust extraction porposes....it is simply not strong enough and will collapse.
I have read a story about a bloke that went to great effort to build an extraction system built on metalic down spout......all went well......till the closed all the blast gates with the blower running........the whole lot collapsed.........sucked in:D

It can be argued however that placing foreign metalic earths particularly point sources like bolts and wires in and on ducts increases the small ( none, not one example) risks that exist.....among other risks.

Any static suppression efforts will simply be for the sake of comfort.

Consider also connecting anything to the electrical earthing system is electricains work and there are very specific rules about this.

as to why the static is on the outside of the duct........insulation resistance and high static voltages..........static voltages ( while largely harmless) are very high voltages....thousands of volts.........the plastics used have limited and finite insulation properties and there is no discharge path....no current flows.....so the charge builds up on the outside of the pipe.


the same issue exists in your home vacume cleaner, built in home vacume systems and a number of other situations.

now if you want something to worry about.....worry about your dust bag......is it smouldering while you sleep........did that nail you hit throw a spark into your dust bag........you have been cutting a heap of green timber......has it heated up enough to start smouldering among all the realy dry shavings ( remember the self igniting piles of grass clippings)..........so you were pushing that blunt blade thru that hardwood.......did it create embers and are they in your dust bag.

If you are worried enough to earth your ducting you should be emptying your dust bag every day.

there are many cases where dust bags have caught fire and burned down the workshop.

remember..... not one single case has been produced where static has caused a fire or explosion in small workshop dust systems....... not one.


cheers

Watch out for hearing aids, my father was helping me with some thicknessing he got a little to close to the vertical duct and zapped his hearing aid ...dead.
Luckily his new $1000+ whizz bang hearing aid was in getting fixed(typical that)and it was only an old spare that got zapped!

I think Soundman's post about sums it up. It takes more heat to ignite wood dust then flour particles. I once tried to deliberately create a small scale wood dust "explosion" and it took more effort than i thought. I heard about the phenomenon, so i wanted to have a go at it myself. Plastic jar, bit of wood dust from an orbital sander bag, spark plug, old petrol motor with magneto, vigorously shake the jar and crank up the motor to create sparks. Nothing happened. The mix ratio with air must be just right and i think you rather need really hot particle-like sparks (angle grinder disc sparks or sparks from a nail head inandvertantly being sanded down by a belt sander) instead of "electrical" sparks.

Smouldering in a dust bag is a much greater risk. I've witnessed a belt sander bag been set alight after an unseen nail had been sanded down together with the surrounding wood. There's first a scorching smell and then smoke and all of a sudden there are flames, probably because of the machine's hot air. It really helps when the bag pores are clogged a bit and the machine has been active for a while, so the dust laden air entering the bag is already heated up. With a properly "breathing" bag and a good airflow, the air is colder and the intenser flow will blow out any beginning flames.

By the way Soundman: i really enjoyed your speaker cone repair post. I wish i would have had your experience and knowledge written down in such a story a few years back, when i had a go myself!

As for static shocks, one particular shop vac hose often surprised me with that. I was fed up and took a length of bare copper wire with a clamp soldered at either end. The wire was bound along the hose by rubber bands at every 10 cms, with one clamp gripping the sanding machine's metal frame and the other clamp gripping a large metal object somewhere in the shed. No more shocks from the hose. I stole the idea from fuel tanker trucks (the ones used for airfield or gasoline station logistics) which have hoses with an added earth wire. First they connect the earth wire, than the hose and with the static voltage drained off, there's hardly any chance of sparks igniting fuel fumes.

Many shop vac hoses are now antistatic or very slightly conductive, so they drain off static automatically. Festo was one of the first brands to offer such solutions, since they are a great name in sanding technology and thorough dust handling (automotive and furniture industry, professional painters, interior decorating industries, etc.).

greetings

gerhard

Not all wood dusts should be treated equally. MDF dust ignites and can cause explosions at 200 deg C.

I am a professional hazardous area consultant (and a wood worker in the bit of spare time I have) and have investigation many sites where "incidents" have occurred - from factories to school woodworking facilities.

Best to treat this as a potential hazard people.

As to Mythbusters, their methods are hardly rigorous and I would not put my life or that of others on the line on the basis of their tom foolery.

If anyone wants to know more about actual issues with combustible dusts (wood and others) see my site - www.epeeconsulting.net

Happy crafting!

Paul, please post a documented case of where an explosion has occurred in a standard style HOME dust collector system and been caused by static discharge when cutting normal materials (not bags of flour :rolleyes: ). Despite looking over many years, and asking those who say it is dangerous to post proofs, nobody has been able to.

There are often people who state this is dangerous, and encourage all sorts of "solutions" - some expensive - when there is no proven threat. Home dust extraction systems simply have not been proven to be susceptible to dust explosions. Fires yes, and if money is to be spent then a fire extinguisher would be a good investment.

The biggest proven threat so far is getting 'zapped'.