DC and static charge

[Rob Glynn]

I just had a look at Terry H's shop and was impressed. Being a recycler, I was pleased to see cabinets and shelves given a new purpose.

My question.

Dust, particularly sawdust, is flammible and can be explosively ignited by a discharge in plastic tube. I have heard of a number of ways to earth a system, metalic tape or wire through the inside of all the tubing is one.
My plan, one of these days, is to fit up a system using only light metal tubing. In the meantime I have a mobile DC and just move it from machine to machine.
What do you do about static charge build up in platic/PVC tubing used in a dust collection system?

[Gary]

I personally know next to nothing about the physics of static electricity, and I'm sure you'll get some responses from some experts. However, I (like many others) have read many, many, many posts on this subject (about even with 'Does a shop need a refridgerator?'). The MAJORITY of opinions by those with real credentials is that in a home size system (like under 1500 cfm) static discharge in plastic duct is not a danger, just an annoyance if it shocks YOU.

I believe someone offered a large cash reward for a single CONFIRMED case of an explosion caused by static discharge in a small system, and no one has collected.

That said, you have to do what makes you comfortable.


Gary

[Terry Hatfield]

Rob,

I do nothing!!!! I have had no trouble with static at all. Never even had a shock...never!!! I really truly belive that the amount of static generated in a PVC duct system is directly related to the relative humidity. The humidity is fairly high here most of the time. Even in the winter. That is the only reason that is logical to me for my lack of static. I know others have static build up. I'm not saying it does not happen , I just saying that I haven't had any.

If you do have a problem with static buildup , go to Radio Shack and buy some bare 18ga. stranded wire and wrap it around the ducts with one end connected to the cabinet of the machine and the other connected to the body of the dust collector. Do not run any wires on the inside of the pipe. This is only inviting a plug. One thing to remember is that you can not ground PVC. PVC is a insulator and it is impossible to ground a insulator. You can use a bleed wire like I described. The bleed wire does exactly as it's name suggests it slowly bleeds off the static build up to ground.

The likelyhood of a spark generated explosion in a home shop sized PVC duct system is extremely low. I know that people talk about this all the time but I have yet to see anyone produce any real evidence of this happening. The most common cause of DC related fires is a hot sliver of metal like cutting through a nail being deposited in the dust bin and starting a fire there.

Here is some good reading on the subject.

http://www.gis.net/~dheaton/woodworking/articles/DC_myths.shtml

just my .02,

Terry Hatfield

[Dennis Peacock]

Rob....

This particular topic can really open a can-o-worms.....
Let's look at it this way:

1. Do you ground your shop vac hose? Nope.
2. Do you ground your vaccum cleaner hose? Nope.
3. Do most ground their flex pipe on their 2 hp DC units? No.
4. Is static present in the three above listed uses? Yes.

I get a "shock" all the time when dealing with my flex duct and 2 hp DC. It hurts sometimes and really scares me more than it hurts. I have found that if I put one hand on a machine and use the flex hose to clean up.....no static charge and no shock.

Simply do as Terry stated and this alone will greatly reduce the amount of static charge in the line. I have read a LOT about all this stuff and everything in the engineering docs I have read clearly state that grounding PVC pipe is NOT necessary.

[David Rose]

wrapped a power cord around one end of one pvc line when I set it up. It was just a handy way to store some extra cord. I've never had static noticable anywhere on the system. I've since heard this is a simple way to avoid it.

PVC spark fires are a hoax I believe. When I first setup my system and was pretty ignorant to the hazzards, I had an accident with the "sucker" running full tilt. I actually got smoldering chips in the bag before I knew what happened. That fire sucked all the way from my router. The line was full of dust at the time. With *flame* sucked through the dust there was no explosion. Not even a "poof" as far as I know. I now use a metal can separator before the particles "hit the fan".

David

[Rob Croke]

Rod Cole, who was a member of Badger Pond, wrote an excellent research article about the possibilities of a home shop dust explosion. His finding was that these explosions are nearly impossible. A summary of his article appeared in Fine Woodworking about a year ago or so, I don't remember which issue. However, you may find his complete article at this link:

Grounding PVC and Other Dust Collection Myths (http://www.gis.net/~dheaton/woodworking/articles/DC_myths.shtml)

Rob

[Scott in Douglassville, PA]

Originally posted by Terry H in NW Ar.

The likelyhood of a spark generated explosion in a home shop sized PVC duct system is extremely low. In any system for that matter. I know that people talk about this all the time but I have yet to see anyone produce any real evidence of this happening. The most common cause of DC related fires is a hot sliver of metal like cutting through a nail being deposited in the dust bin and starting a fire there.


Wow, ok - not going into the rest of the debate, but, while home-based DC fires may be exceedingly rare, industrial ones are not. Wife deals with them all the time at her job (involving metal powders), and I know, in my previous profession, we had <i>huge</i> problems with dust fires and explosions (resulting in catastrophic loss and injury) with talc-based systems. All static-related. Also, my dad dealt with MDF-based DC systems, and <i>they</i> had substantial problems.

Perhaps the reason you don't see tons of after-the-fact articles written about them is that (a) they aren't as a matter of course publicized, and (b) they usually fall under secrecy agreements signed when obtaining work (true in both my wife's and my careers). I agree that empirical data would be much more persuasive in this argument, but in its absence, don't totally discount the anecdotal information, either...

FWIW. /topic

[Steven Wilson]

If your PVC DC pipe gives you shocks then wrap a piece of wire around it and ground it; the static will now have somewhere to go.

In a large commercial DC system metal pipe is required. In a small shop DC system PVC doesn't represent an explosion hazzard. The question is, at what size of a system does PVC become a hazzard, or at what size of a system should metal DC pipe be used? Is it 1hp, 2hp, 3hp, 5hp, 7.5hp, 10hp? Is it with 30' of 6" main, 40', 50', 100'? How many cfm? What SP? What's the deciding factor? If you have a 37" wide belt you need metal but with a 25" wide belt you don't? At some combination of the above you should probably switch to using metal DC piping and not use PVC. Using metal pipe for a woodshop DC is never a "bad" or "unsafe" choice, using PVC can be.

[Terry Hatfield]

Scott,

I should have not added the "any sytem for that matter". I will remove it from my post.

I am sure you are correct about the industrial applications. I would think that most of the industrial systems would be operating at much larger volumes and at a higher dust concentrations than any home based system would ever be. I also understand and agree with the reason you gave for the lack of documentation on industrial fires and/or explosions.

This is a well worn topic on all forums. You can go to just about any forum and seach for "static" and get enough information to fill volumes from both sides of the discussion.

I believe that the bottom line is every person has to decide what to do for themselves. When the question comes up I try to answer with my own experiences and with what I have read. I would not and could not guarantee to anyone that it could not happen. I am just stating the facts as I understand them regarding home based systems.

I am truly sorry if I have caused even more confusion on this topic that there already was.

Terry Hatfield

[Scott in Douglassville, PA]

I completely agree with you Terry, and I appreciate your response. FWIW, the comment you made about "If you have static problems" was right on target - this is a topic that, all science aside, people have to resolve within their own comfort levels. I think the info you've given here, and many other times, helps people to do that.

No harm, no foul, and I hope <b>I</b> didn't come across too strongly.
Scott

[Ken Dolph]

You may want to take a look at he following movie. You can see that they used a common dust collection system.

http://www.risktech.se/rdust.htm

All that is necessary is that ALL metal components of the system be common grounded. You cannot ground plastic. Do not bother trying wires or tape. but if the metal parts are grounded, all is fine.

Explosions are not the only problem. Getting a static shock when your hands are near a moving blade can be disarming as well

:p

[Lee Schierer]

Originally posted by David Rose
wrapped a power cord around one end of one pvc line when I set it up. It was just a handy way to store some extra cord. I've never had static noticable anywhere on the system. I've since heard this is a simple way to avoid it.


That's all I have on my system and the static on the pvc pipe won't even wiggle the hair on your arms if you get close to the pipe.

Dust explosions happen with coal mines, flour mills and grain elevators where the air is so thick you can hardly breathe. Hopefully you'll never see this level of dust in the air in your shop. If there was a potential liability here, wouldn't there be warnings on DC systems that are being sold.

My belief is that this is mostly urban myth.

[Doug Edwards]

Unlike some of the posters above, I have direct personal involvement with all types of industrial dust collection systems. Yes there are explosions. Yes there are fires. It can happen pretty regularly in certain industries. But it requires three things: A dust cloud of explosible material of sufficient concentration and small particulate size, oxygen sufficient to sustain the explosion, and an ignition source.

One of the posts above mentions metal powders. This is about the single most fire and explosion prone dust. A pile of freshly milled aluminum will release both heat and hydrogen. It can be enough to self ignite and will explode with great force. Iron can self combust as well under the correct conditions. An MDF plant conveys large amounts of fine wood dust, usually pneumatically in the production process. This is a production process that cannot be compared to woodworking even with MDF. That is like equating baking the cake with cutting it.

There must be a high enough concentration of dust in the air for a deflagration (fancy word for BOOM) to occur. For wood dust this level is 0.04 oz. per cubic foot where 90% by weight of the material is less than 70 microns in diameter.

For the typical home dust collector (about 500 cfm) this means that you would need to create about 20 ounces of uniformly fine powder in one minute. To create such a fine powder you would need to sand at a very fine grit. Maybe 220 or finer. You would also need to sand a one foot square board down by 1/4" in less than a minutes time.

But wait, you say, my planer creates much more than 20 oz. per minute in shavings. Yes. It does. But the large pieces in the shavings dampen the possibility of an explosion. A dust concentration where most of the weight is in large shavings is almost impossible to explode.

In an industrial shop there are several people working on several machines connected to the same dust collection system. They work on the same activity over and over for long periods of time without a break. Even still, there is almost never enough material in the duct to explode. In virtually all circumstances, the explosion occurs in the DUST COLLECTOR!

Second, there must be enough oxygen. I will concede that this is a distinct possibility in most home systems. At least I hope so.

Third, there must be an ignition source. One of the other posts above says that you cannot ground an insulator. If this were the case, then there would be no chance of a spark from the PVC as the spark is a grounding action. This question has been well covered in the "Myths" link above and I will not regurgitate that here.

If an explosion does happen, what is the damage? The explosive force of wood dust is in the medium range. It is about 10% of the power generated by an aluminum explosion. It is about 80% of the power of a dry shellac explosion. It is about 90% of the power of a dried meat explosion. Yes. That's right. Meat has more explosive power than wood.

Well I’ve gone on long enough. Let’s just say that I am about as concerned with a dust explosion in my home shop as I am about meteors crashing into my jointer. No, I am not going to build a meteor shield either.

[Ken Dolph]

You have explained a lot about the explosion power of dust. What most people want to discount is that dry dust, any kind, flowing from one metal part along a nonconductor to another metal part constitutes a van de Graff generator. It would be possible to build, accidently, such a generator that could produce several thousand volts of static.

I too doubt that the static discharge would come in contact with sufficient dust concentration to explode. But it could be enough to startle you when you are cutting. Shavings from a planner will do this as well as fine dust.

I built such a generator in college. With fine tuning we got to 150,000 volts. Sparks would leap over 10 inches! That was enough to make my arm go numb for half an hour.

Is it that hard to just ground all metal fittings?

Some dust collection systems do come with such a warning.

The most important thing to read about Rod Coles article is that this is not his field. Throughout his article he correctly answers the wrong questions.

[David Rose]

Why is the PVC generated static reduced or nullified by the electrical (magnetic?) field created when a power cord is wrapped around the pipe? Or is the cord just a convenient source to ground before the current builds? I would think it would take enough current to be shock producing well before it would jump through the insulation. But I guess for the body to be the ground, insulation in shoe soles has to be overcome.

David

[Lee Schierer]

Originally posted by Ken Dolph
Y You cannot ground plastic. Do not bother trying wires or tape.
:p

Actually you can ground plastic. Try this simple experiment. Tear off a small piece of paper and lay it on your desk. Run a plastic comb through your hair to create a charge. Hold the comb near the small piece of paper. It will lift off the desk onto your comb. Now do the same thing again, but connect an alligator clip and wire to the comb with the wire going to ground. Comb as much as you want, but you'll never lift the paper off the table .

Static electricity does not follow the normal rules for electric current flow. Insulators can be grounded.

[Ken Dolph]

I believe I understand why but working out all of the vectors of it has my head hurting.

If the effect is true, I believe, (do I have enough disclaiming here) that it has to do with the flow of charges carried by the particles interacting with the coil of wire. There would be electrical and magnetic effects at right angles.

OR it could be that the dust faireys are repulsed by the copper gnomes. Take your pick.

Lee,

If the comb is long and the clip is at one end you could effect the plastic on the desk for a short time. You are right that the residual charge would bleed off over time through the clip but if this were a large effect no insulator would work.

[Dale Thompson]

Rob,
Listen to the folks in this post. As a fire protection engineer, I can assure you that there is virtually NO chance of an explosion in your system. Fire (an explosion is nothing more than a slow moving fire) requires three things: Oxygen. Is it available?-YES! Fuel. Is it available?- MAYBE(but probably not)! Heat/energy. Is it available?-NO!! A million volts (static electricity)will straighten your hair but will not hurt you because it takes AMPS to create energy. No amps-no power-no explosion!
You can curse the "shocks" from your shop vac but they won't hurt you unless you jump back and bump you head on the cast iron of your PM 66.:(:):eek:

[Ken Dolph]

it is static electricity that ignites your gass grill. Roughly 1,000 volts caused by squeezing a tiny quartz crystal.

But I still doubt that it has ever happened in a home shop.

[David Rose]

Ken, sorry about hurting your head over this. Hopefully, it wasn't such a shocking question as to cause you to hit your head on your PM66.

Dale, now I know why I don't have a table saw! I had a tiny one in our business when our daughter was young. She first gashed to top of her head running into it. A year or so later it was her chin. Maybe Roses should look ahead.

David

[Perry Schmidt]

I too have a relative (father) in the industry of air systems who dealt w/ exploding dust. But again, on the industrial scale.

And I read a lot of the posts above about PVC myths in a home dust collection system. I started talking w/ my father before putting in my ducting and was trying to argue for PCV. Just seemed easier to work with.

His point was simple - OK, the risk w/ PCV is neglegable. But why even take that negligable risk?? It's not THAT much more for Home Depot purchased HVAC piping, and it's not that much harder to install. So why take the risk? I think that everyone agrees that metal IS better wrt static. The 'risk factor' between the two might be very very small, but I've never heard anyone say 'metal ducts will cause a static spark which could ignite the dust.'

The biggest thing that scared me about metal ducting was sealing the leaks and it was actually very simple - we just used aluminum duct tape and I had a very well sealed system. Tested it w/ a smoking candles..no leaks. Maybe I got lucky. I read some posts about people having endless problems w/ sealing the leaks on metal ducting, and trying to use a silicone sealant. I agree - that would be hard. But the aluminum duct tape was a piece of cake. After running some short runs, I really wasn't 'scared' of metal ducting anymore.

So why even take the risk - no matter how small - w/ PVC?? If metal is 'just as easy' to install, the only advantage of PVC is cost...and the cost of 'box' ducting wasn't that expensive - IMHO.

Just my 2cents.

Perry

[Steven Wilson]

There is another difference in home/small shop dust collection systems piped with metal (spiral pipe) vs PVC; the fittings. When designing and implementing metal duct work you're generally working with fittings that are appropriate for dust collection systems (long radius, smooth walls, etc), many PVC designs rely on fittings that are really inappropriate for dust collection (in general too tight a radius) and result in system resistance much higher than designed, this can lead to poor system performance. Finding the correct (long radius) PVC fittings can be difficult.

[Rob Glynn]

Thanks to everyone for the information.
My fears as to static caused blasts or fires are allayed and I will replan my, still in the future, DC system based on PVC or some other plastic tubing. I note the comment about sharp bends. I am aware of a particular type of tubing that runs to longer gentle bends and will check that out as an option.
In Oz all electrical items, except lights and a few low current portable items, are earthed back through 3 pin power plugs and 3 part cords to a common earth point, usually a steel or brass rod driven deep into the ground and our electrical appliances and machines and our houses and shops are wired accordingly.
We have 240 volt, 50 cycle power as our single phase source and 450 volt as our 3 phase source. Earthing the machines happens as a matter of course for us, the moment they are connected to a power supply.