PVC and Dust Collection a way to find out without blowing up your shop

[Rod Sheridan]

Matt, I have only one experience with non metal ducting and it was with a length of 5" flex hose that ran from the ceiling to the hood on my planer.

I leaned forward to take the wood from the out feed end of the planer and had a nice arc jump from the hose to my head. Startling to say the least.

I now use metal flex, which is grounded through the metal duct work.

I know the static shock wouldn't be an electrical hazard, however I don't need any surprises when operating machinery.

Regards, Rod.

[Chris Padilla]

Matt,

I feel comfortable in saying your assessments are fair. Will YOU get shocked by PVC ducting on a DC system: most likely YES. Just as Rod stated above.... However, over time, as dust cakes on the inner walls of the pipe, the friction between the air/wood dust causing localized charge pools will decrease and the shocks become less and less....

[Eric Gustafson]

I have worked in the electronics industry for over 35 years. Static electricity damages components, although when I began my career we didn't know that. Plastic can be rendered static safe for components with the application of a electrically conductive coating. The simplest substance for this found in nearly any home, is fabric softener (diluted.) There is also a product called Static Guard. You spray it in your dryer to keep synthetics from clinging to themselves.

My Clearvue cyclone was only clear before the first time I used it. Then it became the color of mud. Not very clear! As an experiment, I sprayed some Static Guard inside. This was months ago, and the cyclone's plastic is clear as glass! Therefore, there is no longer any static build-up. I think you could treat the inside of your PVC pipes by spraying Static Guard or fabric softener while the DC is winding down. I would think it should wear off after awhile, but it doesn't seem to.

[Mike Gager]

mythbusters used coffee creamer to make an explosion but i dont remember saw dust


however here is a cool video of a sawdust explosion

http://www.youtube.com/watch?v=Va1sJF5_f-8

[Ken Fitzgerald]

I would suggest that nobody is using creamer or a road flare with their DC so it shouldn't be a problem.

[Anthony Whitesell]

Unless you ground every square inch,

You can't ground an insulator beyond the point of contact. That's why it's called an insulator. It won't conduct electricity and it won't conduct GROUND!

I don't generally chime in on these discussions, but I was recently reminded of something that is an aspect of directly related to this discussion.

I agree that a static discharge may cause wood dust to ignite. Whether it could, would, or can't I don't know. I'd like to set that aside for the moment and talk of the basic principles and physics of static electricity and insulators. This aspect is usually glossed over in these discussions.

Wood is for the most part and insulator and we all agree tha PVC is as well. When they rub together you get a build-up of static electricity. I think we all knew this part.

For those that have run or give into running that ground wire in or on the PVC ducting. Why does that work to dissipate a static charge? Because static electricity isn't completely static. Once you cover a surface in a charge, it will seek to provide equal potential across the entire surface (self-leveling if you will). If you then supply a single point (your finger or a screw) that is conductive to ground (or a lower potential) then you will get current flow and zapped. But not a second time because the charge is gone. If you attach the screw to the surface ahead of time and then try to build a charge, you won't be able to because you are providing a constant path to ground. Then charge that tries to build nearest the screw are shorted to ground and the higher charge furthest away will dissipate in the direction of the screw. It works alot like bucket with the hole in the bottom. If the hole is plugged and the bucket filled, then the water comes rushing out. If the hole is open and someone tries to fill the bucket, then there is less water near the hole and more further away. But never as much as there was when the plug was pulled on the full bucket.

Theoretically, a single point would be able to ground an entire PVC ducted system. But that is only if the connections between segments are equally as conductive and the charge builds and dissipates evenly. With all the seams, bends, and joints, I think we can agree that not going to be the case. Therefore, a ground wire should touch each segment in atleast one place. Whether you use a screw, duct tape, metal tape, or just run the bare wire inside and let it flop around. Inside or out it won't matter (there are two polarities to static electricity (one is on the inside of the pipe the other the outside). This is not saying the system is grounded and will always maintain a zero potential relative to ground. The grounding point provides a path to minimize and dissipate the static charge and keeps it below the level that will cause it to spark.

With that said, I still don't know if a spark will/would/could ignite wood dust moving at 400-1600 CFM. But knowing the physics behind and the requirements for dissipating a static charge, I'm going to be providing a ground path in my plastic ducted DC system.

[Ken Fitzgerald]

Anthony,

I might be wrong but I believe PVC is commonly used as a insulator on electrical wiring. You can't ground an insulator. It will neither conduct electricity or ground. Otherwise it wouldn't be an insulator. The ground wire will only work at the point of contact and maybe a extremely small distance around it.

[Dan Friedrichs]

Why does that work to dissipate a static charge? Because static electricity isn't completely static. Once you cover a surface in a charge, it will seek to provide equal potential across the entire surface (self-leveling if you will). If you then supply a single point (your finger or a screw) that is conductive to ground (or a lower potential) then you will get current flow and zapped. But not a second time because the charge is gone. If you attach the screw to the surface ahead of time and then try to build a charge, you won't be able to because you are providing a constant path to ground.

This is absolutely not true. There is no requirement that charge be uniform on an insulator (on a conductor, yes, but not an insulator!). This is easy to prove - go get a new piece of PVC pipe, rub it vigorously with a cat, touch one end while grounded, then go touch the other end. You'll get a zap both times.

The confusion emerges because people think that inserting grounded screws the length of the PVC pipe (or putting a grounded wire inside of it) completely removes the charge from the pipe. That's not true - it just decreases the electrostatic potential at which discharges occur. By allowing discharges to occur within the pipe, it's less likely that I'll get a strong zap by touching the pipe.

[Jimmy Coull]

PVC can cause static electrcity and that's a fact, but MIT did a study on the use of pvc on systems uner 3hp and found out that there is pretty much no chance of an explosion.

But my question to most of you is what do you use to collect your dust in, alot of people have cardboard drums or plastic bags. There is more of a chance of picking up a screw or small piece of metal in DC system, when that hits the metal impeller it can cause sparks or make the metal extremely hot as it lands on your saw dust, which can then start a fire.

this is just a little more info to keep the worry warts worrying.

Did you know that only female mosquitoes suck your blood, because they need it for having babies. But only the male mosquitoes make a buzzing sound as the females are quiet. So if you hear a buzzing while laying in bed don't worry because it's a male mosquito, but if you hear nothing at all.............;>)

Jimmy

[Dan Friedrichs]

PVC can cause static electrcity and that's a fact, but MIT did a study on the use of pvc on systems uner 3hp and found out that there is pretty much no chance of an explosion.

If you're referring to the source that I think you are, it wasn't MIT, but rather an employee of an MIT-affiliated laboratory who drew this conclusion. If we're not talking about the same thing, can you provide the source?

[Jim C Bradley]

Ken, you said it all. Give us evidence. Until then I shall continue using my SD-35 PVC dust collecting system. All of my joints are light friction fit so I can change the layout whenever I desire. I have no leakage that I can detect. I have no ground system.

Someplace out in electronic land I saw a very complete report by a person with all kinds of credentials who explained why we cannot get a spark explosion in a DC system. He had all of the data about air to dust ratios, the chemistry necessary to start the fire under DC conditions, etc. The article was in technical and explained in human terms. After reading it I just went PVC.

That is my 2 cents worth.

Enjoy,

Jim

[Ken Fitzgerald]

Jim,

Here's the link to the study they are discussing above: http://home.comcast.net/~rodec/woodw.../DC_myths.html

[Bart Leetch]

Rather than have Mythbusters set up an artificial test, how about we have thousands of small shop woodworkers use their equipment on a regular basis and see what happens?

Oh, wait. We've already done that and we've learned that there is no danger.

Darn, I wanted to see an explosion.

We already have for at least 10 years this topic has been kicked from pillar to post on most WW boards it hasn't happened & won't there isn't the right combination of oxygen, Heat, or fuel . This isn't like flour or grain mills the size of the duct is much smaller & so is the fan system.

Read this:

http://home.comcast.net/~rodec/woodw.../DC_myths.html

Ok Ken your faster on the trigger then I am.

[George Bregar]

I love these threads. They're like the "best oil to use" threads on my BMW forum

I will be using PVC in my new shop. Either nothing will happen, or I will save the rest of you from disaster and go down in history posthumously. Win-win.

[Anthony Whitesell]

I'm going to say my last piece and then wander off.

From my physics textbook. "For any regular uniform shape (flat, spherical, cyclindrical, etc. not conical or T-shaped, or a bend) the static charge will find a uniform potential across the surface of the insulator." Or a hundred years of physics is being disproved. Keep in mind that this is a discussion on static electricity, for a static charge residing on the surface of the shape. If you momentarily apply a singluar discharge point, then the charge will dissipate from the immediate region and the remaining charge will redistribute across the surface. This only applies to regular surfaces. In the case of a 90 degree bend, there will actually be less charge build up at the inside corner due to the repulsitory nature of the static charge trying to maintain an even distribution (can have too many electrons in one place). And I never thought I would use all those years of classes in electromagnetics. Zero will the pipe ever be at zero potential with no charge on it? No. Will the charge be reduced? Yes, and most likely below the level at which a sudden discharge will occur.

What is being missed is that insulators build the charge on the outside of the body and don't pass current (easily) through the material. Where as a conductor easily passes current through the body of the material.

Ken you got me thinking even more. To take this train of thought one step further, all (modern) wires are covered by insulators. If the insulation/insulator can't be grounded, why don't we get zapped every time a cat rubs up against an extension cord?

Thank you for your time.