Question about static electricity & dust collection systems

[LatakiaLover]

I have the industry standard How To book, but nowhere does it cover whether or not "loose" hoses need a ground wire inside them. By "loose" I mean the only thing at its end is a collection horn that's attached to a wooden table or similar, NOT to the exhaust outlet of a powered, debris-producing machine like a router, table saw, or disk sander.

Hours of re-work depends on the answer to this question, so no guessing.

Thanks

[caskwith]

Most dust extraction grade hoses use a metal coil for strength. I have no idea if you need to ground things etc, I never have on my own and no problems yet.

[andrew]

I don't have grounding on my system. I get shocked every once in a while. I've heard it can be a problem with really fumey finishes (possible ignition).

andrew

[andrew]

You can ground the pipe (plastic or otherwise) with copper wire and clamps.

andrew

[LatakiaLover]

Most dust extraction grade hoses use a metal coil for strength. I have no idea if you need to ground things etc, I never have on my own and no problems yet.

Indeed.

I've done some Net searching since posting, and it appears the need to ground small, home-sized systems is probably unnecessary. That the subject has gained a sort of Urban Myth status based on fear and untested extrapolation.

This article is by an EE who works with commercial/factory-sized ducting systems for a living:

Home-shop dust-collection systems have become increasingly popular, but their safety has been hotly debated. The primary issue is whether PVC pipe is safe for use as ductwork. Many claim that sparks in PVC pipe due to static electricity may ignite the dust cloud in the pipe. The specter of a giant fireball consuming a shop and home is repeatedly raised. Others claim you can ground PVC, thus ensuring its safety.

Two years ago I had to decide for myself: PVC or metal ducts for my basement shop. Being both an avid woodworker and a scientist, I made a concerted effort to understand the issues. Fortunately, I have the resources of the library at the Massachusetts Institute of Technology (MIT) and a professor just down the hall who’s an expert in the physics of lightning. I studied static discharge from insulators, as well as the more general topic of dust ignition. I found that it’s extremely unlikely for a home-shop-sized system to have a dustcloud explosion. Commercial-sized systems have had dust-cloud explosions, but different phenomena come into play in larger systems, and 4-in.-dia. PVC is too small for use in such systems, where the airflow is much greater than in a home shop.

Sparks are unlikely in 4-in.-dia. PVC pipe In my research I turned to the Journal of Electrostatics, a publication that covers the effects and interactions of static electricity, particularly in commercial applications. This journal has published a number of studies on the combustibility of dust clouds by electrostatic sparks. The researchers were able to determine some of the conditions necessary to create sparks and ignite a dust cloud.

Sparks can be caused by a variety of conditions—one of which is static electricity. However, sparks are unlikely inside a standard 4-in.-dia. PVC pipe that would be used in a home shop, and more importantly, any such sparks are extremely unlikely to be strong enough to cause an ignition. I can’t say it is truly impossible, but it is very close to impossible, and I do not know of a single instance.

The difference between metal and PVC is that one is a conductor (metal) and the other is an insulator (PVC). A conductor allows electrical charges to flow freely. If any excess charge is not given a path to ground, it can arc, creating a spark that in certain conditions can ignite a flammable substance such as dust. Grounding provides a path for this excess charge to flow harmlessly to the earth, which is why dust-collection systems in all commercial shops are required by code to be grounded. However, an insulator is a very poor conductor of electricity. While it’s possible to get a static shock from the outside of a PVC pipe, it is highly unlikely for sparks to occur inside.

Dust collectors with 3 hp or less pose little danger I published my findings on my web site. Rob Witter, a representative at Oneida Air Systems Inc., which makes dust-collection systems, said he largely agreed with my research. “We as a company have been trying to trim away at these misunderstandings for years,” he said. He added that plastic pipe will “probably never cause a problem” in a home shop. Finally, he pointed out that the National Fire Protection ssociation NFPA) puts no regulations on dust-collection systems of 1,500 cu. ft. per minute (cfm) or less.

All of this discussion applies to home-shop-scale systems. Larger systems, complete with ducts and filters that move more than 1,500 cfm, require at least 3 hp and are not found in most home shops. Larger systems need larger ducts, and with that you have to begin to worry about more complicated forms of static sparks.

The real hazards In a home shop, the dust-collection fire hazards you need to worry about are not in the ductwork but in the collection bag or bin itself. A fire may be caused by a spark, which can occur when a piece of metal is sucked into the ductwork and strikes another piece of metal, or by embers from a pinched blade. The spark or ember settles into the dust pile to smolder, erupting into a full-blown fire hours later, often after the shop has been shut down and no one is there to respond. For this reason, my most important recommendation is to empty the collected dust every day or at least keep it in a closed metal container.

[baweaverpipes]

I got shocked like crazy when I blasted above 110psi.
I finally ran a wire from the cabinet to the floor. This was after i got shocked close to my wanker. It was too close for comfort as I didn't want my little feller zapped, nor my nads. That would sting!

[Sasquatch]

SECRETS OF BLASTING by Bruce Weaver....

[Sasquatch]

To be a little more useful, all hoses should be grounded - inside the hose is recommended, outside the hose will usually do. Some materials (the vinyl they usually use on the flex hoses for example) don't build up static the way a metal tube or a PVC tube will. You can tell. So I'd use the machine, put your hand near the hose and see if there's any build up, any "hummy" feeling, anything untoward. If not you're probably fine.

[scotties22]

Heath made me ground my system with copper wire inside the tubes. He demanded it, actually...doesn't want me to burn the house down and we get pretty staticy during the winter.

[Nate]

I have copper stranded wires running inside and coiled around my dust collection ducting. I haven't had the need for anything on my blast rig yet. I do have a grounding strap, but haven't needed it. I have on others' blast setups though.

[Vermont Freehand]

what I learned from the 3 people I know who have had their shop burn to the ground (given they were cabinet shops), they all say you must connect all machinery with a ground wire that runs from each machine, through the tubing/piping and connected to the blower/sucker motor, and also connected to the collection tank/bin/trailer. You can drill a tiny hole through the piping at the junctions, so you can wirenut the seperate lines together yet outside of the piping, which alleviates intersecting wires inside the piping. Then back in for the straight runs.

[scotties22]

That is exactly what I did. Everything is connected and my circuit is on a GFI.

[WCannoy]

I don't have a dust collector these days, but I had one in my old shop. Here's an old pic (of my brother making a pipe in my shop) that shows the pvc collection piping under the bench in the lower right hand corner of the pic. It was wrapped in braided copper wire on the outside connected to a braided copper wire that was run through the inside. It was all grounded to each machine and to earth through the shop electrical system. It was quick, easy, and not very expensive, and having it done never caused me any problems with the dust collection system. The risk of a dust explosion might be small, but it's so easy to guard against it.

[WCannoy]

Here's 200ft on ebay. I recall back in the day it was almost impossible to find. I had to order it from Grainger...

http://www.ebay.com/itm/JETSTREAM-JT142 ... 1240965127?

[LittleBill]

Below is a link to an article from a guy who used to be on some of the wood working forums I used to frequent. It is more about the likelihood of blowing yourself up than avoiding shocks. If you want to avoid shocks, grounding is important. If you are only worried about leaving a smoking crater in your neighborhood, it probably isn't a big concern.

Personally, if I am doing something where a static charge is going to build up (usually vacuuming), I ground myself by the simple expedient of holding onto something metal like my table saw, lathe, or whatever is close. If you need both hands for whatever you are doing, I knew a guy who used a wire and two alligator clips. He clipped one end to his nether regions (rear pocket or something like that), the other to something metal, and went merrily on his way.

http://home.comcast.net/~rodec/woodwork ... myths.html

The article is long and deep. I have quoted the summary below for anyone who does not feel like plowing through to the conclusions.

Here are just a few of the myths I have read regarding DC dangers and grounding PVC in particular.

1. The number one myth must be that PVC ducts are dangerous. As both theory and practice show, home shop DC explosions are somewhere between extraordinarily rare and nonexistent. The volume of a typical run of 4 inch duct, say 20 feet, is about 1.7 cubic feet or equal to a cube 14 inches on a side. I do not know the explosive power of this volume of dust, but I do not think this is going to level your shop.

2. The number two myth must be that you can ground PVC. You simply can't ground an insulator. There are things you can do to reduce the odds of a strong discharge, especially to your body, but they are far from perfect.

3. The number three myth is the unstated corollary to myth #1: the only thing of concern in a dust collector are the ducts. As seen above, the collected dust pile and the collection bag are greater hazards than the ducts. Fortunately, in practice home shop sized dust bags have shown themselves to pose little explosion hazard.

In no particular order:

4. The external ground wire works by reducing the static on the outside of the PVC. There is little or no static on the outside of the PVC unless you are rubbing the outside for some reason; the static is on the inside. The electric field due to the static charge in the pipe can cause a discharge on the outside, but this in no way means there is static on the outside. Indeed, if there were lots of static on the outside, say due to lots of charged dust floating around from poor quality filter bags, the static on the outside will be the opposite charge from the static inside since opposite charges attract each other. Thus the electric fields from the two will tend to cancel each other as discussed above in shielding. Outside static helps protect you! The charge on the external ground wire due to the electric field from the charge in the pipe will also be of the opposite sign. The outside static and the ground wire have the same sign; they repel each other! Static on the pipe is not drawn to the ground wire. There is no reduction in outside charge due to the external ground wire. And, if you believe that not enough charge will go through the pipe to be of help, how is it that the charge will go along the pipe? It is an insulator either way.

5. The external wire must be bare. The effectiveness of an external ground wire is not "lab tested", but if you believe that a ground wire separated from the static charge by a 1/16 of an inch to 1/8 of an inch of insulator does some good, certainly adding an additional couple hundredth of an inch of insulator on the wire is not going to make much difference. As seen in the specific proposed mechanisms above, the extra insulation does not significantly hinder the value of the external ground wire.

6. Grounded screws can not help as they are too far apart. The maximum distance from pipe wall to internal ground wire is the four inches across diameter of the pipe. The maximum distance from pipe wall to the screw is the square root of 42 + (spacing/2)2 or a little under 4.5 inches for a screw spacing of 4 inches. Four inches vs. four and a half inches is a very minor difference. In addition, the screw point leads to a stronger electric field since it has a smaller radius than the wire, so in fact the screws may well work better than the internal wire.

7. Grounding works by removing charge from the dust. The dust is an insulator just as the PVC is. The dust is in contact with the grounding for only a fraction of a second, even in a metal pipe. You will remove very little charge from the dust.

8. Metal ducts keep the dust from charging. Dust charges perfectly well in metal ducts. Grounding metal ducts keeps the ducts at an equal potential so you don't get metal to metal sparks. Further, to the extent that you are grounded, you won't get a spark to your body either.

9. Any spark will ignite the right dust mixture. Dusts are much harder to ignite than gas mixtures. Electrostatic discharges come in many varieties, and only a few will ignite even the most easily ignited dust mixtures. It is very unlikely that you can generate a discharge strong enough to ignite the perfect dust mixture; this pretty much requires a careful lab set-up for scales as small as your home shop DC.

10. Grounding PVC works by removing charge at a point, and since charge must be uniformly distributed, it therefore removes charge everywhere. There is no requirement that charge on an insulator be uniformly distributed.

11. Getting a discharge outside the ducts, say to your finger, means you also have discharges inside the ducts. As explained above, the electric field outside the pipe is generally much stronger than inside the pipe, so discharges outside are much easier to generate. In addition, you are a conductor and your finger is rather pointed. The the electric field at the end of your finger is especially strong for these reasons. This is why you generally gets discharges to your finger, rather than other parts of your body.

[LatakiaLover]

Yeah, I gave up trying to separate fact from fiction on this one. This subject seems to be the ShopWorld's version of bowl coating.

I ended up simply soldering a length of copper wire to the vinyl tubing's coil spring at each end of its run, and attached the soldered lead to the machines. Called it Officially Good at that point, and went back to doing Pipe Stuff. If the Pipe Gods wanted to electrocute me or blow me up, they'd have done it by now, I figure.

Thanks to everyone who responded, though. COLLECTIVELY, you arrived at what I think is the right answer. Home-sized shops don't need to worry about this.