Grounding metal pipe duct system

[Bob Borzelleri]

This might sound a bit counter intuitive, but stay with me for a bit. I am designing my dust collection ductwork system and have come upon the following question while considering the "all metal" option.

If we accept, for a moment, that grounding for static charge within any duct system (plastic or metal) is important and that using metal is generally seen as preferable to plastic as plastic is an insulator, wouldn't it be necessary to ensure that the metal pipe grounding circuit is complete through to the ground for the main electrical circuit?

If I understand this correctly, most metal pipe systems "terminate" into a plastic flex pipe that is often fixed to a plastic hood that either comes with the tool or is added by the owner. Unless the metal pipe has some form of conductive connection to and through the flex pipe as well as the plastic hood through to the machine and eventually to the electrical system main ground, I am thinking that the static charge ground system is not complete.

I have read two different "guides" for designing systems and neither mentions completing the static charge circuit to the metal frame of the woodworking machine. Am I missing something here? Do most folks make these connections even though at least some of the guidance out there is silent on the subject?

I am aware of the argument that one main benefit of metal pipe is that static charge is less likely to build in the first place, however less likely is not the same as "will not" and most metal pipe systems seem to have plastic flex pipe to the machine which is apparently thought to be more likely to contribute to static charge buildup. For the purposes of this question, I am assuming that part of the benefit of the metal pipe is to allow for grounding of whatever static charge builds up in the pipe as a result of fine particles colliding with each other and that for grounding to really occur, one must complete the circuit through the metal frame of the machine.

Any thoughts on this?

...Bob

[Ken Fitzgerald]

Bob....most machines are grounded via their power connections. I'd just run a single ground wire from the dc metal frame and connect it to the metal duct. The plastic hoods and the flex tubing will have ground via the metal duct and via the machines to which they are connected. Just my 2 cents worth!

[Jim Becker]

For those who really want to create a continuous "ground", the better flex hose has a metal drain that spirals within it. When you cut the hose, you can expose the drain and connect it to both the tool and the duct work. Personally, I have not bothered with that. Not necessary in a home shop, IMHO.

[Bob Borzelleri]

After reading several replies from different forums, I am getting the impression that my concern about "continuous" ground might only be an issue if the flex pipe is long enough (and has enough flow disruption properties) to become the main source of static charge.

When I factor in the fact that there are many folks who are using straight plastic without a ground, it might be that all the suggestions so far would offer some increased degree of protection from static charge ignition.

Since I'm setting this thing up for under floor installation, I'm going to try to design this system to have as little flex hose as possible.

Thanks for the replies.

...Bob

[Effie Lever]

I agree with Jim and Ken, when it comes to the short flexible hoses I would not bother with grounding. As for the "Continuous" ground, assuming that all your shop metal ducted are securely connected together all you need is one ground connection (normally done at the dust collector) to discharge any static from the system

[Dennis McDonaugh]

Bob, why are we accepting, even for a moment, that continuous grounding is required in a home system? From what I've read, there is a large body of research showing nonindustrial sysetms cannot generate enough static charge to create a fire hazard. Even if they could, wouldn't any static charge built up in the short plastic flex pipe be discharged where they contacted the metal pipe which is connected to your collector which is grounded?

[Ole Anderson]

Bob, you could solve the arguement. Hook it up without the final ground connection, run it for a while under conditions that normally would generate static, then wet a finger on each hand, touch one to a grounded outlet or water pipe and then the other to the metal DC pipe (make sure to touch the pipe after you are grounded). You will know shortly (get it?) if you need to run the ground wire.

[Bob Borzelleri]

Bob, why are we accepting, even for a moment, that continuous grounding is required in a home system? From what I've read, there is a large body of research showing nonindustrial sysetms cannot generate enough static charge to create a fire hazard. Even if they could, wouldn't any static charge built up in the short plastic flex pipe be discharged where they contacted the metal pipe which is connected to your collector which is grounded?

Dennis...

Your point is well taken. My original premise was to try to understand the electrical dynamics assuming that continuous grounding was desirable.

As it turns out, my apparently unending need to research this thing brought me to one publication that makes the case for continuous grounding for both metal as well as plastic. "Dust Collection Basics, Recommendations for Home Shop Systems, 3rd edition" by Woodstock International (makers of ShopFox stuff) writes on page 24 the following:

"Since metal pipes act as a grounding circuit, static electrical build-up will be dissipated to each woodworking machine frame assuming there are no grounding interruptions. Each machine frame should be continuously grounded by a grounding wire from a power cord to the electrical circuit to the grounding terminal in your electric service panel. No grounding wires need be added accept in the case where nonconducting fittings, flex-hose and hoods attach to the metal duct or machine frame. In these instances, all non conducting components must be bridged or jumped with a ground wire to ensure a completely grounded system."

Go figure... this stuff might just boil down to black magic and juju...

...Bob

[Dennis McDonaugh]

Dennis...

Your point is well taken. My original premise was to try to understand the electrical dynamics assuming that continuous grounding was desirable.

As it turns out, my apparently unending need to research this thing brought me to one publication that makes the case for continuous grounding for both metal as well as plastic. "Dust Collection Basics, Recommendations for Home Shop Systems, 3rd edition" by Woodstock International (makers of ShopFox stuff) writes on page 24 the following:

"Since metal pipes act as a grounding circuit, static electrical build-up will be dissipated to each woodworking machine frame assuming there are no grounding interruptions. Each machine frame should be continuously grounded by a grounding wire from a power cord to the electrical circuit to the grounding terminal in your electric service panel. No grounding wires need be added accept in the case where nonconducting fittings, flex-hose and hoods attach to the metal duct or machine frame. In these instances, all non conducting components must be bridged or jumped with a ground wire to ensure a completely grounded system."

Go figure... this stuff might just boil down to black magic and juju...

...Bob

Bob, I agree with the quote from the manual 100 percent. Continuous grounding will dissapate a static charge built up on the plastic flex. I'm saying static charge is not a fire hazard in home systems using PVC throughout and is even less a consideration in those using primarily metal ducting. That doesn't mean you won't get an occasional shock from the system and a continuous ground would eliminate that problem.

[Bob Borzelleri]

Dennis...

I think I might agree with your point that a static caused fire is not a likely event in a home shop, but I must admit that I don't necessarily understand all the variables that come into play to support that position.

The most apparent factors would seem to be volume of dust material moving through the system (for the potential charge) and/or residual dust in the ductwork (as fuel for ignition). Production shops should have more of each than an average home shop, but if residual dust is the issue, then it seems to me that the passage of time might eventually bring the home system closer to whatever risk would be present in an ungrounded production scenario, particularly if neither system is cleaned out regularly.

Are you aware of other variables that contribute to the notion that ungrounded PVC systems in a home shop setting are not a significant risk for dust ignition?

...Bob

[Jim Becker]

Production shops should have more of each than an average home shop, but if residual dust is the issue, then it seems to me that the passage of time might eventually bring the home system closer to whatever risk would be present in an ungrounded production scenario, particularly if neither system is cleaned out regularly.

Making the assumption that a properly sized system is working correctly, there shouldn't be anything to "clean out"!! Dust collection is about moving air. If there is enough air moving at appropriate speed, there will be no material left in the system. Explosive concentrations of fines must be very, very high...something unlikely in either a home shop or pro shop situation, again with a properly designed and performing system.

[Kevin Arceneaux]

Making the assumption that a properly sized system is working correctly, there shouldn't be anything to "clean out"!! Dust collection is about moving air. If there is enough air moving at appropriate speed, there will be no material left in the system. Explosive concentrations of fines must be very, very high...something unlikely in either a home shop or pro shop situation, again with a properly designed and performing system.

I agree with Jim.

The odds of any of us getting to the stage where it would explode is slim to none. I saw this earlier and called on of our plant electricians. He laughed, he also does some woodworking, and said that anything a small home shop would need would be for the guy/gal to walk over and touch it once in a while when the humidity is reall low, like in winter down here. He said that anything else is throwing money out the window.

[Rob Russell]

Bob,

You mentioned in one of your posts that you're doing underfloor ducting. There have been posts from folks with underfloor ducting who wished they hadn't gone that route. The main reason is the likehood that you'll want to rearrange your shop/machines as the shop changes/grows.

The static issue's been covered, so I won't beat that horse.

Rob

[Steven Wilson]

Bob, why are we accepting, even for a moment, that continuous grounding is required in a home system? From what I've read, there is a large body of research showing nonindustrial sysetms cannot generate enough static charge to create a fire hazard. Even if they could, wouldn't any static charge built up in the short plastic flex pipe be discharged where they contacted the metal pipe which is connected to your collector which is grounded?

Large amount of research????? Just one article, not peer reviewed ,does not constitue a "large body of research". Define "home system", define "industrial". Some hobbiests have Timesaver wide belt sanders. Any argument that throws out the very undefined terms "home sytem" or "industrial" is pure BS. Now, I'm assuming that you're refering to the possible hazard of a spark within DC pipe (PVC) igniting dust and causing an explosion or fire. I will agree that it is highly unlikely to occur in a smaller DC system of the size typically used by hobbiests and small pro shops (i.e. something 5hp and under running under less than 10" main pipe). There is another fire hazard associated with PVC and static and that is dust build up on the outside of pipes. If you install a PVC pipe system you may notice a bit of static discharge when you first install it. After awhile it tends to decrease or go away. What happens? Dust collects on your pipe. There are incidents of dust buildup on pipe acting as a secondary explosion when a DC catches on fire (usually an ember in the dust bin). IIRC there are a couple of papers from Oregon OSHA describing incidents and also the secondary fire hazzards associated with PVC piping. I would give you a link to the source but it's been a few years (I'm not going back and redoing the search).

Anyhow, as for providing an electrical connection between metal pipe and the machine, use the flex hose with wire inside - it's actually designed for the purpose. IIRC it's really only needed if your flex hose is greater than 5'.

[Doug Edwards]

Actually, a wire in the flex does not techically make it grounded flex. This wire is for stiffening the duct so it doen't collapse under vacuum. Special flex is made for staic dissipation using a conductive polymer in the plastic. This hose is generally marked "SD" in the product liturature and cost about three times the standard stuff.