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.

[Doug Edwards]

It has been my experience that the bulk of industrial wood shop dust collector fires have been related to shop workers using the duct system as an ashtray.

[Bob Borzelleri]

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

Rob...

I have been thinking about that possible downside and have come to the conclusion that since I have a raised floor with considerable crawl space, the most likely complication from a desire for future changes will be having to patch "up" holes I not longer wish to use. So far, I haven't come up with other issues. I'd be interested to hear if I am overlooking something.

I really prefer the notion of having free space overhead so I can easily break light fixtures while swinging long and cumbersome stock.

:D

...Bob

[Dale Thompson]

:D
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.
Bob

Bob,
Save yourself some money and some "sheet metal" cuts on your hands and elsewhere. Go with PVC!! Also, minimize the "flex" hose. It's not a hazard but it does cost you some system efficiency. In most cases - A LOT!! :mad:

The static potential that you will build up in your system will NOT be a problem. :) Forget about grounding! It's a waste of time and material. Static electricity is only voltage. Voltage will NOT start a fire or an explosion (Theoretically the latter are the same. An explosion is nothing more than a rapidly moving fire). If you figure out a way to make an explosion in your DC, there are a lot of fire protection experts who would like to know your secret. ;) :D

In fact, as I have posted before, even the dust explosions that occur in those massive grain elevators are 90% due to open fires from overheated motor bearings or conveyor components. :eek:

Just a thought from a retarded old relic who has been involved in the fire protection engineering business for about forty years. :confused: :)

Dale T.

[Steven Wilson]

So Dale, I guess you believe that the NFPA code 664 is a bunch of garbage then. For the rest of you you might want to download http://www.nfpa.org/assets/files/PDF/ROP/664-02-rop.pdf and have a look. Even though it does apply to commercial facilities the regulations apply to systems rated greater than 1500CFM, some of us have systems of this capacity. The published regulations will cost you $30 but the link above is to a fairly recent draft. The first portion of the document contains proposed changes (a rather interesting read), the second portion contains a draft revision of the NFPA code 64 Wood and Cellulosic Materials Processing.

[Dale Thompson]

So Dale, I guess you believe that the NFPA code 664 is a bunch of garbage then. For the rest of you you might want to download http://www.nfpa.org/assets/files/PDF/ROP/664-02-rop.pdf and have a look. Even though it does apply to commercial facilities the regulations apply to systems rated greater than 1500CFM, some of us have systems of this capacity. The published regulations will cost you $30 but the link above is to a fairly recent draft. The first portion of the document contains proposed changes (a rather interesting read), the second portion contains a draft revision of the NFPA code 64 Wood and Cellulosic Materials Processing.

Steven,
First off, I don't think that NFPA 664 is a "Code". It is probably a "Standard". There is a difference. Also, it is written for "Commercial Facilities". My shop could hardly be defined as a "Commercial Facility". My wife is probably closest when she refers to it as a "MESS"! :)

NFPA 664 is not "garbage"! It is, however, a "concensus" standard. That is the case with all of the NFPA Codes/Standards. What this means is that the contents are not necessarily practical but they are all "compromises". The fact is that many of the members of the various committees are from manufacturers who have an interest in selling their products. I know because I was one of them on several NFPA Committees. I can recall spending two or three days arguing over the use of the word "should" vs. "shall" in a single sentence of a given standard. :eek: That may sound stupid but the choice could mean many dollars to the manufacturers involved. :)

Whatever, if anyone is REALLY concerned about their shop DC blowing up their home, I would suggest that they contact an "unbiased" and "non-compromised" source. Find a website for Fenwal, Inc. or some other major manufacturer of explosion systems and ask to talk with an explosion-suppression engineer or technician. After they get off the floor from laughing, I'm sure that they will answer any question that they have. ;) :cool: :)

Dale T.

[Jim Becker]

I found the document interesting, particularly in that they acknowledge that small shops with small, under 1500 CFM systems including cyclones can accomodate certain things that are total no-nos in their view for larger installations...like having the system indoors, etc.

[Dennis McDonaugh]

You are comparing apples to oranges Steve. Since NFPA seperates industrial units and "small" or "home" units at 1500 CFM I guess we can too. Call those smaller units whatever you like. If your unit is larger, by all means follow NFPA 664. I doubt many of us here have that problem in our "home" shops and I don't think most of us need to provide a continuous ground for our piping, but like you say its cheap peace of mind if you are worried about it.

[Doug Edwards]

Dale: "First off, I don't think that NFPA 664 is a "Code". It is probably a "Standard". "

Yes, it is a standard. but this standard has been adopted as code in many, many locations. It is also a requirement for the clients of the big industrial insurance companies. I know you know this Dale, but many others don't.

Dale: "There is a difference. Also, it is written for "Commercial Facilities". "

This is a very important distinction. The production processes in an industrial facility are very different than the home shop user. In several locations, the committee comments indicate that units under 5,000 cfm are not a big deal and that the safety records do not indicate a need for increased regulation of "open bag" type collectors. this describes virtually every collector mentioned on this forum. In reading through this document and the older NFPA version I have. It is pretty clear that they are not even addressing the typical small cabinet shop or furniture manufacturer. The bulk of the standard, and all of the heavy hitting requirements are for plywood, chipboard, MDF and other plants which use very flammable glues in the process and also for production finishing rooms using very flammable finishes.

Dale: “NFPA 664 is not "garbage"! It is, however, a "consensus" standard. That is the case with all of the NFPA Codes/Standards.”

Being a consensus standard, the committee will look to any available outside source to avoid having to make a serious decision. Since the NFPA standard for the conveying of combustible particulate solids (NFPA 650) requires metal duct and grounding, they will too. The duct requirements in this section are modeled very closely after that section and are consistent nearly all requirements. In none of the standards is non-metallic ducting recommended for combustibles except in cases where the conveyed material will react with the metal ducting. It is simply easier to require here what is required elsewhere than to make an exception for a small wood system.

As Dale implied, the risk of an explosion in the home system is miniscule. Even then the biggest risk is in the collector, not the duct. If you are really concerned about explosion, then you should investigate ways of protecting against damage from a DC explosion. This would require you to put you collector outside several feet from the shop with an XP damper between the collector and the shop. The fan would need to be post collector and be of spark resistant construction. There are several other requirements for this type of protection, but the NFPA decided that the risk was so low that they made an exception (the only significant one I know of) for small wood systems.

[Steven Wilson]

Dennis,

What I found interesting were the proposed exceptions for small comercial shops (some of the ones Oneida submitted) and that they were mostly rejected. As for the 1500cfm dividing line, well that's bascially between the 2HP and 3HP Oneida commercial systems (3HP being 1800CFM, 2HP at 1400CFM) - a fair number of hobby shops are close to that line so it's probably worth a read when designing your system. Too many people are dismissive of the risks associated with so called "hobby" dust collectors. I agree that the static discharge explosion risk is very small in the < 5HP systems but there are other risks that one should know about and then decide if you want to mitigate them. The NFPA guidelines are a good place to start if you want to know what some of those risks are and how to mitigate them if you choose.

[Dale Thompson]

;)
Dale: "First off, I don't think that NFPA 664 is a "Code". It is probably a "Standard". "

Yes, it is a standard. but this standard has been adopted as code in many, many locations. It is also a requirement for the clients of the big industrial insurance companies. I know you know this Dale, but many others don't.

Dale: "There is a difference. Also, it is written for "Commercial Facilities". "

This is a very important distinction. The production processes in an industrial facility are very different than the home shop user. In several locations, the committee comments indicate that units under 5,000 cfm are not a big deal and that the safety records do not indicate a need for increased regulation of "open bag" type collectors. this describes virtually every collector mentioned on this forum. In reading through this document and the older NFPA version I have. It is pretty clear that they are not even addressing the typical small cabinet shop or furniture manufacturer. The bulk of the standard, and all of the heavy hitting requirements are for plywood, chipboard, MDF and other plants which use very flammable glues in the process and also for production finishing rooms using very flammable finishes.

Dale: “NFPA 664 is not "garbage"! It is, however, a "consensus" standard. That is the case with all of the NFPA Codes/Standards.”

Being a consensus standard, the committee will look to any available outside source to avoid having to make a serious decision. Since the NFPA standard for the conveying of combustible particulate solids (NFPA 650) requires metal duct and grounding, they will too. The duct requirements in this section are modeled very closely after that section and are consistent nearly all requirements. In none of the standards is non-metallic ducting recommended for combustibles except in cases where the conveyed material will react with the metal ducting. It is simply easier to require here what is required elsewhere than to make an exception for a small wood system.

As Dale implied, the risk of an explosion in the home system is miniscule. Even then the biggest risk is in the collector, not the duct. If you are really concerned about explosion, then you should investigate ways of protecting against damage from a DC explosion. This would require you to put you collector outside several feet from the shop with an XP damper between the collector and the shop. The fan would need to be post collector and be of spark resistant construction. There are several other requirements for this type of protection, but the NFPA decided that the risk was so low that they made an exception (the only significant one I know of) for small wood systems.
[color=black]


Doug,
Thanks for the knowledgeable response to my post! :cool: My only minor disagreement to your entire post is, "Being a consensus standard, the committee will look to any available outside source to avoid having to make a serious decision". ;) I agree with the humor of that statement but I must add that, "sometimes they do and sometimes they don't!".

On my committees, we had two "weasel phrases". The most popular one was when we couldn't figure out a specific phraseology, we could always use the old standby, "Acceptable to the AHJ (Authority Having Jurisdiction)". The more recent "weasel phrase" is "use good engineering judgement". DUH? I have an engineering degree and have no judgement whatsoever - even BAD!! :eek: :)

Doug, I have even noticed lately that they are backing off of their historical theme of "Save Lives First". They are now saying that the preservation of life AND property are of equal value. Maybe they were thinking of my wife who realistically categorizes me as "property"! ;) :)

As you have probably noticed, I am not a big fan of NFPA. :( They are interested in making money and most of their folks do not know "jack" about fire protection. However, since they are the only kid on the block, I would have to admit that they are better than nothing - just barely!! ;) :)

Dale T.

[Ken Fitzgerald]

Boy have we learned more about DCs, NFPA, concensus by committee and all the guy wanted to know was should and how could he ground his duct.


Due to the relative low humidity in my area and the increasing size of the seat of my pants, everyday of the year even when it's raining, when I get out of my company van if I forget to grab the door before sliding off the seat to get out....I get a good static shock!

I don't have a DC yet I will in the future. While I have little or no concern about a DC fire caused by static build up I, for one, will probably run some checks to determine if my metal ducting is grounded and if not I'll probably run an small wire say....18 gauge copper to a single point on the metal duct and ground it to the dc metal frame. I will do this not to prevent fires but to prevent me from touching the ductwork and discharging the static buildup, if any, through my chubby body to ground. The plastic hoods/adapters should be grounded at the metal body of the tools to which they are attached. IF the tools are properly grounded and any metal ductwork is grounded....static shouldn't buildup in the first place....just my 2 cents worth....... :o

[Bob Borzelleri]

Boy have we learned more about DCs, NFPA, concensus by committee and all the guy wanted to know was should and how could he ground his duct.


Due to the relative low humidity in my area and the increasing size of the seat of my pants, everyday of the year even when it's raining, when I get out of my company van if I forget to grab the door before sliding off the seat to get out....I get a good static shock!

I don't have a DC yet I will in the future. While I have little or no concern about a DC fire caused by static build up I, for one, will probably run some checks to determine if my metal ducting is grounded and if not I'll probably run an small wire say....18 gauge copper to a single point on the metal duct and ground it to the dc metal frame. I will do this not to prevent fires but to prevent me from touching the ductwork and discharging the static buildup, if any, through my chubby body to ground. The plastic hoods/adapters should be grounded at the metal body of the tools to which they are attached. IF the tools are properly grounded and any metal ductwork is grounded....static shouldn't buildup in the first place....just my 2 cents worth....... :o

Almost makes me feel like an enabler of sorts.

...Bob

[Ken Fitzgerald]

Bob....are you sure you didn't mean "instigator"? :eek: ;) :D

[Bob Borzelleri]

Bob....are you sure you didn't mean "instigator"? :eek: ;) :D

Ken...

Another faithful implementation of the long revered, "Let's you and him fight" strategy.
;)

...Bob