Dust Collector and Static Electricity

AnotherJim

Jim
Corporate Member
I just finished reading a long article on static electricity in PVC ductwork:


With some sometimes tortuous explanations, the author discusses the issue of static electricity generation in ductwork. He uses logic, basic physics, and some equations to describe electron density, migration, and grounding within typical 4" PVC ducting attached to dust collectors. He concludes that it is effectively impossible for the static charge inside the duct to cause a spark, combustion, or explosion within the flowing air and entrained wood chips. I think he said (I read it a few minutes at a time, when time allowed) that it is entirely possible to receive a static shock from the outside of the ducting.

I started reading because I'm wanting to install some 4" PVC ducting on my (eventual) dust collector. But in forums and YouTube presentations I keep seeing wires wrapped spiral-fashion around PVC ducts; references to wires strung down the insides of PVC ducts; screws driven through the PVC duct walls at 4" spacing; and even a case of copper tape stuck to the length of the ducting, inside, and grounded at the joints. (Makes one imagine a horizontal Van de Graaff generator working there in the garage.)

So, can anyone recall and describe a personal experience with a static shock, fire, or explosion from his or her own PVC ducting?

Thanks,
 

Michael Mathews

Michael
Corporate Member
I've had 4" PVC pipe for DC in three shops now and none have been wrapped or had any kind of ground wire. I've read about it and heard pros and cons. I never get shocked from my pipes or anywhere on my DC. Call me lucky or call it a hoax. Now, my pipes are also embedded under the floor of my shop through the concrete and tied to the rebar so maybe that's where the grounding is taking place.
 

tvrgeek

Scott
Corporate Member
I have a clear-view which is plastic. I think ESD took out my first bin level sensor. A bit of foil tape to drain the worst of it and no more problem. I use standard HVAC duct now I simplified my layout.

As far as my research goes, there had never been a documented case of an explosion or fire. All hype and theory. Basically, the dust in the air stream is not dense enough. I get far worse shocks off my shop vac. So, it is more of a nuisance than danger.

The last thing I would do is stick screws into the ducts to cause turbulence and catch dust. Run a strip of foil tape the length of them on the outside and tie to ground will drain the most of it if you think it is a problem.
 

gmakra

George
Senior User
I have gotten a jolt a couple times though its not that bad. A little bit worse than the kind you get walking on carpet.
 

tri4sale

Daniel
Corporate Member
So, can anyone recall and describe a personal experience with a static shock, fire, or explosion from his or her own PVC ducting?

I can attest on my CNC machine hooked to my DC it creates a nice shock before I grounded the system. Created enough static charge that it actually interfered with the USB communication between the CNC and computer, once I installed grounding wire in the DC no more communication errors.

Theoretically yes it is possible for static to build up and cause a spark/explosion/etc. Realistically more likely to start a fire w/ a DC by it sucking up hot piece of wood from cutting, especially with a CNC machine.
 

Rwe2156

DrBob
Senior User
I've had the hair on my arms stand up when close to the pipe, and I've been zapped a couple times by an aluminum blast gate. I gtrounded the blast gates since then.

The static causing explosion/fire/spark thing has been thoroughly debunked. What Daniel described seems possible.

On another note, please describe your proposed system, specifically the blower size. 4" ducting is going to dramatically reduce performance for any blower 1 1/2HP or larger!
 

JimD

Jim
Senior User
My last shop had ducting of 4 inch S&D PVC, my current shop has 5 inch snap lock metal pipe. I never got shocked by my PVC ducts but it did get dust stuck to it, presumably by static electricity. My metal pipe is not grounded but I don't see any dust on it. I would have used PVC again but I wanted 5 inch and had no ready source for 5 inch PVC piping and fittings. But I think metal may be a little better, but kind of a hassle to install. I think snap lock needs the seams sealed which I did with duct mastic and tape.
 

Alan in Little Washington

Alan Schaffter
Corporate Member
Dr. Rod Cole's article has been the gold standard since 2000(?)

The problems with PVC is that it is a non-conductor, so metal tape, wire wrap, etc. will only drain static for close to the wired, not the whole PCV surface. You can ground metal blast gates but that will do little for the PVC duct. Often PVC will quickly lose much of its ability to become and remain charged as the interior gets scoured and coated with dust.

As far as hot metal or wood sparks igniting the dust- two factors make that nearly impossible when the DC is running in home DC situations- first the concentration of dust generated in a home shop environment is not sufficient to allow and sustain a fire, and the velocity of the air, typically 4000 fpm, will extinguish sparks and cool the metal.

If getting zapped, the easiest thing to do is make sure the machines and metal blast gates are grounded, and wrap the sections of duct where you are getting zapped with metal screen which has been grounded.
 

kelLOGg

Bob
Senior User
I want to resurrect this thread because there is something very basic I do not understand.

How does a wire embedded in a pipe neutralize static charge? The charged particles cannot contact the metallic conductor because it is actually insulated by being embedded. So, why is the conductor embedded or on the outside of the pipe? I can understand charge collection on the inner surface of a DC pipe but in order for it to be neutralized shouldn't it be in contact with a grounded conductor?
 

McRabbet

Rob
Corporate Member
In order to dissipate the static charge build-up from flexible duct hose, the wire must be grounded at some location; usually at one end. If there are multiple segments of hose, the segments can be linked with wire jumpers and then grounded at a single point. As long as dust is passing through the ductwork, a static charge is built up, but the ground wire will continuously remove charge from those sections.
 

JNCarr

Joe
Corporate Member
I want to resurrect this thread because there is something very basic I do not understand.

How does a wire embedded in a pipe neutralize static charge? The charged particles cannot contact the metallic conductor because it is actually insulated by being embedded. So, why is the conductor embedded or on the outside of the pipe? I can understand charge collection on the inner surface of a DC pipe but in order for it to be neutralized shouldn't it be in contact with a grounded conductor?
Static charge is an accumulation of electrons that have been ripped off of atoms during mechanical friction. Depending on the two materials' position in the triboelectric series, shown below, one will accumulate electrons and the other holes (lose electrons). Since wood is in the upper third (positive) and PVC is very near the bottom (negative), the PVC gains the electrons. The electrons spread out throughout and around the PVC tube because like charges repel. Under no other influences, the charge will very quickly equalize over the entire tube and stay that way. The normal external influence is the humidity in the air which will dissipate the charge - this is why static is more of a problem in dry weather. The moisture in the wood also helps significantly. In any event, if the charge is generated faster than it can be dissipated, it accumulates. A conductor (wire, metal tape, etc) run inside or outside of the tube will very quickly dissipate the charge in the immediate area of the wire. Now the area NOT near the wire is more charged than the area near the wire, so those excess electrons move toward the wire to equalize the charge density across the entire tube - and they too get swept away by the wire. So surface area not in direct contact with the wire also gets discharged. This takes a certain time based on a number of factors, including geometry and the material's conductivity and permittivity.

Grounding near sensitive equipment or sensors is a good idea.
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tvrgeek

Scott
Corporate Member
I want to resurrect this thread because there is something very basic I do not understand.

How does a wire embedded in a pipe neutralize static charge? The charged particles cannot contact the metallic conductor because it is actually insulated by being embedded. So, why is the conductor embedded or on the outside of the pipe? I can understand charge collection on the inner surface of a DC pipe but in order for it to be neutralized shouldn't it be in contact with a grounded conductor?
Because there is some leakage. Just as no conductor is perfect, no insulator is perfect.

It does not "neutralize" it "drains".
 

kelLOGg

Bob
Senior User
I got a better understanding now (I hope). I was trying to understand it from the viewpoint of a chemist (my field) where charged particles (ions) had to contact an electrode (wire in this case) in order to be neutralized. Not the case in dust collection where there are not charged particles but charged surfaces due to friction between the surface and wood dust. Is it like walking across a carpet on a dry day and touching a surface and getting a shock? The surface you touch may be a good conductor like a doorknob and you get a big shock or poor conductor like a wooden door where you get smaller shock. In either event the charge is "drained".
 

tvrgeek

Scott
Corporate Member
I got a better understanding now (I hope). I was trying to understand it from the viewpoint of a chemist (my field) where charged particles (ions) had to contact an electrode (wire in this case) in order to be neutralized. Not the case in dust collection where there are not charged particles but charged surfaces due to friction between the surface and wood dust. Is it like walking across a carpet on a dry day and touching a surface and getting a shock? The surface you touch may be a good conductor like a doorknob and you get a big shock or poor conductor like a wooden door where you get smaller shock. In either event the charge is "drained".
Add to that you are a pretty decent size capacitor, so regardless of the floor or your shoe rubber, you still get a shock. Back when I got Radius and Apple certified, I knew about how many uF.
 

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