DC Static Electricity and Fire - How Real Is The Threat?

[Glen Blanchard]

We have all read the recommendation to run copper ground wire through our DC ducts to discharge any static electritiy to ground and prevent fire. However, I have never read a post by anyone who witnessed a fire caused by his/her DC, and I would think there are those who have not grounded their systems. How real is this threat? How likely is the problem?

Just wondering.

[Dennis McDonaugh]

Glen, there have been several articles in the woodworking mags and there are several internet articles which conclude home dust collection units pose no fire threat from static electricity. Seems they just don't generate enough static charge to be dangerous. Larger, industrial collects are another matter.

[Jeff Sudmeier]

I have never heard of a home shop having this problem, however, it CAN happen with major quantities of dust. Grain elevators used to be well known for spontaniously exploding due to the large quantities of dust. Any spark could set them off.

(My system is not gounded and I have not received so much as a shock from it).

[Jim Becker]

http://mywebpages.comcast.net/rodec/.../DC_myths.html

[Keith Christopher]

This myth was debunked on the Discovery Channel show MythBusters. They used standard PVC pipe and used a sandblaster to try and build up an enormous amount of static. while they could generate a nice shock from it, the amount of combustable particles in the air in the tube could not be reached. The myth was totally debunked. While possible, not likely probable.



Keith

[Glen Blanchard]

Thanks guys. This is what I had suspected for a long time, but this is the first I have seen anyone comment on it myself. Thanks again.

[Bill Lewis]

And yet in a related vein. On page 8 of the most recent issue of Fine Homebuilding (sister mag to FWW) there is a reader feedback letter titled "Shop vacuums and sparks are a dangerous combination"
The letter warns that using a grinder on tile and/or plater can produce sparks, this combined with a shop vacuum containing sawdust can produce the kind of mythical ignition that WWr's have been debating for years. In fact the author states that it happened to him.
While I don't disbelieve the author of the letter, I tend to be a skeptic about the whole issue too. I too saw the Mythbusters episode, and tend to agree (at least for entertainment purposes) with their results. Though it just occured to me that they left out one important element of their testing, namely humidity.
Later in the issue of FHB the warning is repeated in the "Tips and Techniques" section by another contributor with his suggestion of a dust-isolation box for cutting out tile (page 20).
My only related experience has been that I have gotten repeatedly shocked by static electricity while using a shop vac to pick up both drywall and plaster dust.

[Brad Olson]

Just note there is a difference between static discharge and cutting non-wood materials.

Hobbiest dust collectors are designed for wood dust and shouldn't be used for metal cutting (or dry tile blades) since if a hot spark gets into the dust bin you can start a smoldering fire (same goes for shop vacs)

IMHO, floor sweep pickups are bad also because if you accidentally sweep in a screw or piece of metal you can have a collision with the impeller or DC body that sparks and also starts a smoldering fire.

In reality you should check your DC at the end of the day for fire because it is very easy for a tiny ember to start a fire and be very cautious what you sweep into your floor pickup. I personally will not install a floor sweep on my system, but you can use one if you are careful

And yet in a related vein. On page 8 of the most recent issue of Fine Homebuilding (sister mag to FWW) there is a reader feedback letter titled "Shop vacuums and sparks are a dangerous combination"
The letter warns that using a grinder on tile and/or plater can produce sparks, this combined with a shop vacuum containing sawdust can produce the kind of mythical ignition that WWr's have been debating for years. In fact the author states that it happened to him.
While I don't disbelieve the author of the letter, I tend to be a skeptic about the whole issue too. I too saw the Mythbusters episode, and tend to agree (at least for entertainment purposes) with their results. Though it just occured to me that they left out one important element of their testing, namely humidity.
Later in the issue of FHB the warning is repeated in the "Tips and Techniques" section by another contributor with his suggestion of a dust-isolation box for cutting out tile (page 20).
My only related experience has been that I have gotten repeatedly shocked by static electricity while using a shop vac to pick up both drywall and plaster dust.

[Dale Thompson]

Glen,
As has been noted above, there IS no problem! Your chances of winning the zillion dollar lottery twice in two days would be better than having an explosion in your home shop DC. Even if there was one, the bags would act as "vents" to relieve the damaging pressures caused be an explosion. Pressure is the hazard in a true "explosion", not fire. Your hot water heater is a much greater "explosion" hazard than is your dust collector. Running wires through, in, around and over your ducting is a laughable waste of time. They not only interfere with the flow though your ducts but they could also be a problem in and of themselves.

Jeff mentioned the problem with grain elevators. He is right. Grain elevators occassionally do some really awesome "explosions". The fact is, however, that 95-99% of the sparks that cause these explosions are not due to static electricity. They are due to overheated conveyor bearings or drive-motor failures.

Brad mentions "floor sweeps" as being a potential hazard. He is also right. If you have a plastic or a steel impeller, you don't have a problem. If you have an aluminum impeller made of one of the 4000 Series (or is it the 2000 Series?)of Aluminum alloys, you may get a spark. Even if you get a spark, however, I would still bet on a lottery victory over an explosion. Dust explosions are VERY difficult to create, even with an OPEN flame.

There are two materials that scare me with regard to explosions and neither should involve home shop dust collection: Hydrogen and an Oxygen/Acetylene mixture. Mess with these and you can expect to take up residence, very quickly, in another state or country!

Dale T.

[Bill Lewis]

Well I never wanted to suggest that I thought there was any danger with using PVC or any other plastic pipe. I really don't want to perpetuate the myth of the exploding dust collection system. I also think there is even less of a risk that a spark generated by an impact of a metal particle and an impeller than there is with static discharge (another good reason to get a cyclone).

I guess I was drawing to much from my memory on how Mythbusters used several sources of ignition, and several types of media to attempt to "prove" it right or wrong. Funny thing, I have seen the experiment with the coffee can, a candle, a small pile of flour, and a long straw performed. Something they could not accomplish in a high tech way. BTW, I didn't mean to suggest that static discharge and sparks from a grinder are the same. While both are a possible source of ignition, the energy in a concentration of grinder sparks (and related heated particles) is indeed much greater.
I thought it was interesting, and somewhat misleading, that FHB featured a letter from a guy who had his shop vac catch fire. Again, if I recall correctly, it was FWW who put an article out disproving the exploding PVC ducts and static theory.

[Dale Thompson]

Funny thing, I have seen the experiment with the coffee can, a candle, a small pile of flour, and a long straw performed. .

Bill,
In my experience, the best way to replicate a "dust explosion" is to put about two teaspoons of coffee creamer (I think that the stuff at Burger King works the best. I don't know why but that's what my students told me ). Put the creamer into a funnel and attach a clear plastic tube to the bottom of the funnel. Have someone blow on the tubing while someone else is holding a flame over the top of the funnel. With luck, you will get a "dust explosion".

WARNING!! Don't do this in your shop unless your shop lights, ducting, water pipes, gas pipes, etc. are a LOT more dust free than mine!!

Dale T.