explosive situation?

[Andy Dingley]

I (and every professional electrostatics expert I've spoken to) disagree.

Got the numbers ? What's the discharge mechanism ?

[Ken Muldrew]

~10^10 joules

Thunderstorm.

;-)

Ken Muldrew [e-mail] (remove all letters after y in the alphabet)

[replyonline]

Look ... if you think it could be a problem, grab some wire and connect the two together. You only get the spark where there is a difference between the two conductors. Wire keeps them roughly equal.

If you then blow a breaker (or fry the wire) when you turn the saw on, that aint static ... that's 'real' juice and you need to find out where it's coming from and fix it.

This issue is a perpetual football here on the wreck and the simple answer is the complete one. Ground it. The cost is small, the effort relatively trivial, the peace of mind immeasurable. (My plastic pipe lays on a cement basement floor and stays drained.)

The person who said that kits for bleeding the static from plastic pipe were hokum doesn't understand capacitance. They do work ... but they are solving a non-problem.

But the whole issue is probably moot since 1) you probably don't have dry enough circumstances to allow you to build up sufficient static charges to matter 2) you don't deal in a dense enough cloud of fine enough dust particles to matter even if there was a spark plug every six inches in your ducting and 3) your machines are not running long enough to build up those charges even under Arizona humidity / worst case scenario.

Relax, cut some lumber. Have fun.

Bill

[Anon Ymous]

Bill Pentz has done a lot of research into dust collectors:

has some info on static concerns: reading much on the subject my feeling is that a shop system will not be a problem. A dust explosion is a rapid burning. The static spark will have to produce enough BTUs to start a spec of dust burning then (almost instantaneously) the spec of dust must be close enough to another speck and produce enough BTUs to get that burning, and so on and so on. Impossible, no but not very likely.

The real danger in a shop situation is what happened when YOU feel the shock from a static discharge. Does your hand suddenly and involuntarily jump into that spinning saw blade? Do you best to eliminate static for this reason.

RayV wrote:

[fredfighter]

Most of the theoretical calculations posted here indicate such an explosion is unlikely under stady-state conditions. Maybe if a clump of dust gets thrown into the air all at once, like when changing dust collector bags or filters and there is a pilot light nearby.

A few years back here in rec.woodorking someone posted that the Ann Arbor, Michigan Fire Department had documented an average one such accident per year, over the past 25 years, in home workshops, with the most common ignition source being a spark from an older open-frame electircal motor.

I've not been able to find the article in the Google Archives.

That's not at all the same as a static spark. One supposes these may have actually been vapor explosions though the topic of the thread was dust explostions.

If you throw a handful of sawdust onto a fire you can get a nice fireball. So there is no question that sawdust and air can be ixed and ignited. That's a far cry from your garage suddenly exploding while sanding down a coffee table.

[fredfighter]

Why would that work with oak and not pine? Of course it would and this is my point: That is plainly not plainly not the ONLY way to get a wood dust explosion.

Suppose you sand away with your ROS or belt sander and collect a nice large volume of dust. Then when you are emptying your dust collector you drop it and the dust you created over the course of several hours all goes airborne at once.

Now, that could make a nice impressive fireball--but only if there is an ignition source like a pilot light or a spark from an electric motor. A static spark may not be enough, but static sparks are not the only ignition sources.

One should be careful not ot give an overly broad answer to a rather specific question.

[fredfighter]

Ever hear of lightning?

If a lightning bolt strikes your dust collection system, duck!

[Edwin Pawlowski]

Any documented cases of that happening?

[Minwax Mac]

Ray,

This topic has been hashed since I was a regular over 6 years ago on the wreck. I can tell you that I had a similar occurrence with my original setup (which is similar to yours). I grounded everything and never took the chance. Could it happen? Under the right circumstances

- sure. Do you have the right circumstances for it to happen? Why take the chance. $10 for the ground wire and attach it to your TS or whatever and it's done. I had a sanding table that created some extremely "fine" dust and never had a problem. The $10 was just an insurance policy that gave me that added peace of mind. Lew Hodgett is a very well read and versed guy who give can you probably all the stats you'd ever need. I've read some of his responses and he's correct in his writings. If there's a doubt - remove it and ground them. Best of luck to you.

Jim

[George]

Once again, the big picture. If your ignition source is continuous, it will burn away the smaller amounts as they come within range, preventing an explosive situation. You've got a ways to go to figure out how to get a bolus of properly dispersed dust to duplicate that in a tube....

[fredfighter]

Yes.

I've thrown hanfulls of sawdust into a fire.

That's pretty much the same thing as dumping a load of dust on the floor next to a gas water heater with a pilot light, right?

[fredfighter]

What is to stop the flame front from progessing through the dust cloud after the edge of the cloud reaches the ignition source?

The answer is nothing.

So don't dump your dust collector out near a continuous ignition source. Seems straightforward enough for me.

But it is wrong to make an overbroad statement implying that you cannot light a cloud of sawdust on fire. I've done it.

[fredfighter]

Sure it will.

I've seen some impressive sparks generated by a prototype pneumatic conveyer using PVC pipe. A dust collector is a pneumatic conveyor. A foot long spark means hundreds of thousands of volts. But I don't know how much current. No one was hurt and no fires or explosions resulted so I expect the current was miniscule.

As others have noted, grounding the system is a good idea to prevent being zapped by the sparks. They won't kill kill you, but they might shock you into doing something that will.

[Edwin Pawlowski]

Did the room explode?

[fredfighter]

Yes. In fact I was killed.

Seriously, I did it outdoors.

[mjmwallace]

It was in Fine Woodworking. Search their archives for the article. Pretty much said, as I remember it, that running a grounding wire is not needed.

MJ Wallace

[Robert Bonomi]

Well, *that's* the problem. you listened to the experts. Me, I just _did_ it. reliably and repeatably. around 35 years ago.

It was a high school 'science fair' project.

Plexiglas 5-sided cube, about 8" on a side, with a circa 3" spike through the middle of one face, the outside of which was connected to a fair-sized grounding cable (heavy-duty automotive jumper cables).

Then I had a fairly hefty Van de Graff generator -- one which could pull a 'spark' somewhere around 9". Agreed, this is getting well towards 'artificial lightning', but it _is_ still just static electricity.

Throw some cake flour in the box, hold a cover piece over it and 'shake well', set it back down, slide the cover off, and promptly take the 'wand' connected to the top of the Van de Graff and bring it across the box, from the edge opposite where the spike was. ***KA-BOOOOM***!!!!

At around minimum explosive density it was merely "_really_ loud". And it got more so, as the dust density increased.

Even without any dust, the static discharge alone made a pretty fair amount of noise.

Note: then there were they guys building the *big* (as in 8-foot-plus tall) Tesla coils. _Those_ things were dangerous! While admittedly not 'static' machines, *anything* that can maintain and sustain an atmospheric arc (a "Jacob's ladder") in excess of 4 _feet_ is deserving of some serious respect. :)

[todd]

Sounds like they had a nice little toy there. In a previous career, I designed and tested high-voltage insulators. I tested using an AC set that could generate 1000kV and create an arc over 15 feet long. We also used an impulse generator which simulated lightning and switching surges that could go up to about 2500kV. All of that was very interesting.

If you want even more fun, do high *current* testing and run 20kA through some hardware. Ka-boom!

todd

[lwasserm]

Not to diminish any dangers or recommend against any safety practices, but it's often difficult to get a lit match to light a flammable liquid, let alone a handful of dust.

[fredfighter]

Dead wrong!

It is easy to light a _flammable_ liquid (e.g. gasoline) with a match, that follows from the formal definition of 'flammable'.

It is not easy to light a _combustible_ liquid (e.g. kerosine) with a match as you have to heat it to the flashpoint before it will ignite.