Plastic DC hose and static -- OUCH!

[Carlos]

I was using the drum sander, put my hand on top of it, and got a huge zap that made my arm go weak. I looked down to see a big spark jumping from the coils in the hose to the hose clamp every two seconds or so. It was blue and about 1" long.

Needless to say, I've run a ground wire through it now.

Oddly, it only built up energy and sparked while actually running material through it, not while the DC and machine were running with no wood in it.

[eccentrictinkerer]

It's the friction between the moving material and the hose that causes the voltage buildup. It's spectacular, but harmless (unless you jump back and smack your elbow on a drill press table). DAMHIKT

I sold Fenwal explosion suppression systems for many years to industrial plants.

Our engineers were adamant that a home dust collection system poses no threat. There's lot of info on the 'Net regarding this topic. (A lot of mis-information,too. )

[drumpriest]

Carlos, what's your DC again? I have rarely gotten little shocks, but it's happened. With my Jet 1100CK on the planer with air assist, it's happened.

[TheChief]

Actually, it's not that surprising. The material being moved (wood) and the material it's moving in (the hose, unless it's metal, and air most likely dry) are insulators. Being of different materials, the moving wood dust gives up electrons to the hose creating a difference in electrical potential. Eventually there is enough difference for those transferred electrons to discharge when a ground path becomes available -- in this case, you.

Think of it as someone dragging thousands of tiny tiny feet against the inside of the hose then waiting to touch you on the nose When dust isn't going through the hose, no foot dragging - much smaller chance for shock.

It is possible to move a large amount of dry air over a large enough a surface for long enough to build up a decent potential. After driving for a while in the winter around here when the humidity is low I'll often get bit when touching the door when getting out of the car. With DC through a hose it would probably take a long time for air alone to build up enough to notice, but under the right conditions with a big DC I imagine it could happen.

[LCHIEN]

has the ground wire helped? To me it sounds like it really won't help much, the problem is everything is insulated - the ground wire ties the two machines together and to ground but the moving material is still striking the hose. If the hose is being charged and charging the spiral wire, it will still arc to the frame of the tool since there's no ground path. You need to ground the spiral wire to the clamp and/or to the frame of the DC and or planer/tool.

There's not enough energy in the arc to start a fire due to heating. The possibility of ignition of the dust is the parimary cause of dust explosions, the woodworking dust is generally not fine enough and dense enough in quantity to be explosive.

The static discharge if not properly conducted to ground can be distracting and uncomfortable, enough reason to try and control it, but I believe it has been proved its not an explosive/fire hazard in home WW systems.

[Carlos]

Carlos, what's your DC again?

Powermatic 3HP, it's a monster.

has the ground wire helped?

Sure, it removed the problem because everything involved is now grounded. There is no way for a spark from the hose wire to the clamp because the ground is connected to both, to the machine frame, and to the DC (via the metal DC pipes, which are all connected to the DC frame). I had everything grounded before except for the metal coils inside the hose, and that built up the charge. I exposed the end of that coil and made a loop to hold the grounding wire.

There's not enough energy in the arc to start a fire due to heating.

Agreed, and also agreed on the extreme unlikeliness of having the right conditions for an explosive dust mixture. It was enough energy to make a burned spot in the plastic hose, but still probably not enough to cause a dangerous situation. The jolt was painful, but as a former TV repair guy, I've been hit with worse.

I wasn't posting this as a warning or anything, but it was interesting.

[GoFish104]

Carlos, if you can increase your humidity it will help also. I can get a pretty good jolt off my shop vac hose in the winter when we're dry. If I just spray a couple shots of water from a spray bottle set on mist it will stop. Jack

[Carlos]

if you can increase your humidity

Y'all ain't from 'round here, are ya? It's pretty dry, and the garage-shop wouldn't hold humidity for long. Usually we work with the doors open anyway to keep fresh air flowing.

[SARGE..g-47]

You shouldn't have any problem at this point if you are indeed grounded from stem to stern. I run a 3 HP cyclone that has about 10' of PVC coming off of it over-head and down a forward wall. Then I have 16' of clear 6" hose that does have wire spiraling through it.

I drilled a hole in the cyclone where the PCV starts and wrapped copper wire around the PVC all the way to the end.. the PVC got holes drilled every 6" and a screw inserted through. I just wrapped the copper wire around the PVC and also made a twist around each exposed screw head.

At the machine end I drilled a hole and mounted a ground screw with a copper wire coming off with a female connector on the end. I use the flex hose to connect directly to 4 large machines that all now have ground screws. I use a male connectors at the end of the clear hose with the spiral wire to connect and dis-connect quickly from one machine to another. Plug in or un-plug the male-female connector.

Since doing this.. I have never even gotten a mild shock that is common with PVC from static.

[Carlos]

You shouldn't have any problem at this point if you are indeed grounded from stem to stern.

Yeah, I've looked around for "sneaky" problems like that ungrounded metal coil in the hose, and think I've got them all. I haven't had any other shocks or seen sparks.

[Tom Miller]

I wasn't posting this as a warning or anything, but it was interesting.

Well, it's not a bad thing to warn about, either -- just not for the reasons some might expect. When you get a zap that you're not prepared for, it could lead to trouble. The last time I felt an appreciable shock was when I was edge routing with a handheld router, and with a 4" DC hose jury-rigged to the setup. Luckily I don't throw stuff in the air when startled like the wife does.

Like Loring, though, I'm a little incredulous about the effect of grounding wire. Obviously, something is working for some people, but given the resistivity of the hose material, not to mention PVC, it's hard to imagine anything short of a full metallic coating keeping the static down.

Regards,
Tom

[Carlos]

This particular hose is the type with a metal coil instead of plastic coils. I think what's happening is that the charge dissipates into the coil from the nearby plastic, charging the coil to a potential that is high enough to arc to the nearest grounded object. This happened even with a grounding wire inside the hose, but the metal coil in the hose was not connected to the grounding wire.

Good point on the potential for injury if you jump. I did, as the jolt was pretty good, enough to leave my arm numb for a bit. Had I been using a bladed tool instead of a sander there's a bigger potential for danger.