Help me understand/prevent static shocks! NEW INFO

[Perry Holbrook]

I can't seem to find a way to prevent my sponge sander from shocking me with a static jolt. First on all a sponge sander is a horizontal double shaft motor with a soft sponge 6" long mounted on each arbor. An idler pulley is behind the arbor. A 6x25 sanding belt is installed between the arbor sponge and idler pulley. It is one of the best sander buys I have made. Mine is a Grizzly but has been discontinued, General has them. Anyway, the sander has a plactic dust hood, then a pvc 4" hose to a 6" pvc pipe to the main dust line to a 5HP Clear Vue Cyclone. Air return from the cyclone happens to behind the sander at eye level. The return filter is a 4' x 4' x 1'ft wood plentium with 10 12"x24"x2" nano filters mounted on the front and sides. You stand on anti fatigue rubber mats while operating the sponge sander.

A copper ground wire is attached to a rod driven in the ground outside and contacts the pvc, dust hood, metal parts of the sander, and inside the 4" hose inside. In spite of this, I get a considerable static charge while using the sander. I can actually feel my hair standing up after about 1 minute and I get a good shock when I touch anything metal.

I have tried several things. Standing on a metal plate grounded to the copper drain wire. Wearing different shoes. Touching the sander base with my foot as I sand. Opening other blast gates to reduce air velocity.

Some made a little difference, but nothing solved it except that if I actually held the copper wire in my hand while sanding, but that is not safe at all.

I haven't had a problem with this until this winter. But the return air system was installed this season.

The sander is certainly grounded, but the sponge is probably an insulator, keeping me from being grounded thru the sander.

I think the combination of the return air filter being so close , less than 2ft, the high velocity of the dust intake and being in contact with the moving sanding belt, insulated from the sander via the sponge are all somehow involved.

I just need to find some ideas on fixing this. How do I ground myself so the charge is passed to the ground and I don't get shocked?

Ideas? and thanks.

Perry

After some of the post below, I tried one other thing. The idea of draining the charge from the belt seemed to make sense. So I hung large metal washers on a grounded copper wire so they actually were in contact with the belts, thinking they would drain the charge.

Sanded for about 30 seconds and touched the sander, got a really good shock. Hopefully the grounding straps attached to me will work.

Ouch

Perry

[Rod Sheridan]

Hi Perry, grounding yourself is in fact safe.

It's the safest method of preventing you from being charged with static electricity.

The friction from the belt is creating a static charge on the workpiece, and of course you who are holding the workpiece. Your shoes and the rubber matts are an insulator, allowing you to become charged with respect to ground. Grounding yourself will remove the charge, I suggest a ground wire from you to a grounded point such as the ground rod you mentioned.

At work we wear an anti-static wristband which is a conductive band around your wrist, grounded with a wire.

I would suggest maybe an ankle strap and ground wire so it doesn't get caught in the spinning belt.......Rod.

[Perry Holbrook]

That's basically what I thought. I just thought that standing on a grounded metal plate would do it. I'll come up with something that allows me to move freely but not be a safety hazard. I'm concerned about tripping on something attached to my feet/legs or something that could get caught on the spinning sander if it is attached to my hand/arm. Maybe different shoes and the metal plate idea?

Perry

[Ken Fitzgerald]

Like Rod, I carry an ESD kit and use it when when working on equipment as static electricity is one of the biggest reasons for damage to sensitive circuitry.

If you want an idea of the cheaper ways to prevent it, here's one of the cheaper 3-M kits : http://www.specialized.net/Specialized/09039-Desco-Elastic-Wrist-Strap--6-Cord-Kit-4mm-5241.aspx Note: I've never dealt with this place but found it using a Google search.

Like Rod suggested, I'd find someway of hooking it up your ankle to keep it away from the moving sander.

Good Luck!

[Pat Barry]

Be careful with grounding straps. You need a series resistor of approx 1 Meg Ohm in order to bleed static charge build up to ground and in order not to create a high current pathway thru yourself to ground. A plain old copper wire tieing you to ground effectively creates something similar to an electric chair. You must have a series resistor! You touch a live wire with a hard grouund strap and the very best you will escape with is a shock. Worst case we will never hear from you again.

[Dan Friedrichs]

Pat has a good point. If you accidentally touch a hot wire when UN-grounded, you may only feel a slight shock because you (wearing your rubber shoes, etc) don't present a very good path to ground. If you tie a grounded wire around your ankle, you WILL present a good path to ground, and significantly more current could flow through you.

The wrist-straps (like Ken linked to) have a large (megaohm) value resistor in series to prevent this.

[Ken Fitzgerald]

You guys would bring that up after I returned all my annually calibrated test equipment to tool depot since I retired Friday .......but.....I had a cheap analog meter and I checked my 3-M kit and it has the resistor built in to it.

[Richard Wolf]

You guys would bring that up after I returned all my annually calibrated test equipment to tool depot since I retired Friday .......but.....I had a cheap analog meter and I checked my 3-M kit and it has the resistor built in to it.


Nice stealth gloat Ken. Good luck.

[Perry Holbrook]

Thanks, just bought a couple of the ones Ken showed.

Perry

[Harvey Pascoe]

I had the same problem with my stationary buffing machine, shocks the hell out of me till I started wearing a ground strap. Not worried about grabbing any hot wires while buffing. Not going to climb any power utility poles either.

[Bob Riefer]

I previously worked at a company called Techni-Tool as a product manager for their static line. The company is among the US leaders in static prevention for the electronic production (and similar) industry.

I can tell you that the private label offering they carry (the "Techni-STat" product line) is equivalent to 3M or Desco or any of the other brands, and you'll save a boat load.

Heel grounding is probably a better fit for woodworkers as I'd be concerned that a wrist strap could get entangled in machinery.

Also, there are ESD mats (again, the Techni-Stat brand is great, and WAY less expensive) an sprays that can be effective in some applications (for example, spraying the outside of PVC pipe).

I don't work there anymore so I have nothing to gain with this recommendation. Use www.techni-tool.com (http://www.techni-tool.com) and try their "virtual catalog" from the homepage.

[John Coloccia]

Was going to suggest heal grounding and technitool. If I could see the actually machine in front of me, I could probably devise another solution that would mitigate the problem. Most of the solutions would take the form of wrapping something with foil or wire so the part you're touching is somewhere around ground potential, and if there's a discharge it will be to the metal and not to you.

[Mike OMelia]

You could direct all that charge to hollow metal balls above your sander. Every now and again, enough charge would build up and create a mini lightening bolt between them (assuming grounding on one of them). Very entertaining. And anybody wandering by would think you were some kind of mad scientist. Tilt your head up and say "It's Alive!" ;)

Congrats to Ken on retirement and I think you have now isolated the problem.

Mike

[Perry Holbrook]

Please see my new info in the first post.

Perry

[Anthony Whitesell]

I was just going to suggest a heel strap. From Ken's referenced website try this link http://www.specialized.net/Specialized/Desco-static-heel-grounder-Desco-esd-heel-ground-8901.aspx

How long does it take for the charge to build? Have you tried running the sander for a minimal amount of time to check if it is the DC or the sander?

The more I think of it and I'll think about it some more. But grounding yourself probably isn't helping the situation. If the sander is building the charge, the you are at enough of a different potential to cause the static charge to discharge through you. Grounding yourself, may just increase that potential. Along this line of thought, you would need to reduce the static charge build up at its source, the sander. An ioizer may help with this situation, such as http://electrostatics.com/product4.html. The other option would be to raise the humidity level in the area to help reduce the build up of the statis charge.

[Dan Hintz]

you would need to reduce the static charge build up at its source, the sander.
Exactly... don't complicate the situation with ESD setups. Find the offending link on the machine itself and ground it. Maybe the table isn't well grounded like the machine's case, maybe its the substrate you're working on, etc. Solve that problem, don't add a new one.

[Dick Strauss]

Heal straps only work on special conductive flooring. As Dan wrote...locate the offending area and ground that area only.

[Perry Holbrook]

I've got ground wires running all over this machine, including the washers dragging on the belt and it is still shocking me, even with high humidity (it's raining). The charge is in me, I'm pretty convinced the sponge is the cause. If I hold the ground wire in my hand as I sand I don't have the problem. I'm ordering heel straps and trying that.

Perry

[Dan Hintz]

Perry,

As mentioned before, heel straps will do nothing for you unless you have a conductive floor mat to walk on (and they're not cheap).

[Rob Lindquist]

As far as grounding conductors go, this static string has been very useful, moreso then a bare wire even on some very challenging static issues we needed to solve at work. www.stopstatic.com

[Ken Fitzgerald]

Perry,

If holding the ground wire in your hand prevents the shocks, the wrist straps you ordered should remedy the problem. The problem with them is I wouldn't want the cords or straps to become entangled in the sander. However, from someone who has worn them for 30 years or so, typically the problem is that the cord pops off the strap too easily. If you could wear them around your elbow...away from your hands it might be safe.

[Perry Holbrook]

Perry,

As mentioned before, heel straps will do nothing for you unless you have a conductive floor mat to walk on (and they're not cheap).

I had cancelled the wrist strap order when someone suggested trying to ground the belt. When that didn't work, I ordered the heel straps today. My plan is to lay a grounded sheet metal pad on top of the anti fatigue mat. When I ordered it I told them my idea and they thought it was a good one, made no attempt to up sale me. In fact they even sent me the strap as a no charge sample. Not bad.

Thanks for the input.

Perry

[Dan Hintz]

If you could wear them around your elbow...away from your hands it might be safe.
No reason you could wear it around your ankle... assuming you're going to use the same machine for a while, else it would become a pain real fast.