Following on from the recent DC discussion ... what is the deal with static charge buildup with all this dust flying around. I've heard recommendations to add wire spirals around the tubing and connect it to DC and machine grounds at each end.
I imagine this is only a problem with plastic tubes/conduits ... metal would seem to be proof against the whole thing. What do the experts have to say on this?
I can only imagine the static build up that will happen. I am in the middle of putting up my collector system and the amount of plastic (ABS) shavings that stick to the cut pipes is amazing. The stuff is everywhere. Stuck to the saw, the walls, the piping, me; I mean everywhere. I will definitely have to try to ground this somehow. I was thinking of grounding to nearby
110V outlet cover plate screws. Is this a viable way to go?Paul
If your tubing is metal, that will dissipate the static charges. I have plastic tubing with a spiral wire embedded. I used to get static shocks before I grounded the wire.
That will work if the screw is indeed grounded, but it is better to use a pipe you know is buried into the ground.
I had a lot of electrical "book learnin'" in kollege but my practical knowledge is very low. In this situation, grounding a DC to a good earth ground, should there be a current-limiting resistor somewhere between the water pipe and wire going through the DC system? If so, what size?
Thanks.
-- Mark
This is really stupid when you think about the fact that you're wrapping wire around an insulating material.
Slide a wire tap along the coil, and tune radio stations!
Barry
It's not necessary since the connection keeps the wire at the same potential (or very close to) ground. There is a very small current flowing through the wire. To install a resistor would guarantee that the wire has a small potential on it.
Only make the connection at one point. The most logical point would be to ground the wire at the DC end. Run wires out all the way to the ends where your machines hook up to it but don't connect it at that point. Ground should only connected at one point.
The pipe itself is the conductor in metal systems and should still be grounded at one point.
You can ground the system if you have problem with getting shocks from it. Its not necessary, from a safety point of view. I liive in Houston where the humidity is relatively high and I have no problems at all running ungrounded.
Bob
I got the idea from (IIRC) the "grounding straps" sometimes used when repairing computers. It's my understanding these all have a resistor in them to limit the current flowing through the repair-person in the event of current-to-ground.
As a kid I was badly shocked by 1960's era tools (*) and I admit I may be overly concerned about a direct circuit to ground (especially since the ground wire of a standard 3-prong outlet does the same thing if I understand home wiring correctly). But suppose I'm concerned about doing something really stupid like grabbing the DC grounding wire in a sweaty hand, then biting through the cord of my Skilsaw. Would a properly-sized resistor between the DC grounding wire and ground help in this situation?
-- Mark
(*) Most memorable: I turned on a sheep shearing machine, got a whale of a shock, couldn't turn it off and had a hard time letting go. :-( Fortunately the floor of the sheepshed was fairly dry and I was finally able to drop the d*mn thing. If the manure had been fresh it may have killed me.
No.
Resistors in grounding systems are only there when grounding personnel. It's not needed for static, but it can save lives. An accidental mains-voltage electric shock can be painful in a typical situation, but fatal if you have an arm grounded with a low-impedance path to ground. A mere few megohms makes no difference to grounding effectiveness, but it's a significant barrier to mains voltages.
It builds up. If this is a nuisance to you, then ground it.
There's no safety hazard to static discharge (in a woodworking DC). The two real safety risks are static discharge mixed with flammable solvent vapour, or sucking up a hot ember into the dust stream.
Grounding is hard to do reliably. An internal wire will suffer dust erosion. The worst case for static buildup is a metal flange in the plumbing that's insulated by plastic pipe (it creates a capacitor), so make sure that such things are earthed by external bonding wires.
Suppose I'm (irrationally) afraid of doing something stupid, like wrapping the DC ground wire 12 times around my arm, then shooting a 16d nail through my thumb into the live hot wire on my Skilsaw. What are the specs for a resistor to put between the DC grounding system and the water pipe to limit the current flow yet allow for DC static discharge?
Or can someone talk me out of a fear of an open megaamp path-to-ground throughout my shop?
Thanks.
-- Mark
To date none of the experts have come up with a single itty-bitty bit of evidence that PVC piping will lead to a dust collector explosion.
That's the facts, the whole facts and nothing but the facts.
UA100, stopping at the Lancome' counter after suiting up to go fly with Grandma, over the Sinai of course...
Techs don't ground themselves when working on live power which is the only way that current could flow. That would be dangerous. Grounding is used to dissipate static electricity so that circuits aren't damaged. These are circuits that are being worked on with no power. It would serve no purpose to put any kind of a current limiting resistor in the ground path to dissipate static electricity. In fact, it would be counterproductive.
No resistor would help that kind of foolishness. BTW - your body is more of a resistor than the wire, so in effect there is already a current limiting resistor in the circuit. I think you should stay away from electrical appliances though - at least until you get your fear of doing stupid things under control a bit...
Your shop is protected by circuit breakers or fuses of lets say 20 amps (depending on the branch circuit). Megaamps as you call it won't happen in a direct short from hot to neutral. Your body is in effect a limiting resistor if it makes the connection. The bottom line is adding some resistor in the system will only cause problems for you and will not prevent you from mutilating yourself if you put your mind to it.
Frank
On Sat 13 Nov 2004 02:57:13p, "nigel" wrote in news: snipped-for-privacy@f14g2000cwb.googlegroups.com:
Did anybody catch that Mythbusters show where they tried to make the Static Canon?
The myth is that a construction worker finds a really nice piece of large diameter, about 8-10 inch, plastic pipe and wants to use it but it's all dirty so he sandblasts it at the site and builds up such a huge static charge on it that when he walks around its end, it shoots a lightning bolt out of it and blows him across the compound.
They tried everything. Couldn't even make the thing spark. They had a voltmeter on it, and it kept building up a little charge and then dissipating.
As someone else has said, the stuff is an insulator. Grounding the outside grounds only the outside. Don't need a resistor, you're only sending a few volts to the ground. But any charge inside the pipe is still there.
I've also seen articles on attempts to duplicate a shop explosion. They found that the dust particles have to be a certain size, and be dispersed in the air in such a way that each ignited particle ignites the particle next to it, and in such a manner that it creates an explosion-like combustion. They found that when they finally got the conditions correct for a dust explosion, the air was so full of fine-particle dust they could barely see. And any air currents in the area screwed it up. The dust particles weren't close enough together. Their conclusion was, if you have the correct conditions to create a dust explosion, you should be more worried about breathing than an explosion.
Want me to hunt around for the article? I think it might have been posted to this very newsgroup.
Dan
Agreed that "megaamps" won't happen from hot to neutral. "Could" they happen from from hot to the DC ground? (Yes, I'm an idiot on these things.)
Thanks.
-- Mark
So the general word is that there's no safety hazard involved - saw dust is obviously different to flour, for example.
That's what I was looking to be sure about! My new shop will be in a building that doesn't belong to me and as the lawyers here in europe are starting to get almost as trigger happy as in the USA I wanted to be sure I wasn't not making myself negligent through ignorance. Thanks people ....
Not so sure it's stupid - this is static charge we're talking here, not anything with a significant current.
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