Static buildup in DC systems ?

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On Sat, 13 Nov 2004 21:59:49 GMT, "Mark Jerde"

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.
--
Smert' spamionam

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Andy Dingley wrote:

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
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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
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Frank Ketchum wrote:

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
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Absolutely not. The circuit breaker (or fuse, as the case may be) limits the current that can flow through the hot conductor -- without regard to where it's flowing to.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.
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On Sun, 14 Nov 2004 05:21:41 GMT, "Mark Jerde"

Since the neutral and ground are bonded in the box and there is less than an ohm of difference between their points on a solid copper line. Whatever won't happen to one won't happen to the other. <g>
Static discharge is from whatever ungrounded area to either neutral or ground, since they are the eqivalent to any major static force. Yes, static happens, but it doesn't blow up DC systems in gar^H^H^Hshops.
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On Sun, 14 Nov 2004 00:35:06 GMT, "Mark Jerde"

They're complicated. It's a few megohms (9M from memory) but like any of these high value resistors, you have to account for construction and surface paths as much as the resistor itself, and especially guarding against failure modes that could cause a short. Usual HT practice is to never use a single resistor anyway, but to use smaller ones in series. If one fails short, it's just a small change in resistance, not an overall short.
UK practice is to buy a plugtop. This fits in a standard bench power outlet, is bright yellow and has a number of press-stud connectors on the top. Inside is the right sort of resistor, bonding you to the mains ground. If your bench was built for electronics work, then it might have such connectors built in anyway. Static wrist strap

For the question of "What sort of minimum resistance makes me vague safe ?" then it's hard to answer. But a resistor of just 27 kilohms would limit current to under 10mA, and that's going to reduce the serious hazard massively.
NB - I'm not suggesting static straps with 27k resistors !
So I hope that shows that putting in even a relatively low resistance to the ground path can keep the fault current non-lethal.
As a rather gruesome example, there was a recent UK case (discussed at length in uk.d-i-y) where the daughter of a TV presenter (?) was electrocuted in their kitchen. A metal pan rack had been installed over the cooker and the mounting screw had nicked a cable in the wall (the cable had been improperly routed and not correctly protected). For some time the family had experienced a tingle from this rack, but done nothing about it. Finally the victim was killed by it, because one day her ankle also happened to be touching the well-earthed and low-impedance case of the dishwasher (there was a contact burn on her ankle aferwards).
--
Smert' spamionam

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Andy Dingley wrote:
Thanks for the informative post.

So my fears aren't, um, groundless. ;-)
-- Mark
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I'll try. I don't know if this will talk you out of your fear, or increase it to the point that you won't ever walk into the shop again, but here goes. :-)
Every metal-framed tool in the shop that has a three-conductor power cord plugged into a properly grounded outlet constitutes "an open megamp path-to-ground".
The same is also true of most, if not all, of your major appliances such as your refrigerator or your washer.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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I don't know about the "megaamp" part of it, but I'll take a shot at talking you out of it; Every powertool in your shop with a 3 prong plug, every water pipe, EMT, BX cable, and who knows what else is a path to ground.
--

Larry Wasserman Baltimore, Maryland
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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.
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This is really stupid when you think about the fact that you're wrapping wire around an insulating material. <G>
Slide a wire tap along the coil, and tune radio stations!
Barry
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Not so sure it's stupid - this is static charge we're talking here, not anything with a significant current.
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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.
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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
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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.
--
Smert' spamionam

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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...
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Well, what I heard is that Saddam's entire WMD program is based on this theory and that's why we be having such a hard time finding them WMD's, 'cause we just walk right on by all a these PVC pipes laying around all over the place, not realizing that they only need to be hooked up to a dust collectorto be turned into explosive deevices, which is debilishly clebber when ya thinks about it.
wrote:

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Interesting. So.....
In the book Woodshop Dust Control, Sandor Nagisygnskfiwef(something-or-other) recommends that the reader NOT use plastic pipe for the DC system, and shows a lot of detail on grounding plastic pipe to prevent an explosion. He seems to consider it a real risk.
Yet the concensus here is that it's not a risk in a small-shop environment. (also other threads have pointed to some good sources of info on why an explosion is unlikely).
In that book, he also recommends that a DC not be located near a furnace or water heater - appliances with a pilot light. Is he being overly cautious in this warning, too? As long as the DC has decent bags so it's not blowing dust right through, how likely is it that there would be a high enough concentration of dust to be ignited from a pilot light?
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environment.
or
blowing
Well - even though I'm not answering your questions, let me throw another twist into the game. Explosive environments, and by that I mean truly explosive environments, require electrical switches that are rated for that environment, so that the spark that typically occurs when you throw your basic household light switch, does not cause an explosion. So - when was the last time you heard of a dust explosion in a woodworking shop from a light switch being thrown? I realize you could make an argument that an open flame is a higher concentration of heat than a spark, if the threat of explosion was truly as great as is claimed by some, wouldn't you think there would be at least a mention of explosive environment light switches?
--

-Mike-
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