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.
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).
True, but MDF dust is not much different than flour.
Lou
nigel did say:
Sorry to hear that.
Thanks for the informative post.
So my fears aren't, um, groundless. ;-)
-- Mark
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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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)
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On Sat, 13 Nov 2004 13:57:13 -0700, nigel wrote (in article ):
I read somewhere on-line (the dust collection info site?) about a factory using PVC and how everyone would get nasty static shocks from it. They tried the usual tricks of wire, etc but none really worked. What finally did work was a strip of the metal (aluminum ?) duct tape in a strip along the inside of the PVC and another strip along the outside. The two strips were then connected together with several holes drilled through the pipe wall and fastened with a bolt and washers. This was then earth grounded. Something about the greater surface area of the conductor in the pipe doing a better job of collecting those pesky extra electrons....
-Bruce
Musta been fun getting that tape on the inside of the pipe...
-Doug
First, only a very tiny current through the heart is likely to be fatal. You don't need megaamps, or amps, 60 milliamps can do it. Second, while techs don't ground themselves when working on high voltage, they do ground themselves while working inside computers, and the grounding devices (wrist straps are one kind, but there are others) always have resistors in the cord specifically to prevent such an accident.
Not good enough to prevent death though.
I'm not sure I understand why you pointed that out. It wasn't a point under discussion.
I was a tech for a lot of years before making career changes and we used direct ground when working on computers. I've worked on mainframes where we literally climbed inside the frame all the way to common PC's. I have seen the resistive wrist straps, but they are far from the only grounding technique. When concerned for static discharge, which is the primary concern for computer techs, there is no need for the resistive component. A direct ground path is equally effective. The accident that the tech is concerned for when working on a circuit board is the accidental discharge of static electricity through the board. No need for a resistive element to address that issue.
Did you read what he posted? With the type of foolishness he suggested, there is nothing that can prevent death.
On Sun, 14 Nov 2004 05:21:41 GMT, "Mark Jerde" calmly ranted:
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.
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.
----------------------------------------------- I'll apologize for offending someone...right after they apologize for being easily offended.
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Safety was being discussed. Terms like "megaamps" were being thrown around.
Be kind enough to tell me where one can obtain a wrist strap that does not have a resistor installed, and if the purpose of the resistor is not safety then what is its purpose? Yes, there are other devices with other safety measures that go with them. But they all have some means of ensuring that you don't get a low resistance path to ground.
This is true, but beside the point, which is that one's body is not a good enough resistor to prevent electrocution.
The wrist strap is not the universal grounding method. There are pads that techs stand on and slap bars the are used to discharge static - the pad preventing the buildup of static after the discharge. There are grounded pads that components are placed on. There is the method of clamping ground straps/braids directly to the object. There are even wrist and foot straps that do not contain any resistive elements. The point being - you do not need a resistor in series to safely discharge static electricity. Don't confuse what you can find in tech repair areas where they deal with risks (to the component, not to themselves) from extreemly minute levels of static discharge with what you need to discharge static in other environments.
Never claimed it was. Just pointed out the foolishness of his scenario.
I think you misunderstand the purpose of the resistor; AFAIK it is not to safely discharge *static* electricity, it is to prevent you from much higher chance of death in the event of electrocution, for example if you should touch the mains. One of the most dangerous scenarios is when one hand touches the mains and the other is grounded; this puts the current directly across the heart. Wearing a wrist strap is just begging for trouble in the event of an accident. A resistor mitigates this risk.
PK
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?
environment.
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?
Well, I understand this, but I would have to wonder why anyone would wear a wrist strap when working with the mains. Touch the mains with one hand and ground with the other and a resistive ground strap isn't going to help you one bit. How much current is any resistor built into a wrist strap going to take? How long is that going to afford any protection - if it afforded any in the first place? If you wanted a device which provided a better path to ground than through your heart, you would want a non-resistive path. A simple ground braid. The only purpose I know of for a wrist strap is specifically for static electricity. I can be educated though - am I missing something?
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.
I agree my scenario was (purposefully ) absurd -- wrapping a ground around your body then biting a live hot wire. That's probably too stupid even for a Darwin award. Yet as the quote below from another post in this thread illustrates, household current can be deadly and there are surprising ways to get your body in the circuit. I have been shocked several times, never enjoyed it, and do the prudent things to keep myself safe, like replacing cracked or nicked cords.
In the category "What I Don't Know Could Kill Me" I was uncertain about the
*real* risks of a large, open ground running everywhere in my small shop. In college I chased phasors around LRC circuits and became a close personal friend of Kirchoff, but I know very little about practical electricity. The answers here are that grounding the DC ducts are not more dangerous than any other part of home wiring.Thanks to all who replied.
-- Mark
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