Static buildup in DC systems ?

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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?
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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
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On Sat, 13 Nov 2004 21:01:39 GMT, "Paul in MN"

That will work if the screw is indeed grounded, but it is better to use a pipe you know is buried into the ground.
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Phisherman wrote:

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
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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.
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Frank Ketchum wrote:

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.
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of
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.

understand
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...
--

-Mike-
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Mike Marlow wrote:

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.

--
--John
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in
event
only
used
path
You
I'm not sure I understand why you pointed that out. It wasn't a point under discussion.

(wrist
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.

be
something
then
more
Did you read what he posted? With the type of foolishness he suggested, there is nothing that can prevent death.
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Mike Marlow wrote:

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.
--
--John
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where
grounding
component.
discharge
to
safety
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.
--

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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
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wrote:

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?
--

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Ahh... yes, a few things in this particular case, which is OK, we all have mental blocks sometimes.
- It isn't about working on the mains, but about accidents that happen. A power supply might be plugged in and wired incorrectly, for example. The oddest things happen. I remember working on a darkroom timer (digital) once; it had the grounding plug cut off and was wired with the neutral to the case. The plug was in upside down and I got quite a jolt from the *case* (this is why it was being serviced). If I had a non-resistive wrist strap on, the unpleasant experience could have become a fatal one. Rule one of working with power: Accidents happen. - The resistor limits current a lot. Lets say your body is zero resistance; a 5 megaohm resistor will limit current to microamps. It does'nt have to "take" current; it limits the current simply (consult ohm's law). - Of course if you have a hand on a ground and touch the other to the mains, you are in trouble. The point is for the wrist strap not to add *new* problems; it doesn't solve old ones. That is why the resistor is there. Again, consult ohms law. Note on really high power stuff, they trained us initially to put one hand in our back pocket to stop us from doing stupid things. Here is something to think about: Lean up against a table or other waist high item; what part touches the table? That is why you see some techs with their ass stuck well out :)
Again, you are missing the point a bit about the wrist strap. Yes, it is to provide protection against static. However, having a ground around your wrist introduces *new* health hazards, i.e. an increased chance of electrocution. The resistor mitigates this risk.
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wrote:

wear a

and
you
going to

any
to
Who me???? Mental blocks???? Sometimes????????? You're too kind...

The post I replied to mentioned mains, that's why I comment on them.

Correct, but in this case no wrist strap would have been better than a resistive one. The original symptoms called for checking voltage on the chassis to ground. This is not really a wrist strap issue.

Yes they do and doggoneit how comes it always seems that they never happen to the other guy?

The basic problem here is you should not be attaching anything to your body in the hope it will conduct current like that. You stay away from making complete paths to ground from high voltage. Again - in this situation, no wrist strap is better than a resistive one. Consider - the only way your resistive wrist strap is going to be beneficial is if you have no other path to ground through your body. You're insulated at the floor and the only path to ground is that strap. No strap - no path to ground - no current flow. Conversely, perhaps you're on a conducting surface and you do somehow have a path to ground besides through the strap - that's the path current is going to take - not through the 5Mohm resister.
--

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Mike Marlow wrote:

From the viewpoint of safety, certainly. From the viewpoint of protecting an expensive piece of machinery from static damage not so.

And no disspation of static.

And again you're missing the point. You wear the wrist strap to dissipate static. Because you are wearing a grounded electrode attached to your body, if you should come in contact with high voltage, you will get a shock due to that ground path. The resistor is there to keep you from getting killed if while wearing a wrist strap for the purpose of dissipating static you inadvertently come in contact with high voltage.
I don't know why you're having so much trouble with this.

--
--John
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shock
static
You don't? Hell, I admitted to the thick headed syndrome in a previous reply. Ok - it really is a mental block thing - I got focused on the "fix one problem at a time and deal with hot chassis as a power problem before worrying about static" way of thinking. I do see your point about mitigating the inherent risk of a tether to ground - which in fact is one of the points I made earlier when I said they were only useful in draining static and that they shouldn't be used for troubleshooting power problems. I think the point you were trying to make got lost because that's all I was seeing. I really can see things pretty well, it's just that sometimes it takes a while...
--

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Paul Kierstead wrote: <snip>

That was an excellent explanation, Paul.
Of course, the resistor is only a good idea for those of us that don't plan our accidents far enough in advance to remove the wrist strap prior to accidentally contacting operating voltages.
R, Tom Q.
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Mike Marlow wrote:

One doesn't. But power has a way of appearing unexpectedly on the bench.

Touch the mains with one hand while wearing a NON-resistive ground strap on the other and then what?

Well, typically the resistor is 1 megohm 1/4 watt, which should be within its power rating up to 500 volts, at which point the current through it would be 500 microamps.

Unless you're working with more than 500 volts it should provide protection indefinitely. And when it fails it tends to fail open and break the circuit rather than fail short.

That is not the purpose of the device.

Yes, you are. The purpose is to eliminate static. The use of the wrist strap, however, introduces a new danger--a path to ground through the wrist strap. The resistor is there to eliminate that specific danger by limiting the amount of current than can flow through that path to a level that is not dangerous.
--
--John
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Mike Marlow wrote:

I agree my scenario was (purposefully <g>) 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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