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?
On 13 Nov 2004 12:57:13 -0800, "nigel" <[email protected]> wrote:
>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?
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.
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 ....
In article <[email protected]>,
nigel <[email protected]> wrote:
> So the general word is that there's no safety hazard involved - saw
> dust is obviously different to flour, for example.
True, but MDF dust is not much different than flour.
Lou
In article <_Kxld.2531$GV5.1353@trnddc04>,
Mark Jerde <[email protected]> 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
>
>
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
[email protected]
On Mon, 15 Nov 2004 21:36:38 -0700, [email protected] wrote:
>steel pipe is the preferred installation. for home shops, PVC is very
>convenient.
And MUCH less expensive. I just bought a bunch of the PVC for installing a
cyclone currently on order from Bill Pentz.
Comparitive pricing between a local plumbing supply house for 6" ASTM-D2729 S&D
PVC and the web prices from a well-known woodworking supply house:
Pipe: PVC $8.50 for 10 ft length, Steel $28.00 (+ shipping) for 2 x 5 ft
lengths
Wye: PVC $12.00 each, Steel $60.00 (+ shipping)
Well, you get the idea. Comparative costs for other fittings are similar. But,
there is a wider variety of fittings available in the steel.
Tom Veatch
Wichita, KS USA
On Sat, 13 Nov 2004 13:57:13 -0700, nigel wrote
(in article <[email protected]>):
> 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 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
Keith Carlson wrote:
> "Unisaw A100" <[email protected]> wrote in message
> news:[email protected]...
> > >What do the experts have to say on this?
> >
> >
> > 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...
>
> 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?
I could be that Sandor Nagyszalanczy is referring to a commercial wood shop and
avoiding a law suit.. The book is not just written for the hobbyist.
--
Jack Novak
Buffalo, NY - USA
(Remove "SPAM" from email address to reply)
"J. Clarke" <[email protected]> wrote in message
news:[email protected]...
>
>
> 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.
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...
--
-Mike-
[email protected]
"Mark Jerde" <[email protected]> wrote in message
news:_Kxld.2531$GV5.1353@trnddc04...
>
> Or can someone talk me out of a fear of an open megaamp path-to-ground
> throughout my shop?
>
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
"Unisaw A100" <[email protected]> wrote in message
news:[email protected]...
> >What do the experts have to say on this?
>
>
> 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...
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?
On Sat, 13 Nov 2004 21:01:39 GMT, "Paul in MN"
<[email protected]> wrote:
>
>"nigel" <[email protected]> wrote in message
>news:[email protected]...
>> 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
>
That will work if the screw is indeed grounded, but it is better to
use a pipe you know is buried into the ground.
"Mark Jerde" <[email protected]> wrote in message
news:ptvld.1341$GV5.185@trnddc04...
>
> I had a lot of electrical "book learnin'" in kollege but my practical
> knowledge is very low. In this situation, grounding a DC to a good earth
> ground, should there be a current-limiting resistor somewhere between the
> water pipe and wire going through the DC system? If so, what size?
>
It's not necessary since the connection keeps the wire at the same potential
(or very close to) ground. There is a very small current flowing through
the wire. To install a resistor would guarantee that the wire has a small
potential on it.
Frank Ketchum wrote:
> "Mark Jerde" <[email protected]> wrote in message
> news:_Kxld.2531$GV5.1353@trnddc04...
>>
>> Or can someone talk me out of a fear of an open megaamp
>> path-to-ground throughout my shop?
>>
>
> 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.
>
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
"nigel" <[email protected]> wrote in message
news:[email protected]...
> 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.
>
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.
> 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?
The pipe itself is the conductor in metal systems and should still be
grounded at one point.
"Mark Jerde" <[email protected]> wrote in message
news:yawld.1482$N_5.603@trnddc03...
>
> 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.
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.
>
> 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?
>
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-
[email protected]
"nigel" <[email protected]> wrote in message
news:[email protected]...
> 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
In article <FXBld.1912$BX4.883@trnddc08>, "Mark Jerde" <[email protected]> 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.)
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.
On Sat 13 Nov 2004 02:57:13p, "nigel" <[email protected]> wrote in
news:[email protected]:
> 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?
>
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
Andy Dingley wrote:
> On Sat, 13 Nov 2004 21:59:49 GMT, "Mark Jerde"
> <[email protected]> wrote:
>
>> 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?
>
> 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.
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
"Paul Kierstead" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>,
> "Mike Marlow" <[email protected]> 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?
>
> Ahh... yes, a few things in this particular case, which is OK, we all
> have mental blocks sometimes.
Who me???? Mental blocks???? Sometimes????????? You're too kind...
>
> - It isn't about working on the mains, but about accidents that happen.
The post I replied to mentioned mains, that's why I comment on them.
> 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.
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.
> Rule one of working with power: Accidents happen.
Yes they do and doggoneit how comes it always seems that they never happen
to the other guy?
> - 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).
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.
--
-Mike-
[email protected]
On Tue, 16 Nov 2004 11:44:28 +0000, Andy Dingley
<[email protected]> wrote:
>On Mon, 15 Nov 2004 21:36:38 -0700, [email protected] wrote:
>
>>home shops never involve anywhere near the volume of dust needed to
>>generate a hot static discharge into a dust cloud dense enough to
>>sustain it.
>
>Nor does a commercial DC. It's not the volume that's the issue, it's
>the energy per discharge.
dust volume *and* the speed that dust is travelling do have a bearing
on the static charges built up in plastic pipe. high volume + high
speed = large static energy being generated. in systems handling tons
per day like grain silos and millwork factories the risk of static
discharge becomes real. in home shops it's pure unadulterated urban
mythology...
>
>>>Plastic piping, and unearthed too,
>>>is in regular use for the flexible sections of DC hookups.
>>
>>that's ABS flex, not long straight sections of PVC
>
>How are machines like panel saws connected to a DC in the USA ? Here
>we commonly see ten-twenty foot drops of clear flexible PVC.
and that one machine is unlikely to make enough dust to be a problem.
it's the main trunk lines that do.
Keith Carlson wrote:
>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.
As I said,
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.
And this includes Sandor.
UA100, who is mentioned by name on the acknowledgements page
of Sandor's book Power Tools...
Mike Marlow wrote:
>
> "Mark Jerde" <[email protected]> wrote in message
> news:yawld.1482$N_5.603@trnddc03...
>
>>
>> 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.
>
> 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.
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.
>> 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?
>>
>
> 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.
Not good enough to prevent death though.
> 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...
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Mike Marlow wrote:
>
> "J. Clarke" <[email protected]> wrote in message
> news:[email protected]...
>> Mike Marlow wrote:
>>
>> >
>> > "Mark Jerde" <[email protected]> wrote in message
>> > news:yawld.1482$N_5.603@trnddc03...
>> >
>> >>
>> >> 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.
>> >
>> > 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.
>>
>> 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.
>
> I'm not sure I understand why you pointed that out. It wasn't a point
> under discussion.
Safety was being discussed. Terms like "megaamps" were being thrown around.
>>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.
>
> 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 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.
>> >> 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?
>> >>
>> >
>> > 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.
>>
>> Not good enough to prevent death though.
>>
>
> Did you read what he posted? With the type of foolishness he suggested,
> there is nothing that can prevent death.
This is true, but beside the point, which is that one's body is not a good
enough resistor to prevent electrocution.
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Mike Marlow wrote:
>
> "Paul Kierstead" <[email protected]> wrote in message
> news:[email protected]...
>> In article <[email protected]>,
>> "Mike Marlow" <[email protected]> wrote:
>>
>> > The point being - you do not
>> > need a resistor in series to safely discharge static electricity.
>>
>> 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
>
> Well, I understand this, but I would have to wonder why anyone would wear
> a
> wrist strap when working with the mains.
One doesn't. But power has a way of appearing unexpectedly on the bench.
> Touch the mains with one hand
> and ground with the other and a resistive ground strap isn't going to help
> you
> one bit.
Touch the mains with one hand while wearing a NON-resistive ground strap on
the other and then what?
> How much current is any resistor built into a wrist strap going
> to
> take?
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.
> How long is that going to afford any protection - if it afforded
> any
> in the first place?
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.
> If you wanted a device which provided a better path
> to
> ground than through your heart, you would want a non-resistive path.
That is not the purpose of the device.
> 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?
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
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Andy Dingley wrote:
> On Mon, 15 Nov 2004 12:57:49 -0700, [email protected] wrote:
>
>>let's make a clear distinction between production shops and home shops
>>here.
>
> Why ? The physics is the same.
But the volume of dust handled by the collector may be different. CWG has a
planer that is bigger than my truck. Somehow I think that when it's
turning a piece of 8/4 lapacho into 5/4 it produces a little more in the
way of shavings and dust and whatnot than my 13" Delta.
Whether the volume is high enough to constitute an explosion hazard in the
event of a static discharge in the system, I have no idea.
> If plastic piping is a no-no for commercial workshops in your
> jurisdiction, then that's to reduce the _consequences_ of a fire (and
> they do happen, for many reasons). Plastic piping, and unearthed too,
> is in regular use for the flexible sections of DC hookups.
>
> If I had a roof mounted central DC system, I'd want steel pipe too.
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Mike Marlow wrote:
>
> "Paul Kierstead" <[email protected]> wrote in message
> news:[email protected]...
>> In article <[email protected]>,
>> "Mike Marlow" <[email protected]> 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?
>>
>> Ahh... yes, a few things in this particular case, which is OK, we all
>> have mental blocks sometimes.
>
> Who me???? Mental blocks???? Sometimes????????? You're too kind...
>
>>
>> - It isn't about working on the mains, but about accidents that happen.
>
> The post I replied to mentioned mains, that's why I comment on them.
>
>> 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.
>
> 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.
>
>> Rule one of working with power: Accidents happen.
>
> Yes they do and doggoneit how comes it always seems that they never happen
> to the other guy?
>
>> - 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).
>
> 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.
From the viewpoint of safety, certainly. From the viewpoint of protecting
an expensive piece of machinery from static damage not so.
> 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.
And no disspation of static.
> 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.
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
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Andy Dingley wrote:
> On Mon, 15 Nov 2004 21:36:38 -0700, [email protected] wrote:
>
>>home shops never involve anywhere near the volume of dust needed to
>>generate a hot static discharge into a dust cloud dense enough to
>>sustain it.
>
> Nor does a commercial DC. It's not the volume that's the issue, it's
> the energy per discharge.
You're right that it's not the volume, it's the concentration. To get an
explosion you need x quantity of suspended particles per unit volume and
you need an ignition source. If the concentration is high enough for an
explosion to occur, then it is not a good idea to rely on the hope that any
discharge will be of too little energy to cause ignition. In a home shop
it's not likely that you'll get that kind of concentration. In a
commercial shop it might be a different story.
>>>Plastic piping, and unearthed too,
>>>is in regular use for the flexible sections of DC hookups.
>>
>>that's ABS flex, not long straight sections of PVC
>
> How are machines like panel saws connected to a DC in the USA ? Here
> we commonly see ten-twenty foot drops of clear flexible PVC.
>
--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
Mike Marlow wrote:
>>> Did you read what he posted? With the type of foolishness he
>>> suggested, there is nothing that can prevent death.
>>
>> This is true, but beside the point, which is that one's body is not
>> a good enough resistor to prevent electrocution.
>>
>
> Never claimed it was. Just pointed out the foolishness of his
> scenario.
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
> 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).
Andy Dingley wrote:
Thanks for the informative post.
> 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).
So my fears aren't, um, groundless. ;-)
-- Mark
"Paul Kierstead" <[email protected]> wrote in message
news:[email protected]...
> In article <[email protected]>,
> "Mike Marlow" <[email protected]> wrote:
>
> > The point being - you do not
> > need a resistor in series to safely discharge static electricity.
>
> 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
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?
--
-Mike-
[email protected]
>What do the experts have to say on this?
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...
On 13 Nov 2004 12:57:13 -0800, "nigel" <[email protected]> wrote:
>Following on from the recent DC discussion ... what is the deal with
>static charge buildup with all this dust flying around.
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
On Sun, 14 Nov 2004 00:35:06 GMT, "Mark Jerde"
<[email protected]> wrote:
>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?
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
>Or can someone talk me out of a fear of an open megaamp path-to-ground
>throughout my shop?
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
On Mon, 15 Nov 2004 12:57:49 -0700, [email protected] wrote:
>let's make a clear distinction between production shops and home shops
>here.
Why ? The physics is the same.
If plastic piping is a no-no for commercial workshops in your
jurisdiction, then that's to reduce the _consequences_ of a fire (and
they do happen, for many reasons). Plastic piping, and unearthed too,
is in regular use for the flexible sections of DC hookups.
If I had a roof mounted central DC system, I'd want steel pipe too.
--
Smert' spamionam
On Mon, 15 Nov 2004 21:36:38 -0700, [email protected] wrote:
>home shops never involve anywhere near the volume of dust needed to
>generate a hot static discharge into a dust cloud dense enough to
>sustain it.
Nor does a commercial DC. It's not the volume that's the issue, it's
the energy per discharge.
>>Plastic piping, and unearthed too,
>>is in regular use for the flexible sections of DC hookups.
>
>that's ABS flex, not long straight sections of PVC
How are machines like panel saws connected to a DC in the USA ? Here
we commonly see ten-twenty foot drops of clear flexible PVC.
--
Smert' spamionam
On Tue, 16 Nov 2004 09:53:19 -0500, "J. Clarke"
<[email protected]> wrote:
>> Nor does a commercial DC. It's not the volume that's the issue, it's
>> the energy per discharge.
>
>You're right that it's not the volume, it's the concentration.
It's the concentration and the energy. If there's inadequate energy
in the discharge, it won't ignite. If the concentration is inadequate,
it won't propagate.
However, even low concentration is not a guarantee of safety for wood
dust handling. There's a problem with smouldering and a burning
particle _may_ settle out in a dead-air zone where there's likely to
be a build up of other dust. In forensic examinations of many dust
collector fires, they began not in the filter or the collecting bin,
but at stagnant corners and sudden pipe expansions.
For a real treatment of this problem, read Luttgens & Wilson's
"Electrostatic Hazards" or a similar industry-standard handbook. I
still haven't found one that has evidence of static discharge caused
fires in wood dust.
--
Smert' spamionam
On Tue, 16 Nov 2004 03:42:35 +0000, Andy Dingley
<[email protected]> wrote:
>On Mon, 15 Nov 2004 12:57:49 -0700, [email protected] wrote:
>
>>let's make a clear distinction between production shops and home shops
>>here.
>
>Why ? The physics is the same.
home shops never involve anywhere near the volume of dust needed to
generate a hot static discharge into a dust cloud dense enough to
sustain it. that takes a dust collector in the 40 HP range pulling
from multiple high volume dust producing machines at once. while in
theory a home shop could create the right conditions, in practice it's
never happened and never will.
>
>If plastic piping is a no-no for commercial workshops in your
>jurisdiction, then that's to reduce the _consequences_ of a fire (and
>they do happen, for many reasons).
not just to reduce the conscequences- also to reduce the sources.
>Plastic piping, and unearthed too,
>is in regular use for the flexible sections of DC hookups.
that's ABS flex, not long straight sections of PVC
>
>If I had a roof mounted central DC system, I'd want steel pipe too.
steel pipe is the preferred installation. for home shops, PVC is very
convenient.
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.
On Sat, 13 Nov 2004 20:26:54 -0600, Unisaw A100 <[email protected]>
wrote:
>>What do the experts have to say on this?
>
>
>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...
"nigel" <[email protected]> wrote in message
news:[email protected]...
> 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?
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
On Sun, 14 Nov 2004 11:22:12 -0700, Bruce wrote:
> 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
In article <[email protected]>,
"Mike Marlow" <[email protected]> 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?
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.
In article <[email protected]>,
"Mike Marlow" <[email protected]> wrote:
> The point being - you do not
> need a resistor in series to safely discharge static electricity.
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
Phisherman wrote:
> On Sat, 13 Nov 2004 21:01:39 GMT, "Paul in MN"
> <[email protected]> wrote:
>
>>
>> "nigel" <[email protected]> wrote in message
>> news:[email protected]...
>>> 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
>>
>
> 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
"J. Clarke" <[email protected]> wrote in message
news:[email protected]...
> Mike Marlow wrote:
>
> >
> > "Mark Jerde" <[email protected]> wrote in message
> > news:yawld.1482$N_5.603@trnddc03...
> >
> >>
> >> 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.
> >
> > 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.
>
> 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.
I'm not sure I understand why you pointed that out. It wasn't a point under
discussion.
>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.
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.
>
> >> 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?
> >>
> >
> > 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.
>
> Not good enough to prevent death though.
>
Did you read what he posted? With the type of foolishness he suggested,
there is nothing that can prevent death.
--
-Mike-
[email protected]
On 13 Nov 2004 12:57:13 -0800, "nigel" <[email protected]> wrote:
>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.
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
On Tue, 16 Nov 2004 10:03:07 -0700, [email protected] wrote:
>On Tue, 16 Nov 2004 11:44:28 +0000, Andy Dingley
><[email protected]> wrote:
>
>>On Mon, 15 Nov 2004 21:36:38 -0700, [email protected] wrote:
>>
>>>home shops never involve anywhere near the volume of dust needed to
>>>generate a hot static discharge into a dust cloud dense enough to
>>>sustain it.
>>
>>Nor does a commercial DC. It's not the volume that's the issue, it's
>>the energy per discharge.
>
>dust volume *and* the speed that dust is travelling do have a bearing
>on the static charges built up in plastic pipe. high volume + high
>speed = large static energy being generated. in systems handling tons
>per day like grain silos and millwork factories the risk of static
>discharge becomes real.
In the grain elevator explosions with which I am familiar, static was not
the cause of the explosion, it is usually either a faulty electric motor
that generates a spark (much longer duration and thus more energy supplied
to start the ignition), or an open flame.
On Sat, 13 Nov 2004 21:59:49 GMT, "Mark Jerde"
<[email protected]> wrote:
>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?
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
In article <_Kxld.2531$GV5.1353@trnddc04>, "Mark Jerde" <[email protected]> wrote:
>
>Or can someone talk me out of a fear of an open megaamp path-to-ground
>throughout my shop?
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)
Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter
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You must use your REAL email address to get a response.
"Keith Carlson" <[email protected]> wrote in message
news:6m2md.408724$D%.261058@attbi_s51...
> "Unisaw A100" <[email protected]> wrote in message
> news:[email protected]...
> > >What do the experts have to say on this?
> >
> >
> > 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...
>
> 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?
>
>
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-
[email protected]
On Sun, 14 Nov 2004 05:21:41 GMT, "Mark Jerde"
<[email protected]> calmly ranted:
>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.)
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.
-----------------------------------------------
I'll apologize for offending someone...right
after they apologize for being easily offended.
-----------------------------------------------
http://www.diversify.com Inoffensive Web Design
On Mon, 15 Nov 2004 13:41:22 GMT, "Keith Carlson"
<[email protected]> wrote:
>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?
Very unlikely. The research I've seen (Sorry, I don't have a cite immediately
available) indicates that in order for a dust cloud to support a flame front,
the visibility in the cloud would be on the order of a meter. That's a pretty
thick dust cloud. Based on that, the rule of thumb I use is - If I can still see
the far wall of the shop through the dust, an explosion is the least of my
worries.
Tom Veatch
Wichita, KS USA
On Mon, 15 Nov 2004 13:41:22 GMT, "Keith Carlson"
<[email protected]> wrote:
>"Unisaw A100" <[email protected]> wrote in message
>news:[email protected]...
>> >What do the experts have to say on this?
>>
>>
>> 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...
>
>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.
yup. he bought the whole load of BS too. it's a shame, 'cause he's a
pretty good teacher.
>
>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?
>
let's make a clear distinction between production shops and home shops
here.
in production shops there are large machines, multiples of them,
running continuously, each making lots of dust. this is an environment
where plastic piping is a hazard and AFAIK illegal.
in home shops this is not the case.
there are plenty of non-static discharge hazards associated with dust
collectors in home shops. big bags of sawdust are a ready source of
fuel just waiting for a source of ignition. vacuum up a cigarette butt
or a nail that sparks off of some metal part inside the DC and you
could have a smouldering fire inside the DC that erupts into full
ignition hours after you've shut off the lights and gone to bed.
I fail to see how a pilot light could do this short of sucking big
piles of dust through the piloted device, but someone somewhere might
be able to pull it off ; ^ ) more likely is that the DC would blow
out the pilot...
"J. Clarke" <[email protected]> wrote in message
news:[email protected]...
> Mike Marlow wrote:
> >
> > 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 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.
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.
> >
> > Did you read what he posted? With the type of foolishness he suggested,
> > there is nothing that can prevent death.
>
> This is true, but beside the point, which is that one's body is not a good
> enough resistor to prevent electrocution.
>
Never claimed it was. Just pointed out the foolishness of his scenario.
--
-Mike-
[email protected]
Frank Ketchum wrote:
> "Mark Jerde" <[email protected]> wrote in message
> news:ptvld.1341$GV5.185@trnddc04...
>>
>> 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?
>>
>
> 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.
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.
Paul Kierstead wrote:
<snip>
>- 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.
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.