Static buildup in DC systems ?

[Paul Kierstead]

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.

[bridger]

yup. he bought the whole load of BS too. it's a shame, 'cause he's a pretty good teacher.

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...

[Nova]

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]

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.

[Mike Marlow]

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.

[Unisaw A100]

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...

[Tom Quackenbush]

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.

[Andy Dingley]

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.

[bridger]

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.

not just to reduce the conscequences- also to reduce the sources.

that's ABS flex, not long straight sections of PVC

steel pipe is the preferred installation. for home shops, PVC is very convenient.

[J. Clarke]

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.

[J. Clarke]

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.

[Andy Dingley]

Nor does a commercial DC. It's not the volume that's the issue, it's the energy per discharge.

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.

[J. Clarke]

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.

[Tom Veatch]

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

[Tom Veatch]

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

[Andy Dingley]

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.

[bridger]

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...

and that one machine is unlikely to make enough dust to be a problem. it's the main trunk lines that do.

[Mike Marlow]

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...

[Mark & Juanita]

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.