Re: I didn't realize what you meant (was RE: WARNING! WARNING! WAR...

[AJSOENKE]

In a message dated 11/23/2007 12:13:55 P.M.  Pacific Standard Time,
[hidden email] writes:
No problem Gary and I'm  glad we agree on that :-)

Sorry if I didn't make that clear. To try to  clarify this.. the basic
premise is that anything you attach to you or your  kit, while you are
building, is bonded to the same point (that's important)  and via a 1M ohm
resistor lead.

I also use dissipative tools, like  screwdrivers, cutters etc etc, but that
is not really necessary, as long as  your body is grounded via a strap.

I repeat, this ground MUST be via a 1M  ohm resistor (or higher, but better
to keep all resistances the  same).


+++++++++++++++++++++++++++++++
 
Sorry if I missed something here, and I'm not trying to confuse issues.  But,
after 40 years in the industry working with the most sensitive of  
semiconductor devices, the only requirement for a series resistance that I've  seen used
is in the devices like a wrist strap that were attached to the  operators
body. The purpose being to protect them from possible contact with  power systems
that could place the operator in the HV ground path. The 500K or  larger
resistor was usually adequate to provide a non-lethal warning of somethin  amiss.
I don't disagree that a big resistor to ground will dissipate a static  
charge, but most of us would prefer a total ground on the chassis. Otherwise, as  I
understand it, you still run the risk of setting up a voltage differential  
that could damage parts. The reasoning is that a static charge will dissipate,  
being of limited source. But an AC voltage delta between the tip of an Iron
and  an earth ground could be several volts, depending on the ground point and
power  system setup. I've seen as much as 40 volts in normal household wiring.
The  addition of a high value resistance in that circuit could raise the
voltage  substatially, AND it is a non dissipative source that could fry more than
a  small semi.  
 
The rule I have always applied at home and industrially is:
1) use isolation transformer where possible,  
2) use a GFCI protected circuit,
3) establish a common ground point that has the best possible path back to  
the house ground.( That is usually at the service entrance and should be tested
 to make sure it works )
4) use a ground tester to make sure the ground is not open at the outlet.  
(Testers are cheap and available in most hardware stores) .
5) Make sure the grounds from common point to work surface and chassis  are
firmly connected - alligator clips always pop off.
6) Wear your wrist strap and avoid synthetic or wool clothing.
7) last and very important - always use an ohmmeter to test the wrist strap  
and other ground jumpers to make sure they are working.
 
A lot of good hardware can be damaged by an open ground wire. A good OP can  
be hurt by a bad wrist strap. With the constant flexing of the  wrist  
cables,etc, the wires will break internally and not be noticed, there's no  antenna
or speaker attached to it to tell you something's wrong.
 
I've done a lot of residential testing in the course of helping run an RFI  
committee for the local club. We have found many RFI cases caused by bad  
grounds
 
In home's I've visited over 30 years, I've found about 1 in 10 homes is  
poorly grounded at the service entrance, and about 1 in 10 has open grounds on  
outlets. Many result from electrical wiring done by the homeowner without the  
help of a licensed electrician or city inspector.
 
Judging from the Hams I have met over the years, the SKs I have known, and  a
lot of the traffic I see on these lists, I have come to the opinion we have  
many in our midst to with disputable sense of values is some areas.   If you
can't afford a little coin on Ohmmeters, new wrist straps, GFCIs and a  visit
from the power company (usually free), you probably don't have adequate  
insurance to be messing with this stuff. n This isn't where you want to play on  the
cheap.
 
nuff said'  Al WA6VNN
 



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[Don Wilhelm-4]

Al,

AFAIK, you are correct - the large value resistor is primarily there to
protect the person wearing the wrist strap.

However, I do not like to do any electronic or electrical work on a
directly grounded and conductive surface - period.  IMHO, there is too
much of a chance to create a short to ground with an open lead or an
accidentally placed tool in contact with a conductive work surface.  
Even though we usually do not work on high voltage equipment frequently
anymore (which is where those practices originated), it is still good
practice since low voltage power (in particular batteries) can produce a
lot of damage from high current levels - I have seen several cases of
molten metal flying around as a result of a shorted low voltage/high
current source.  Think about it a bit - there is a good (and similar)
reason for the instruction to ALWAYS remove the grounded lead from an
automotive battery first (even if it seems non-intuitive at first) - if
one terminal is left grounded, the tool removing the 'hot' terminal can
short to the chassis and cause much harm and injury.  Don't think that
small batteries are harmless either - I once observed a 12 volt pack of
AA sized NiCAD batteries melt wire and burst when subjected to a direct
short - it was a destructive test in a controlled lab environment, but
it convinced me that such things are not harmless.

73,
Don W3FPR

[hidden email] wrote:

>  
> Sorry if I missed something here, and I'm not trying to confuse issues.  But,
> after 40 years in the industry working with the most sensitive of  
> semiconductor devices, the only requirement for a series resistance that I've  seen used
> is in the devices like a wrist strap that were attached to the  operators
> body. The purpose being to protect them from possible contact with  power systems
> that could place the operator in the HV ground path. The 500K or  larger
> resistor was usually adequate to provide a non-lethal warning of somethin  amiss.
>
>  

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