Re: [LUG] OT: Preventing electrostatic discharge

[Keith Abraham <keith@xxxxxxxxxxxxxxxxxxxxxxx>]

Simon Waters wrote:

> Robin Cornelius wrote:
>  
>
> > Basicly ensure you are connected to ground, a very quick and dirty way of 
> > doing this is to touch the PC chassis (when the mains plug is still connected 
> > *BUT SWITCHED OFF*, the metal chasis is connected to earth so you will 
> > discharge any static.
> >    
>
> Minor point, but I doubt having the PC plugged in makes much difference.
>
> "ground" on any modern building is almost certainly connected to a
> circuit breaker that trips should any current flow, it is probably only
> connected to earth via being wired to neutral behind the circuit breaker.
>  
Read the electricity section of the building regulations. Every electrical
supply has to have a valid earth (ground). Just think of a bathroom shower
without a proper earth. Scary!!

> In that sense they are very poor "earths", hence the networking
> "purists" (read rich people) recommending optical fibre (or wireless!)
> between buildings, rather than wire networking cables.
>  
Probably optical fibre because of the lower impedance and greater 
bandwidth.

> Touching the case equalises the potential between you, and the case (and
> hopefully the components), and that should be sufficient to work on them
> safely.
>  
Equalising the potential is the important bit. Static usually has a high 
potential
but a low current. Even this very low current can damage a chip but the 
current
will only flow where the potentials are unqual.
A Texas engineer once told me that antistatic mats and wrist bands were 
mainly
used to keep the girls who assembled the componets from moving around 
and generating a fatal (to the chips) static potential.

Always unplug the computer from the mains. Its the only way you can be sure
you won't get a lethal shock.
I merely roll up my sleeves and rest one arm on a metal part of the 
chassis. (All
motherboards have quite a large earthing pad(s) which will, at some 
point, be
grounded to the chassis.) I've used this method for years and never lost 
a chip.   

> We should of course test this with a Van de Graf generator, and an old
> PC, at the next LUG meeting, because I'm not 100% certain. My experience
> is that it is quite hard to damage most PC components through static
> (accidently!) but that may just be that the weather is too damp in Devon
> to get a really good static build up.
>  
You can build up a potential approaching 30kve (at very low amperage) by
rubbing your hand briskly against your trousers. No jokes please ;-)

Keith



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