Rick Glazier wrote:
>
> Kris Shapar <[log in to unmask]> writes:
> >I've read that you should leave the PC plugged in so it will be
> >grounded (and wear a grounding strip attached to the case), that's
> >not right?

More 2c.  Proper installation of industrial, commercial or government
communications and computer systems require a separate "bonding" to an
earth ground.  In other words, they do not rely on just the ground pin
of the ac power system to provide a path to earth, but instead require a
separate, insulated bonding wire that goes from the equipment to earth.
Now to require that the ordinary consumer be subject to those same laws
would cause an uproar of proportions never before heard.

I would imagine that in a PC workshop, all work benches have proper
polarity, three wire grounded outlets and a fourth (bare or insulated)
wire bonding the metal workbenches together which go to an earth ground
directly without passing through any circuit breakers or switches.  And
the antistatic mats all connect to this 'earth ground' system.  Note,
that because it ties directly into earth, there is no potential on this
fourth line.

Now to the common house and the consumer.  When was the house built?
Problems?  Maybe some time ago, the wiring was damaged and the ground of
the ac wiring is not effective anymore?  Any number of possibilities can
exist.  And there is no requirement to have that fourth bonding wire.
So what to do?

Most people just plug their computer into the wall outlet without even
knowing if the polarity of the outlet is correct and the ground is
correct.  It should be tested (checkers are available at many hardware
stores for about $5.00)

But when it comes to working on something that has AC power applied to
it, ALWAYS UNPLUG IT!  You can never tell if there may be an internal
problem that can allow a lethal (and 120 or 240 for our foreign friends)
is considered LETHAL) voltage to exist within the confines of a
computer.

So what to do about grounding?  You must provide for a separate
grounding path to earth ground so that any static charges will be
properly dissipated.  Do NOT use a wrist strap cord to ground your case.
That cord has a built-in resistor in it so that you, when you wear it
properly, are connected to ground through a large (1 megohm or more)
resistance to limit any fault current through your body to a few
microamperes (I think 25 miliamperes can cause the heart to defillibrate
and 50 milliamperes can stop the heart).  If you use a wrist strap cord
as a grounding device, the unit is NOT directly connected to ground and
if a fault current exists in it, when you touch the circuit, your body
(approximately 10,000 ohms impedance) will become the path of least
resistance and . . . well, you can guess the rest of the story.

So what SHOULD the average consumer do if they want to work inside the
innards of a computer?  First, understand and remember that household
voltages ARE lethal, and you should do everything possible to stay away
from it.  Test the outlets where your computer system is set up.  And if
you are going to move it to a work area, test those outlets, too.  Make
sure the polarity is correct and the ground is correct.  Since the
common household does not have a fourth wire running thru it for bonding
electronic systems to earth, and you should remove all ac from the
computer before you open the cover, the best solution is to go to an
electronic parts store, like radio shack, and purchase a small banana
clip, an alligator clip and a small length of insulated wire (18 ga. or
larger).  Build your own grounding wire.  Connect the alligator clip to
the metal chassis of the computer and plug the banana plug into the
ground pin socket of the ac outlet.  You now have a grounded chassis
without any ac power applied.  And when you use your wrist strap, any
static voltages that may be try to develop on your body will be
dissipated through the grounding wire of the AC power system to earth
ground.

. . . and everyone lived long and happy ever after!

I apologize to those who may think that this was long-winded.  But I
have lived with electricity for over 35 years and it still amazes me how
so many people still do not respect it for what it CAN do.  I will
always be a crusader for safety.

Sincerely
--
Jean Bourvic :>))

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