Whether a case needs to be grounded or not is up in the air, but whether or
not the case should be left plugged into the wall is not. This is simply
just not a good practice. The chances of being hurt or killed are very,
very small, but finite. Why take that chance at all?
If you feel that you need to ground your case, I suggest that you purchase
a three-into-two cheater plug. Plug your computer cord into the cheater
plug and connect the green grounding wire to a well-grounded water pipe.
This will safely ground your computer with no risk of hurting yourself. (Of
course, if there is a lightning storm going on, then you should not even do
this!) You could also just connect your case to a pipe using alligator
clips and a wire. The pipe must be grounded - no plastic pipes between your
connection and the ground.
The main point that I have been trying to convey is that the "ground" on the
wall socket is not necessarily at earth ground, nor is is necessarily safe
to use as an earth ground.
Dean Kukral
PS:
We have discussed this topic in late 2003 and early 2004. Steve Collins
wrote a definitive letter on the topic on Jan 1 2004. I will take the
liberty of copying it from the PCBUILD archives and presenting it here:
Date: Thu, 1 Jan 2004
From: Steve Collins
Organization: AlternateRealities Computers
Subject: Re: Grounding yourself; Was: Adding memory - grounding
yourself
Hi Dean,
After spending 20 years in the semiconductor industry (including FAB and
Probe & Test), I think that I can consider myself a bit of an ESD expert.
I think that the confusion here is being caused by the term "grounding". In
this case, the term is not really being used properly (or maybe it's more
accurate to say that it is not being applied properly). Grounding is
required when the potential of some part of an electronic or electrical
circuit must be set at the same potential as the planet Earth. This is
accomplished simply by connecting (or "grounding") that part of the circuit
or component to the planet through a conductive piece of material with a
resistance of less than 1 ohm.
When dealing with ESD, the concern is not with the grounding of any element.
The concern is with maintaining anything that will touch, or anything that
will come close to each other, at the same electrostatic potential. If you
are grounded through a wrist strap, and your case is sitting at a potential
of 5KV due to electrostatic build-up, then grounding is actually a bad thing
since you will definitely draw an arc as soon as you get close enough to the
case. So, the focus here is to maintain all elements that come in contact
at the same potential:
Connect your wrist strap to the object that you are working on. In the case
of a computer, clip the strap to a bare piece of metal that is part of the
case. If you have a number of objects that you are working on (e.g. a hard
drive that is to be mounted in the case, new memory, etc.), then place all
objects on a conductive mat and connect your wrist strap to the mat. It is
NOT necessary to ground any part of this setup. Remember that the object is
simply to bring all elements to the same potential. We never recommend
using any part of a live circuit (including house wiring) for any purpose
other than providing power to an electrical instrument or appliance. NEVER
connect a living thing to your house wiring. This includes the cat, the
fish, the canary and, oh yes, YOU! If you require a ground reference,
install a separate ground bus that is not part of, or near, your house
wiring. NEVER remove, install or replace components on a device that is
connected to a source of power (i.e. ALWAYS unplug the power supply before
doing any component work).
Follow these simple rules and you and your components will live to see
another day.
Regards,
Steve Collins
AlternateRealities - High Performance Computers, Clusters and
Infrastructures
************************************************
----- Original Message -----
From: "Thomas Mayer" <[log in to unmask]>
Sent: Tuesday, August 15, 2006 11:50 AM
Subject: Re: [PCBUILD] New here, need help BADLY!
Dean
ESD can vary between 200 volts and 2,000 volts. If the computer case is
not grounded, there is a risk that the charge could reach some internal
electronic computer components before dissipating into the atmosphere.
And, once the power supply itself is turned off, there is no electricity
reaching the internals, but the remaining connection to a grounded
outlet provides a ground for the case. Of course working in the power
supply itself is an exception.
See http://www.build-your-own-cheap-computer.com/static-electricity.html
Tom Mayer
Dean Kukral wrote:
> I disagree that to ground through the case (either by touching or by using
> a
> wrist strap) that you should have the power cord plugged in.
>
> Just the opposite. Unless you know what you are doing, you should never
> have the power cord plugged in when you are working inside an electrical
> device!!!
>
> The purpose of touching the case and/or using a wrist strap is to drain
> static electricity off of yourself so as not to damage any integrated
> circuits. There is absolutely no need to have the case plugged in to
> serve
> as a ground. Static electricity (free electrons) will tend to spread
> themselves out on the case and drain themselves off quickly at the corners
> and other sharp points.
>
> Dean Kukral
>
>
> ----- Original Message -----
> From: "Thomas Mayer" <[log in to unmask]>
> Sent: Monday, August 14, 2006 11:56 PM
> Subject: Re: [PCBUILD] New here, need help BADLY!
>
>
> You can either get a grounding wrist strap or, follow Thomas Harold's
> suggestion to touch the case often as you are working. However, to
> ground through the case, the power cord should be plugged in, if it is a
> 3 wire grounded connection, and then turn the power switch off on the
> power supply and not just the case. Of course, no matter which method
> you use, it is always best way is to avoid touching any contact points.
>
> Tom Mayer
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