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PCBUILD Home

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PCBUILD  June 2009

PCBUILD June 2009

Subject:

It is neither necessary nor desirable to ground the case to a wall socket - explanation why

From:

Dean Kukral <[log in to unmask]>

Reply-To:

Personal Computer Hardware discussion List <[log in to unmask]>

Date:

Tue, 16 Jun 2009 08:35:13 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (69 lines)

Somehow a thread in PCSOFT has drifted away from application software into hardware.  I think that if  there  is any more discussion 
on this topic, it should be on PCBUILD, not on PCSOFT.  Those who exclusively subscribe to PCSOFT probably don't want to read it.



On more than one occasion I have strongly asserted in PCBUILD/PCSOFT that you should not use the ground on your wall socket to 
ground your computer while working on it. I think that most electricians who deal with the real world will agree with me. I would 
like to use my own house as an example of why you should never, ever use the ground on the wall to ground your computer while you 
are working on it. (This includes the use of those specially-built cords that have only one wire to the ground.)

My house was built in 1977, which is not so long ago, and, as far as I know, all the work was performed by licensed electricians. It 
has grounded outlets throughout. The electricity comes to a pole at the property edge, and, from there, underground through a 
plastic pipe into the house. The transformer is grounded at the pole outside, so three large aluminum wires come into the house 
underground through the pipe. The grounded, so-called "neutral" wire, and the two out-of-phase 120V wires, which combine to 240V. 
They are connected to a breaker box inside the house. As far as I can tell, there are no other grounding wires around the house, and 
the water comes into the house through a plastic pipe from the well. So, I think that the ground at the outside pole is the only 
ground. It serves as the "neutral" wire to the other two wires, and as the sole ground wire for the entire house. It is possible 
that there is a ground that connects the breaker box to rebar in the house - I am not sure, nor am I sure that it would serve as an 
effective ground from a safety point of view. If such a ground exists, however, it is also connected to the neutral wire at the 
breaker box.

[If any electricians out there want to tell me that this situation is probably not correct, then please let me know.]

In an ideal world, the neutral wire is the shortest, most conductive path to ground.

Therefore, if you use the ground wire (which connects to the neutral wire at the breaker box), to ground your computer, in an ideal 
world, you would be relatively safe, since any other path to ground would likely have a higher resistance.  (On my house, the metal 
screws on the outlet boxes are not grounded. So, connecting to them does nothing.)

Unfortunately, the real world and the ideal world are not always the same.

In my neighborhood, the three houses surrounding me have all had their electricity go out because of corroded supply wires. If a 
little water can seep into the plastic pipe (due to a bad connection, a construction leak, or a leak caused by earth movement), then 
the aluminum wires are more than happy to corrode. (Copper wires will corrode too, but not as rapidly. But, copper is much more 
expensive than aluminum. And, I think rebar - if connected to the moist earth - would be especially ready to corrode.)

Now, suppose I have my computer connected, that is, "grounded," to my wall outlet box. I am actually connected to the "neutral" 
wire, which is every bit as "hot" as the two hot wires.

[If you cut your neutral wire and grab both sides while something on that circuit is drawing power, you will definitely get a 
shock - and perhaps a severe or fatal one. Remember, the current is coming in through the hot wire, proceeding through the 
appliance, exiting through the neutral wire and going into the ground. Just as much current is passing through the neutral wire as 
is passing through the hot wire.]

So, suppose that I am working on my so-called "grounded" computer and I accidentally touch some metal which is connected to ground. 
In the ideal world, not much would happen, because the ground to earth path through the neutral wire is likely of much lower 
resistance than the ground to earth path through me. I suppose that I could get a mild shock, if it were not.

On the other hand, suppose that my aluminum wire has become corroded, like my neighbors', and has some significant resistance. Then, 
the path through me might carry quite a bit more current, giving me anything from a mild shock to a fatal one.

So using as an example my own house, you should be able to see that there is a very small, but finite chance that by using the 
ground on the wall outlet to ground your case, you run the risk of severe injury or electrocution.  You should not trust the wiring 
of your house to be properly grounded!

Furthermore, in is not necessary to ground the case at all!  Why are  you grounding the case?  The whole purpose of all this is to 
ground YOU, not the case!!  Grounding the case does nothing to ground you.  All it does is **help you to come to the potential of 
the wall socket,** for which there is a small chance of danger.  For years, the best way to ground yourself prior to working on the 
insides of your computer has been to use a wrist strap clipped to the case of your computer.  It is still the best way!

Dean Kukral

PS: Are there any master electricians out there that can confirm or deny what I am saying about house wiring?  While I have a MSEE, 
it is in hardware, and it is not the same as knowing anything about wiring a house!

                         PCBUILD's List Owners:
                      Bob Wright<[log in to unmask]>
                        Mark Rode<[log in to unmask]>

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