I left the two previous comments in for continuity, no pun intended.
Usually there are three conductors coming into the panel at a residence, two
hots and a, generally an un-insulated neutral. That gives you 240V, 230V or
208V at your entrance/meter/breakers depending on your utility and their
transformers. The ground is accomplished by a grounded bond from your
service, i.e. breaker panel or meter box if separate to a ground rod that is
generally about 6' into the ground. It can be to a METAL water line but with
so many new water services being plastic a ground rod is the norm. Look
around near your meter for an un-insulated copper wire connected to a rod or
a pipe. If there really isn't one nearby then your home may not be properly
grounded. I believe that code is they be visible. 1977 is late enough that I
would almost guarantee that is the case unless you live in some really
unrestricted, building codeless, no inspector burg. I won't go into bonded
neutrals. Use Google if you're really interested. That is the grounded
neutral circuit that Jim mentioned. Am I an electrician? No. But I do work
as a maintenance mechanic with electricity all the time and mostly 480V so I
know a pretty fair amount about the stuff. If you really are unsure go get
yourself a three prong tester with the status lights and check it out for
yourself. Handy little testers regardless. Easier than my $300 Fluke.
As an aside, my home still only has a 115v service. It's old. But there is
240v to the weatherhead from PG&E. That is the three wires I mentioned at
the top of this note. The second hot is taped off for whenever I get around
to remodeling. I did add a ground rod long ago to replace the one to the
old pipes.
As to the innards of MY computers, I leave them plugged in unless the length
of the power cord is restrictive, that is I need to dig into the inside. If
that is the case, again no pun intended, I just touch something metal to
discharge any static first, usually just the case and then dig in. I've
never had static be an issue. Then again there are only a couple of times a
year when it is highly prevalent in my locale. The exception is if I want to
dig into the power supply, usually to replace the fan. $3 vs. the cost of a
new PS. Yeah, I know about capacitors and stored charges. This is for those
in the know. Safety first, if you don't know, to go there.
Brad Loomis
-----Original Message-----
Hi Dean,
A separate house "ground" wire is provided as a safety feature.
Nothing, but "grounding" is allowed on the ground circuit (per NEC).
The ground wire is not allowed to carry current to devices. It is designed
to only be a safety device.
The home neutral circuit is grounded however it IS allowed and normally does
carry current. The implication here is that if you have a neutral circuit
fault it might affect its connection to ground, hence the neutral should not
be used as a safety device. If you lost your ground on a neutral circuit it
would typically become "hot" (as it normally carries current). Hence a
ground wire would be safer.
Generally your house ground should be grounded at the entry point to your
house (check with your local power company).
In any case, using the neutral as a safety device is not a good idea! If
you use anything use the ground.
Hope this helps.
Jim
--- On Tue, 6/16/09, Dean Kukral <[log in to unmask]> wrote:
From: Dean Kukral <[log in to unmask]>
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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