I am not a licensed electrician. I did, however work for one for
eight years. I have to admit parts of the job I just didn't
understand. One is the fact that the ground wire and the neutral wire
"from" a wall plug connect to the same bar in the breaker box. This,
on it's own would lead me to share some of Deans trepidation.
Searching for answers through a google search turned up about as many
"unplug all wires from the back of the case" as it did "leave the
power cord plugged in but the power supply rocker switch turned off"
(which seems to me to put a lot of trust in it not being a faulty
switch). Confusing!
I always used the "unplugged" method on my builds (6 to date) and the
wrist strap with no noticable problems. But, in my searches, I also
read that partial damage can be done that only occasionally shows up
from a "shakey" device. This makes me wonder about rare or random
problems I've experienced being linked to ESD (Electro-Static
Discharge).
I'm very curious to see where this thread leads. I'm re-thinking how
to go about my next build.
-Phil-
On 6/16/09, Dean Kukral <[log in to unmask]> wrote:
> Jim, that is, perhaps, the way things **should** be according to code. If
> you will go back and reread my post, you will see that
> it is not the way things **are** at my house.
>
> Your point that you should not use the neutral confirms exactly what I have
> been trying to say.
>
> (If you do not subscribe to PCSOFT, then you have not been following the
> hardware thread that accidentally popped up there.)
>
> If my house, which was built by qualified electricians and approved by
> inspectors, is not wired properly, then anybody's house could
> be wired improperly, or just accidentally short-circuited, neutral to
> ground. And, therefore, using the "ground" wire to
> unnecessarily ground your computer case yields a very small chance for a
> very serious accident.
>
> Dean
>
>
>
> ----- Original Message -----
> From: Jim McCrary
> To: [log in to unmask]
> Sent: Tuesday, June 16, 2009 10:27 AM
> Subject: Re: [PCBUILD] It is neither necessary nor desirable to ground the
> case to a wall socket - explanation why
>
>
> 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]>
> Subject: [PCBUILD] It is neither necessary nor desirable to ground the case
> to a wall socket - explanation why
> To: [log in to unmask]
> Date: Tuesday, June 16, 2009, 9:35 AM
>
> 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!
> Bob Wright<[log in to unmask]>
> Mark Rode<[log in to unmask]>
>
> PCBUILD's List Owners:
> Bob Wright<[log in to unmask]>
> Mark Rode<[log in to unmask]>
>
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