24 Jun
2005
24 Jun
'05
9:54 a.m.
>>>> "Bj??rn" == Bj??rn Vermo
<bv at norbionics.com> writes:
Bj??rn> On 21 Jun, 2005, at 12:35, Ethan Dicks wrote:
> On 6/21/05, Paul Koning <pkoning at
equallogic.com> wrote:
>> If your boxes are plugged into grounded outlets, as they should
>> be, then there should NOT be large currents flowing through frame
>> ground cables. If you do observe that, track down the fault
>> before someone gets hurt.
>
> The key word is "should"... If you get some ya-hoo who decides
> they can save a few $$$ and wire up an outlet... well... let's
> just say that the first homeowner's tool I bought was an outlet
> tester, and I found two outlets that had hot and neutral
> reversed...
>
> I worked at a place where the owner plugged a serial cable in
> between his PC and his Mac, and he roached the Mac serial port
> because of the same problem. He had all the outlets tested and
> fixed, but he also made a proclaimation - no computers were to be
> connected via serial cables unless they pulled their power from
> the same wall outlet. A bit inflexible, but guaranteed not to
> have a mismatch.
Bj??rn> In Norway, you will not get mains-powered devices approved
Bj??rn> unless the chassis is isolated. Our AC wiring does not have
Bj??rn> any concept of "neutral", since both wires are supposed to be
Bj??rn> at the same potential relative to ground. That way, everybody
Bj??rn> learns to be careful.
That doesn't address the point I was talking about. While US practice
does distinguish neutral and hot leads (for 110 volt outlets, that
is), equipment doesn't depend on that much anymore. The only place
where it is still used is in power switches, which are often single
pole and in the hot wire only.
In ancient days, designers would connect the supposed neutral to the
chassis, but that's no longer done and probably no longer legal.
Instead, the grounding wire (green wire) connects to the chassis.
That's true worldwide. (In Norway, that would be the connector at the
outer edge of the outlet.)
All this works fine in correctly wired installations, where no current
flows on the ground wire, and all ground wires are connected to the
same common ground as required in the USA by the regulations. If the
installer gets this wrong, you may end up with a voltage difference
between the ground pins on different outlets. That's both illegal and
dangerous -- it can fry equipment, but more importantly it can fry
people.
The same can occur in any other country -- all it requires is that the
electrician screws up sufficiently badly.
paul