Re: strange network problem
On Thu, Nov 02, 2006 at 11:39:57AM -0500, Lennart Sorensen wrote:
> On Thu, Nov 02, 2006 at 01:00:50PM +0100, Erik Mouw wrote:
> > Europe doesn't need such plugs because over here earth and ground are
> > two different and unconnected things, whereas in the US and Canada they
> > are usually connected together at the electrical panel (at least it was
> > like that in the houses I've seen in New York state). The ground wire
> > is the ground for the electrical system. It can and is allowed to have
> > a different potential from the earth (within limits, of course). The
> > earth wire is connected to the earth by a metal wire burried a couple
> > of meters deep into the earth.
>
> Certainly true as far as I can tell. It does seem that ground and
> neatral are considered one and the same for many things here.
Some of the confusion arises because of the difference between what
people call things and what the code calls things. I haven't read the
US code, but here in Canada there are several wires within a residential
installation that are 'grounded'.
Neutral wire from the pole transformer, grounded with a
grounding conductor to a grounding electrode at the pole. This
neutral wire is also grounded at the main panel within the
house.
System grounding conductor to the system grounding electrode
within the house. Also bonding conductors that connect
everything non-electrical that needs to be 'grounded', e.g. gas
pipes, well casings, etc. The only purpose of this is to keep
the potential of the hots from rising too far above ground.
Without this, a two-phase 120-0-120 V could change to a 60-x-180
V which would damage equipment.
Equipment bonding conductors carried in all cables to bond
connected equipment and metal conduits, outlet boxes, etc.
Some equipment has a separate bonding conductor (isolated
ground) that connects a recepticle (socket, outlet) gound
terminal to the main panel separate from the equipment bonding
conductor within the cable and separate from the metal box
enclosure.
The only place where all of these wires are at the same potential is at
the service entrance box where they are all tied together. At the
equipment, if there is no current on the bonding conductor it should be
a ground potential. Neutral will be above ground because of the
resistance to the current flow. Improper wireing elsewhere in the
building (e.g. cross-connected neutrals from different circuits) can
result in a net current flow through a cable which can induce a current
on the bonding conductor raising the potential at an outlet.
There's an excellent publication on all this called:
How to protect your house and its contents from lightening:
surge protection: IEEE guide for surge protection of equipment
connected to AC power and communications circuits.
Wouldn't life be simpler if all computer interconnects where straight
fibre? No ground loops in a light path.
Doug.
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