wrote:
Hi Tom,

Can you explain briefly (technically if needed) why earthing is
not grounding?

Earthing is grounded. Or I should say: "earthing" is UK usage for what
in the US and Canada is called grounding. To say that something is
earthed or grounded implies that it is electrically conductive to
planet earth, which for these purposes we pretend is an infinite sink
for electricity. (In electronics where there is no such thing, the word
"common" is used instead. The "ground" in your car's electrical system
is a "common", since there is no conductance to the earth.)

My understanding is there are 3 wires, hot, neutral, and ground.

Quite right, in a normal North American 120 V circuit or a normal 240 V
circuit elsewhere. Older wiring simply omits the ground.

(In official code books, the neutral is called the "grounded" conductor
and the ground is called the "grounding" conductor. Let's not do that.
Grounding is also called "bonding" because of how all junction boxes
are supposed to be connected to it.)

Hot
and neutral connects to the utility lines, and ground connects to the
ground. Is it the 'difference' between one ground and the service
panels' ground that might be causing some problems?

Neutral and ground are connected together at - and only at - your main
service panel, and that connection is also connected to ground in the
form of a buried rod or something appropriate to the local conditions.

And in fact, I believe the neutral is also connected to a buried ground
rod at the transformer where your power is stepped down to 120 V from
whatever higher voltage the main lines are at. Others may correct me on
that.

You mentioned "A safety ground must connect the appliance ground
prong
to circuit breaker box safety ground - the neutral bus bar", if I am
understanding this correctly, you are saying both hot and neutral
gets
wired to the neutral line? How can this be right? Won't the ground
then be constantly hot?

Not hot and neutral certainly, but ground and neutral yes, which is
probably what you meant.

All the grounds from each circuit go to the ground bus (not the neutral
bus) in your main panel. All the neutrals from each circuit go to the
neutral bus in the panel. Plus, the neutral bus has one connection to
the ground bus, and the ground bus has a big wire going to your buried
ground rod.

With these connections in place, neither neutral nor ground can ever be
hot because they have a (effectively) zero-resistance path to ground.
"Hot" you must understand is a relative term; one thing is electrically
hot relative to another. Normally the reference point is the ground.
Circular but self-consistent.

If your neutral bar became disconnected from ground AND from the
utility's neutral feed, then all the neutrals in your house would be
hot. Things would quit working because the current that feeds them
would have no path to ground. (In fact the "other" hot leg of a North
American two-leg 120/240 service complicates this, but let's ignore
that.)

If your panel's ground bar became disconnected from ground AND neutral
became disconnected from the utility, then all the metal chassis of all
your appliances would become hot, via their ground connection. That's
really bad for anyone touching such a chassis while they're in good
contact with the earth. If you're in slippers on a carpet you're
probably ok but if you're loading dishes from steel sink into a
dishwasher, you're in trouble.

If I connect the ground of a receptacle (using a proper ground wire)
to an actual earth ground (and assuming the connection is good, not
flaky), what exactly is the problem? (I understand the concern about
connecting to water pipes and people taking bath....)

As I understand it, IF you did this "right" and IF the connection is
good, then you're ok, and as I understand it, historically electical
code permitted isolated grounds like this as a means of dealing with
legacy two-prong wiring. But others may correct me on this.

It's the weakness of the "IF" that's the problem. The trouble begins if
your connection is imperfect and has non-zero resistance. Then if your
chassis becomes hot owing to an internal fault, current will flow
through your makeshift ground, maybe not enough to blow the fuse but
maybe enough to overheat your ground wire. And someone near the ground
wire, or even standing on the earth over your buried rod, may be a
better path to ground (perhaps their other foot is in a puddle, or
they're leaning on your meter box, which is well grounded) and the
current will flow through them.

And suppose you've got another circuit "grounded" on the same rod, so
now the chassis of that appliance is made hot because of the fault in
the first one.

So the answer is that the proposal is not prima-facie dangerous, in
that if executed perfectly it would work. The problem is that it's so
hard to execute well it will certainly result in a half-assed mess, so
a better approach is not to try. Code now says that a home has one and
only one connection to ground, via the ground bar in the main service
panel. Even subpanels in the same building - even outbuildings, with
some restrictions - are supposed to have separate neutrals and grounds
and no local ground rod.

Sorry if these questions are too naive for all your troll sensitive
posters. I am just trying to understand this, so if you don't want
to answer, just don't say anything.

I'm confident that my responses will draw at least as many flames as
your questions would.

Chip C