HorneTD has provided good and detailed descriptions of
grounding. I will describe the same thing from a different
perspective. Take three D batteries (ie inside a flashlight
(torch in UK)). Each 1.5 volt battery adds to apply 4.5 volts
to a light bulb. Which part of that circuit is the ground?
Top of topmost battery? Bottom of bottommost battery? Select
any point. Ground can be arbitrarily defined.
But what is the voltage between topmost battery and earth?
Undefined. No connection exists. Voltage could be anywhere
from 0 to maybe 20,000 volts. Now we earth the bottommost
battery. Topmost battery is 4.5 volts above ground.
Suppose we also have inside that flashlight a 1.5 volt
battery. Convention is to connect 1.5 volt lamp to top and
bottom of bottommost battery so that both lights share one
common point. Arbitrary common point - even if it was not
connected to earth - would be called ground. We will call
this light bulb ground.
So now we have defined two grounds. Light bulb ground and
earth ground. We have put both grounds at the same point.
Let's move the earth ground wire to a point between two other
batteries. Now earth ground is not the same as light bulb
ground. We still have two different grounds. This time two
different grounds are at different locations on the circuit.
Wall receptacles must connect back to the neutral bar inside
the circuit breaker box where power originates. This "safety
ground" must be sufficient to trip a circuit breaker. We just
happen to (for good and well proven reasons) connect
building's earth ground to this same point. A concept is
called single point grounding. Done this way much for the
same reason that stereo components interconnect using single
point grounding (which is a but another ground).
Earth ground and safety ground are two different grounds.
They share common wires. But they remain different grounds
with different purposes.
Why is a building's earth ground so important? The neutral
wire inside the transformer had failed. The house also had no
earth ground connection. To get back to that transformer
ground, household electricity used a gas meter as a neutral
wire connection. Fortunately no one was home when gaskets
failed and the house exploded. Just another reason why earth
ground is so important. It does nothing until that rare time
when it is really needed.
Too many instead will say the lights work just fine without
that ground. Therefore that ground is not necessary. Same
thinking that killed seven Challenger astronauts and seven
Columbia astronauts.
Connecting a wall receptacle to earth does not provide a good
conductive connection to breaker box - to trip the circuit
breaker. That 'safety ground' must carry 15+ amps back to the
breaker box to trip the breaker. Earth ground will not
reliably do same. Wall receptacle must make a good, clean,
predictable, conductive connection to breaker box safety
ground. Wall receptacles must connect to the ground of their
purpose - the main breaker box neutral bar, also called here a
safety ground.
wrote:
Can you explain briefly (technically if needed) why earthing is
not grounding?
My understanding is there are 3 wires, hot, neutral, and ground. 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?
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?
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....)