The safety ground is typically a bus bar in main disconnect
box. The central point at which neutral and safety ground
wires meet. This is a ground different from the earthing
rod, which is different from a receptacle safety ground, which
is different from the computer chassis ground, which is
different from motherboard ground, which is different from the
entertainment system single point ground to eliminate hum
between various stereo components. All may be interconnected
- but not by a perfect conductor. And all serve different
functions. Therefore all are considered different grounds.

Take a 50 foot connection from breaker box to receptacle
using a 20 amp wire. That safety ground wire may be 0.2 ohms
'resistance'. However to transients, the same wire may
measure 130 ohms 'impedance'. If trying to earth a trivial
100 amp transient, then the wire would be something less than
13,000 volts from plug-in surge protector to breaker box
ground bus. Clearly wire impedance makes that receptacle
ground all but no connection to earth. Earth ground and
safety ground in that wall receptacle are not same.

Therein also lies reason for a single point ground between
stereo components and why breaker box ground is not same as
earth ground. Wire has electrical characteristics that make
each interconnected ground different. Wire becomes an
electronic component when discussing transient protection.

For human safety, the single point ground of significance is
inside a breaker box. To eliminate hums in stereo equipment,
the single point ground is where all component grounds meet.
To protect computer motherboard from static shock
interruptions, a motherboard ground connects to chassis
ground at only one point. To discharge a static electric
charged human is a ground located underneath the shoe (no
earth ground involved in that static electric discharge). So
that various signals don't interfere, then A/D converters have
separate analog and digital grounds - that meet at a single
point typically at the A/D converter. For surge protection,
the single point ground of significance is central earth
ground. Many grounds. All different even if interconnected.

Again, every ground may be interconnected but each ground is
different because wire is an electronic component. Distance
also determines quality of that earthing - because again, wire
is an electronic component. Plug-in protectors do not 'shunt'
a less than 10 foot connection from each incoming power wire
to earth ground. Therefore they cannot earth that incoming
wire. Distance in that 50 foot wire at 130 ohms impedance
demonstrates why, for example, wall receptacles are not earth
ground.

m II wrote:
In any electrical code that I'm aware of, ground *means* earth ground.
In this neck of the woods it is defined as:

A connection to earth using a grounding electrode.

I don't know what other kinds of 'any ground' can possibly be.
It's either a ground or it isn't.

mike