Grounding question

[nix85]

i still dont see a single valid reason for existence of the 3rd hole

[Floor]

the third prong ....

In a home wiring system, if a hot wire contacts the outer body
of an appliance, it is a dead short to the breaker.  The appliance
body cannot conduct a large current if it is wired to the third prong.

An undesired current available through the appliance body becomes a source
of current through any person's body that completes an electrical circuit from that
appliance body to a grounding point.
... ... ... ... ... ...
The actual earth grounding rod at the electric service points, are / is for lightning
strikes.

In RF ranges and earth grounding there are other things to consider.

[nix85]

let's first dispell the "hot wire" nonsense
there is no such thing as "hot wire", or "phase and neutral"
voltage is same on both terminals with only difference being that
"neutral" is USUALLY (not always) earthed.

MCBs are used for overcurrent protection
RCDs for earth leakage human protection

i know earthing is used mostly for lightning and static build up drain

again, what is the difference if casing of an appliance is connected
to neutral in the appliance or 2 meters away in the breaker panel

[Floor]

"again, what is the difference if casing of an appliance is connected
to neutral in the appliance or 2 meters away in the breaker panel"

In short (pun intended) the third wire is for redundancy.

Current flows between a hot wire and a common wire only after an appliance is used
on that pair (hot and common). 

Current never flows between a hot and a ground unless something is wrong.
The ground wire is for safety only, never to be used as common, always out of the
circuit unless something is wrong. It is to provides a dead short to the breaker.
Breakers help prevent fires.

Current never flows between a ground wire and a common wire unless something is wrong.
They are supposed to have the same resistance and voltage potential.

Common and ground have parallel paths to the common terminal.

The common is connected to earth ground at the main breaker box.
Down the line, it does not have the same potential to earth ground as does a hot wire.

The ground wire is connected to earth ground at the main breaker box.
Down the line, it does not have the same potential to earth ground as does a hot wire.

Common wire or ground wire,  it  will always dead short a hot wire,  because it leads to the center
tap of the line transformer. But also they short a hot wire because they are earth grounded.

Common is supposed to connect to a hot wire only with current that has passed through
the electrical resistance of an appliance.  That connection is to both the transformer and to
earth ground.

When an appliance draws a large current, it is because the appliance has a low
electrical resistance. 

In respect to an earth ground (buried plumbing),  an appliance that draws a large current
( low resistance) may still have a considerable voltage potential to that earth ground even
after the load.

If a path is provided, voltage and current will divide between an earth ground
and the common wire (according to the resistance differences).

If the common wire is connected to the body of an appliance and a person's body connects the appliance to an earth ground, that person becomes part of that divided circuit. 

If an appliance is one that draws a lot of power (low resistance), the voltage and current to
earth ground which is still available after the appliance load can be substantial.

If the common wire becomes disconnected from its terminal at the breaker box,
It has become disconnected from both the transformer center tap and the earth ground.

The appliance body is now completely live in respect to an earth ground, (after the
appliance load).

This is of course less live, than if the hot side of the load were directly connected to the
appliance body (before the appliance load).

Both the common and the ground wire are neutral to earth ground (ideally).

The "common" wire is hot in common, to either side of the transformer.
The "common" wire is neutral to the ground wire and to an earth ground.

The hot wires are hot to both, the earth ground and to the common (transformer).

Again the third wire is for redundancy.
    also
Common and ground wires share the same terminal at the mains breaker box.

In general, in a sub panel connection, there is no local connection present between common
and ground wires. 
                           
The common bar is not connected to the box / body.

A separate ground bar is connected to the sub panel box / body.

The common and ground wires are connected to the same bar only at the mains box.

There are exceptions to this generalization.

[fritznien]

let's first dispell the "hot wire" nonsense
there is no such thing as "hot wire", or "phase and neutral"
voltage is same on both terminals with only difference being that
"neutral" is USUALLY (not always) earthed.

MCBs are used for overcurrent protection
RCDs for earth leakage human protection

i know earthing is used mostly for lightning and static build up drain

again, what is the difference if casing of an appliance is connected
to neutral in the appliance or 2 meters away in the breaker panel

we have been thru this,any fault in the neutral will let the case rise to full voltage and ZAP!what are you trying to do here?you want to cheap out on the cord use non conductive cases.no the ground in a house wiring is not remotely able to handle lightning but it will keep metal safe to touch if there is a fault. telephone equipment has very stringent grounding systems but compared to a toaster the equipmentin a phone system is very delicate.