ill_wind said:
the earth should be at the lowest potential so it should be impossible to get a shock from earthed appliances.
lets get some basic facts straight
lowest potential is A MEANINGLESS TERM IN AN AC SYSTEM and anyway even in a DC system the direction of a potential difference is meaningless from a safety point of view
It is not possible to state the potential of any point in a system as a figure in its own right only the potential difference from a given reference
you get a shock
it should further be noted that AC voltages as well as having a magnitude can have a phase angle but once again this value is meaningless in isolation. It only has meaning to measure the phase difference between one potential difference and another potential differenct in the same (phase synchronised) system. This is what allows you to have 3 phases all 240V RMS from earth but 415V from each other
now lets look again at how we provide protection
EARTHED
EQIPOTENTIAL
BONDING
WITH
AUTOMATIC
DISCONNECTION
OF
SUPPLY
ok first equipotential bonding
we bond metal that either has some form of connection to earth or surrounds live electrics together to keep it at the same potential during a fault
second earthing
the main reason we earth the protective system is to prevent shocks between stuff connected to the protective system and the body of the earth
third automatic disconnection of supply
we obviously don't want fault current to flow for too long and equipotential bonding can never be perfect so we connect the protective system back to the neutral point of the generator or transformer either directly via cabling or indirectly via the earth. This causes current to flow when something shorts to earth tripping/blowing a protective device
from the point of view of electrical shock indoors Equipotential bonding (and note that in this sense the CPC (earth) cores of the cuircuit and appliance cables are part of the equipotential bonding system as well as the earthing system i suspect this is why calling them the earth cores is no longer considered correct) is FAR more important than earthing The system or provideing a return to the transformer star point