Quote Originally Posted by DrAndy
that third wire is run to the consumer unit, where all the other earths meet up

then they are connected to an earthing rod, properly installed
Usually it is as simple as using twin and earth cable... which comes with what should, under most circumstances, be a correctly sized earth. all of which are terminated at the consumer box. calculating the correct size of earth for a supply is a surprising complex job and twin and earth uses a conservative sizing to be correctly or oversized under most circumstances.


As for how the consumer box itself is earthed, that depends upon which of several std setups the local electricity board has used in setting up your local substation/transformer, it could be just an earth rod, it could be more complex and getting it right is rather important.