TT network
In a TT earthing system, the protective earth connections of the consumer and supply source are provided by a local connection to earth, independent of any earth connections at the generator.
The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.
TN−S Network
A TN-S system has the supply source directly connected to earth, the installations are connected to the neutral of the supply source via the lead sheath of the supply cable, and the neutral and protective conductors(i.e. earthing) throughout the whole system performing separate functions.
TN-C-S Network
A TN-C-S system is as the TN-S but the supply cable sheath is also the neutral, i.e. it forms a combined earth/neutral conductor known as a PEN (protective earthed neutral) conductor.
The installation earth and neutral are separate conductors.
This system is also known as PME (protective multiple earthing).
T - terre (French for earth) and meaning a direct connection to earth.
N - neutral
C - combined
S - separate.