There are 3 main commonly used Earthing Arrangements found in domestic installations.
TN-C-S ; TN-S ; TT
The T stands for ‘TERRA’ which is Latin for Earth.
The N means that the connection to earth is via the supply Network.
The C stands for ‘COMBINED’ in relation to the neutral and the earth.
The S stands for ‘SEPARATE’ in relation to the neutral and the earth.
A TN-C-S system has the neutral and earth COMBINED in the Electricity Boards supply cable(s), and SEPARATE with-in the installation, (this being the meter position and onwards). Also commonly known as PME (Protective Multiple Earthing).
A TN-S system has the neutral and earth SEPARATE through-out the entire system, including the Electricity Boards Supply cable(s).
A TT system has the neutral running in the supply cable(s) as normal, and the earth from the installation is connected to an appropriate Earth Electrode inserted into the ground.
An RCD is essential in an installation formed by a TT earthing arrangement.
It is the Electricity Boards responsibility to provide a TN-C-S or a TN-S supply, if requested and available, it is not their responsibility to provide a TT earthing arrangement.
In order for the Electricity Board to provide a TN-C-S supply, they have to construct their network to the standards of PME (protective multiple earthing), this is why a TN-C-S system is often referred to as a PME installation.
Drawn & Written By Lectrician - DO NOT DISTRIBUTE WITHOUT PERMISSION.