Earthing Systems:

These have been designated in the IEE Regulations using the letters: T, N, C and  S.              

These letters stand for:

T    -  terre  (French for earth) and meaning a direct connection to earth. 

N    -  neutral

C    -  combined

S    -  separate.

When these letters are grouped, they form the classification of a type of system. 

The first letter denotes how the supply source is earthed.  

The second denotes how the metalwork of an installation is earthed.   The third and fourth indicate the functions of neutral and protective conductors.

TT SYSTEM

 A TT system has a direct connection to the supply source to earth and a direct connection of the installation metalwork to earth.  An example is an overhead line supply with earth electrodes, and the mass of earth as a return path.

Note that only single-phase systems have been shown for simplicity.

 TN-S SYSTEM

 A TN-S system has the supply source directly connected to earth, the installation metalwork connected to the neutral of the supply source via the lead sheath of the supply cable, and the neutral and protective conductors throughout the whole system performing separate functions.

The resistance around the loop P-B-N-E should be no more than 0.8 ohms.

TN-C-S SYSTEM

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).

NOTES

Older houses in towns use TNS  (solid) i.e. separate earth say cable sheath.

Around Towns new houses use (PME) TNCS i.e. neutral and earth shared.

Single House in country with own transformer uses TT i.e. own buried earth electrode.

Petrol stations, Swimming pools, Changing rooms etc. are not allowed to be PME.