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.
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 as shown below.
Note that only single-phase systems have been shown for simplicity.
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.
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).
The resistance around the P-B-N-N loop should be less than 0.35 ohms.
SUMMARY OF EARTHING SYSTEMS
The TT method is used mostly in country areas with overhead transmission lines. In contrast to the TN-S system there is no metallic path from the consumer's terminals back to the sub-station transformer secondary windings. Because the earth path may be of high resistance, a residual current circuit-breaker (R.C.C.B.) is often fitted so that if a fault current flows in the earth path then a trip disconnects the phase supply.
For protection against indirect contact in domestic premises, every socket outlet requires an RCCB with a maximum rated current of 30mA.
The TN-S system of wiring uses the incoming cable sheath as the earth return path and the phase and neutral have separate conductors. The neutral is then connected to earth back at the transformer sub-station.
Remember in TN-S, the T stands for earth (terre), N for neutral and S denotes that the protective (earth) and neutral conductors are separate.
The TN-C-S system has only two conductors in the incoming cable, one phase and the other neutral. The earth is linked to the neutral at the consumer unit. The neutral therefore is really a combined earth/neutral conductor hence the name PME.
In order to avoid the risk of serious electric shock, it is important to provide a path for earth leakage currents to operate the circuit protection, and to endeavour to maintain all metalwork at the same potential. This is achieved by bonding together all metalwork of electrical and non-electrical systems to earth.
The path for leakage currents would then be via the earth itself in TT systems or by a metallic return path in TN-S or TN-C-S systems.
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.