Types of System Earthing
There are three types of system earthing used today. These are: –
· TN System
· TT System
· IT System
The TN system is the type used in this country. It is commonly referred to as a “neutralised system”. This basically means that the supply neutral is connected to earth at the supply point.
There are three variations of this system; the one we use in Ireland is the TN-C-S system. The two other variations are the TN-C system and the TN-S system.
Although all three are TN Systems the difference between them is in the way that the neutral and protective conductors are arranged.
In the TN-C-S System the neutral and protective conductor functions are combined in a single conductor in part of the system.
· T = Direct connection of one point to earth.
· N = Direct electrical connection of the exposed conductive parts to the earthed conductor of the supply system. In AC systems the earthed
conductor is normally the neutral conductor.
· C = Neutral and protective functions combined in a single conductor ( known as PEN conductor )
· S = Neutral and protective functions provided by separate conductors.
TN-C-S System
The 230 Volt single phase supply to a premises is taken from the secondary winding of a DSO transformer. One terminal of this secondary winding is connected to earth ( ground ). This terminal becomes the neutral for the system. The neutral conductor is therefore at earth potential ( zero volts ).
A two core concentric cable is generally used to connect from the supply network to the premises. The centre core is the phase conductor. The outer core performs the functions of neutral and protective conductors. This conductor is known as a PEN conductor. It is terminated at the DSO main fuse unit.
PEN = PE for protective conductor ( protective earth ), N for neutral conductor.
From this point on, the neutral and protective conductors are separated and must not be connected together, anywhere throughout the entire installation. The protective conductor is referred as the Main Protective Conductor. It is connected to the Main Earthing Terminal of the installation. This means that the Main Earthing Terminal is solidly connected to the DSO neutral.
At each installation an earth electrode is provided. The earth electrode is connected to the Main Earthing Terminal and therefore to the DSO neutral. This arrangement provides the consumer with an earth terminal, which is connected to the neutral conductor of the system, thereby providing a low impedance ( low resistance ) path for the return of earth fault currents.
Impedance is the ratio of AC voltage and current. The Ohm is the unit of impedance. Its symbol is the letter Z.
TN-C-S System Earthing
