EARTHING

Earthing or earthing system is the total of all means and measures by which part of an electrical circuit, accessible conductive parts of electrical equipment (exposed conductive parts) or conductive parts in the vicinity of an electrical installation (extraneous conductive parts) are connected to earth.

Earth electrode is a metal conductor, or a system of interconnected metal conductors, or other metal parts acting in the same manner, embedded in the ground and electrically connected to it, or embedded in the concrete, which is in contact with the earth over a large area (e.g. foundation of a building).

Earthing conductor is a conductor which connects a part of an electrical installation, exposed conductive parts or extraneous conductive parts to an earth electrode or which interconnects earth electrodes. The earthing conductor is laid above the soil or, if it is buried in the soil, is insulated from it.

The various standardized earthing system :

• when designing an electrical instillation , one of the first things to determine is the type of earting system

 TYPE OF EARTHING SYSTEM

The three earting system given official status by international standards (IEC60364)are also stipulated by alarge number of national standards : infrance by the lv installation stanadard : NFC  15-100.

1-The  TN system

Its principle

-          the transformer neutral is earthed

-          the electrical load frames are connected to neutral

      -this type of the system has three possibilities

 a)- TN – C  system 

The same conductor acts as a neutral and protective conductor (fig E-A)

B) TN – S  system

C)- T N –C – S    system

 Its operation

An insulation fault on a phase becomes a short circuit and the faulty part is   disconnected by a  Short-Circuit Protection Device (SCPD).

2- T  T   system

 its principle

-          the transformer neutral  is earthed

-          the electrical load frames are also earthed

its operation

The current of an insulation fault is limited by earth connection impedance. Protection is provided by the Residual Current Devices (RCD)

Its principle

    -the transformer neutral is not earthed, but is theoretically unearthed. In actual fact, it is naturally earthed by the stray capacities of the network cables

     -the electrical load frames are earthed.

Its operation:

- should an insulation fault occur, a low current develops as a result of the network’s stray capacities (see fig. 3a ). The contact voltage developed in the frame earth connection (no more than a few volts) is not dangerous;

- if a second fault occurs on another phase before the first fault has been eliminated

(see fig. 3b and 3c ), the frames of the loads in question are brought to the potential developed  by the fault current in the protective conductor (PE) connecting them. The SCPDs (for the frames interconnected by the PE) or the RCDs  (for the frames with separate earth connections) provide the necessary protection.