Characteristics of different earthing systems:

TN system neutral-connected exposed conductive part

•    Letter T depicts that

            The neutral is directly earthed.

  

•     Letter N depicts that

            The exposed conductive parts of the loads are connected to the neutral conductor.

There are two types of systems, possibly depending on whether the neutral

conductor and protective conductor (PE) are combined or not:

 Case 1: When neutral and protective conductors are joined in a single conductor in this case it is known as PEN. This type system is identified by a third letter C and is called TNC system. PEN conductor eliminates potential rises in the exposed conductive parts if a fault occurs.

(Reference: IEC 60364-5, section 546-2) 

 Case 2: In this case the neutral conductor and protective conductor are different. These type of systems are recognized by a third letter S and is said as TNS.

Characteristics of different earthing systems:

IT system: Specific characteristics:

•     Switching upon occurrence of a double fault is usually generated by phase-to phase fault protective devices (circuit-breakers, fuses, etc.).

•     If the short-circuit current is not sufficient to activate protection against phase-2-phase faults, In        case if the loads are far away, protection should be done by residual current devices (RCD).                               

           

TT system:

Specific characteristics:

•    The installation of RCDs is compulsory.

•     All exposed conductive parts protected by the same protective device should be connected to the same earth.

•     The neutral earth and the exposed conductive part earth may or may not be Interconnected or combined.

•  The neutral may or may not be distributed.

Specific characteristics of TNS and TNC systems:

•  Fault switching is obtained by devices providing protection against phase2 phase faults (circuit-breakers, fuses, etc.).

• both TNS and TNC systems can be used in the same installation. The TNC system (4 wires), however, must never be downstream of the TNS system (5wires).