Earthing system TN and IT, design

Terminology

Various earthing systems has been specified and defined by the International Electrotechnical Committee in standard IEC 364.

The LV earthing system characterises the earthing mode of the secondary of the HV/LV transformer and the ways of earthing the installation frames.

The combination of these two letters gives three possible configurations

Identification of types of earthing system is thus defined by means of 2 letters:

• the first for connection of the transformer neutral:

  • T for "earthed",

  • I for "unearthed" ;

• the second for the type of application frame connection:

  • T for "connected directly" to the earth,

  • N for "connected to the neutral" at the origin of the installation, which is earthed

Note

The TN system, as per IEC 364, comprises several sub-systems:

• TN-C: if the neutral conductors N and PE are combined (PEN) ;

• TN-S: if the neutral conductors N and PE are separate;

• TN-C-S: use of a TN-S downstream from a TN-C, (the reverse is forbidden).

Note

  • TN-S is compulsory for networks with conductors of a cross-section 10 mm2 Cu.

Note

  • Each earthing system can apply to the entire LV electrical installation, but several earthing systems can exist side by side in the same installation.

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