{"id":82749,"date":"2026-09-24T10:44:59","date_gmt":"2026-09-24T13:44:59","guid":{"rendered":"https:\/\/a3aengenharia.com\/?post_type=articles&p=82749"},"modified":"2026-09-24T10:45:46","modified_gmt":"2026-09-24T13:45:46","slug":"tn-tt-it-earthing-systems-nbr-5410","status":"publish","type":"articles","link":"https:\/\/a3aengenharia.com\/en-us\/content\/technical-articles\/tn-tt-it-earthing-systems-nbr-5410\/","title":{"rendered":"TN, TT and IT Earthing Systems: Differences and NBR 5410"},"content":{"rendered":"\n

The TN, TT, and IT<\/strong> earthing systems define how the supply point relates to earth and how exposed-conductive-parts of the installation are connected to protection. This choice changes fault-current paths, automatic disconnection behavior, the use of PE or PEN conductors, application of RCDs, and coordination with equipotential bonding and SPDs. TN, TT, and IT do not describe types of rods or grounding grids; they describe electrical protection architectures.<\/p>\n\n\n\n

In TN<\/strong>, one point of the supply is directly earthed and exposed-conductive-parts are connected to that point by protective conductors. In TT<\/strong>, the supply has its own earth connection and the installation’s exposed-conductive-parts are connected to an electrode electrically independent from the source earthing. In IT<\/strong>, the supply is isolated from earth or connected to it through an impedance, while exposed-conductive-parts remain connected to an earthing system.<\/p>\n\n\n\n

There is no universally \u201cbest\u201d system. Selection must consider the supply arrangement, service continuity, prospective fault current, operating times, maintenance, presence of RCDs, generators or UPS systems, electronic loads, and installation-specific requirements. Incorrect identification of the system can lead to wrong decisions regarding neutral, PE, PEN, RCDs, and SPDs.<\/p>\n\n\n\n\n

How to Read the Letters TN, TT, and IT?<\/h2>\n\n\n\n

The nomenclature identifies two fundamental relationships. The first letter<\/strong> indicates the relationship of the supply to earth. The second letter<\/strong> indicates how the installation’s exposed-conductive-parts relate to earth.<\/p>\n\n\n\n

In simplified form:<\/p>\n\n\n\n

Letter<\/td>Functional meaning<\/td><\/tr>
T in the first position<\/td>One point of the supply is directly connected to earth<\/td><\/tr>
I in the first position<\/td>Live parts are isolated from earth or one point is connected to earth through an impedance<\/td><\/tr>
T in the second position<\/td>Exposed-conductive-parts are connected to an earth electrode electrically independent from the supply earthing<\/td><\/tr>
N in the second position<\/td>Exposed-conductive-parts are connected to the earthed supply point by a protective conductor<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

In TN systems, the letters S<\/strong> and C<\/strong> further indicate whether neutral and protective functions are separate<\/strong> or combined<\/strong>. This gives rise to TN-S, TN-C, and TN-C-S.<\/p>\n\n\n\n

Conceptual relationship among source, exposed-conductive-parts, and earth in TN, TT, and IT systems<\/title>