1. An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes.
2. TT
2.1. the transformer neutral is earthed
2.2. the frames of the electrical loads are also connected to an earth connection
3. TN
3.1. the transformer neutral is earthed
3.2. the frames of the electrical loads are also connected to an earth connection
4. IT
4.1. the transformer neutral is not earthed
4.2. the frames of the electrical loads are connected to the earth
