International standard IEC 60364 describes three families of grounding arrangements, using two-letter codes: TN, TT, and IT systems.
The first letter indicates the connection between the ground and the power-supply equipment (such as a generator or a transformer). The second letter indicates the connection between the ground and the electrical device being supplied.
- T: direct connection of a point with ground (from Latin ‘terra’)
- I: no point is connected with ground (isolation), except perhaps through a high impedance route
- N: direct connection to neutral at the origin of installation, which is connected to the ground