IEC Terminology
International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.
The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
- T
- Direct connection of a point with earth (Latin: terra);
- I
- No point is connected with earth (isolation), except perhaps via a high impedance.
The second letter indicates the connection between earth and the electrical device being supplied:
- T
- Direct connection of a point with earth
- N
- Direct connection to neutral at the origin of installation, which is connected to the earth
Read more about this topic: Earthing System