1. Letter: Type of transformer earthing
- T direct earthing of star point
- I isolated star point
2. Letter: Type of consumer earthing
- T direct earthing of the consumer
- N direct connection of bodies (consumer) to star point
3. Letter: Kind of installing protectiv and mid-point conductor in TN-network
- S protective conductor and mid point conductor are installed separately
- C protective conductor and mid point conductor are combined