Naming Electricity System Earthing Arrangements
The information on this document is based on UK DIY FAQ part written by Andrew Gabriel.
European naming conventions
Mains electricity systems are categorised in the many European countries (Finland, UK, etc.) according to how the earthing is implemented. The common ones are TN-S, TN-C-S and TT.
Note that in these descriptions, 'system' includes both the supply and the installation, and 'live parts' includes the neutral conductor.
Description of letters
T The live parts in the system have one or more direct connections to earth. I The live parts in the system have no connection to earth, or are connected only through a high impedance.
T All exposed conductive parts are connected via your earth conductors to a local ground connection. N All exposed conductive parts are connected via your earth conductors to the earth provided by the supplier.
C Combined neutral and protective earth functions (same conductor). S Separate neutral and protective earth functions (separate conductors).
Valid system types in the 16th Edition IEE regs:
TN-C No separate earth conductors anywhere - neutral used as earth throughout supply and installation (never seen this). TN-S Probably most common, with supplier providing a separate earth conductor back to the substation. TN-C-S [Protective Multiple Earthing] Supply combines neutral and earth, but they are separated out in the installation. TT No earth provided by supplier; installation requires own earth rod (common with overhead supply lines). IT Supply is e.g. portable generator with no earth connection, installation supplies own earth rod.
Ways to provide grounding
Inside or nearby your consumer unit (fuse box) will be your main earthing terminal where all the earth conductors from your final subcircuits and service bonding are joined. This is then connected via the 'earthing conductor' to a real earth somehow. The following earthing conventions are used in UK:
TN-S The earthing conductor is connected to separate earth provided by the electricity supplier. This is most commonly done by having an earthing clamp connected to the sheath of the supply cable. TN-C-S The earthing conductor is connected to the supplier's neutral. This shows up as the earthing conductor going onto the connection block with the neutral conductor of the supplier's meter tails. Often you will see a label warning about "Protective Multiple Earthing Installation - Do Not Interfere with Earth Connections" but this is not always present. TT The earthing conductor goes to (one or more) earth rods, one of them possibly via an old Voltage Operated ELCB (which are no longer used on new supplies).There are probably other arrangements for these systems too. Also, a system may have been converted, e.g. an old TT system might have been converted to TN-S or TN-C-S but the old earth rod was not disconnected.