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

First letter:

  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.

Second letter:

  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.

Remaining letter(s):

  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.