Earthing Types

Earthing Types

BS 7671 lists five types of earthing system:

• TN-S
• TN-C-S
• TT
• TN-C
• IT

Where those letters indicate:

• T = Earth (from the French word Terre)
• N = Neutral
• S = Separate
• C = Combined
• I = Isolated

An explanation can be found here.

A useful IEE document on earthing (presumably most DIYers can ignore the section on earthing caravan pitches)

You can find details on installing a TT earth here.

Earthing types explained

TN-S

Earth connection provided by the supplier.

TN-C-S

Also known as PME, this earth connection is also provided by the supplier. Its direct connection to the neutral brings some added issues that require addressing at the consumer's end.

TT

Local earth connection, either to an earth rod, or in years past to a metal water pipe. Local earths typically have an earth impedance high enough that a short won't trip a 30A fuse, so TT installs have long required whole house RCD or ELCB protection.

TN-C

Not normally used.

IT

Never used for permanent house supplies, this is commonly used for portable generators. Simply there is no reliable earth connection on the system.

Identify your Earthing type

TN-C-S

TN-C-S (aka PME or Protective Multiple Earthing), installations can be recognised by the earth conductor being connected to the cable head end such that it can be connected to the main incoming combined Neutral and Earth conductor (known as a PEN or Protective Earth & Neutral conductor)

In this example the supply enters via a cable at the base of the cutout. Note that a service connector block has been used as a main earth terminal, and the connection to the cable head end can be clearly seen (note that the wire making this connection is actually undersized in this photograph - it should be at least 16mm² and not the 6mm² used here, this is possibly a hangover from a system that was upgraded to PME later in life)

The next example shows inside an external meter box set into a cavity wall. An overhead supply cable can be seen entering the box at the top right hand side, and dropping to the head end. The tails and main earth wire then head off into the bouiding to connect to the consumer unit.

TT Earthing

TT Systems are normally recognised by the lack of any earth connection to the incoming supply cable sheath or to the incoming neutral (i.e. combined earth and neutral) conductor.

An older example:

Here the incoming supply is by way of overhead cables. These terminate on the outside of the building and separate wires with fibre reinforced rubber insulation bring the supply into the building. The neutral connection is made directly to the meter, and the live connection is via a straightforward main cutout fuse and then to the meter.

A slightly more up to date TT installation:

Here the supply is again via overhead wires, but this time is converted to a concentric supply cable from the connection point outside the building to the main cutout. The cutout in this case is a more sophisticated device through which both neutral and live conductors pass. It has provision for the connection of an earth conductor the the incoming neutral when used on TN-C-S (PME) installations (note the same brand is used in one of the PME setups shown above), however in this case you can see that the only connections actually used are the live an neutral feeds to the (electronic) meter.

Main Earthing Terminal

The main earthing terminal (MET) in a building is the place where the main earth, the main equipotential bonding conductors, and the connection to the circuit protective conductors for installation's circuits meet. On smaller installations this will frequently be hidden away inside a consumer unit, where the main earth bus bar typically has addition connection points for the connection to the main earth, and also the bonding conductors.

On larger installations (often ones that employ more than one CU), the MET may be an external terminal block similar to that shown here:

See Also

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