Domestic Earthing Arrangements

TN-S


The neutral at the supply is connected to earth at one point only.


The consumer’s earthing terminal is usually connected to the metallic sheath of the distributor’s cable into the premises.


If a live conductor touches the metal casing of an appliance then the fault current is taken through the earth conductor to the substation causing a circuit breaker or fuse to break.

TN-C-S


The supply neutral conductor is earthed at source and at intervals along its run. This is referred to as PME (Protective Multiple Earth).


The supply conductor is also used to return earth fault currents arising in the consumer’s installation. The distributor provides a consumer’s earthing terminal which is linked to the incoming neutral conductor.

TT


This is similar to TN-S because the neutral of the source is connected to earth, but no facility is provided by the distributor for the consumer’s earthing.


With TT, the consumer must provide their own connection to earth by installing an earth electrode local to the installation.

There are three types of earthing arrangements that you need to know about: TN-S, TN-C-S and TT. In these abbreviations:


T = Terra (Latin for earth)

N = Neutral

S = Separate

C = Combined