Full Scope Electrical Installer

Guide to Safe isolation procedure 

Electrical items we use

     
Earth Systems TT, TN-S-System & TN-C-S-system

TT network 

In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.

The big advantage of the TT earthing system is that it is clear of high and low frequency noises that come through the neutral wire from connected equipment. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.

 T, N, C and  S. 

 These letters stand for:

T    -  terre  (French for earth) and meaning a direct connection to earth.

 N    -  neutral

C    -  combined

S    -  separate.

Earthing conductors

Earthing conductors which are defined in BS 7671 as aprotective conductor connecting the main earthing terminal of an installation to an earth electrode or
other means of earthing must be adequately sized particularly where buried partly in the ground, and be of suitable material and adequately protected against corrosion and mechanical damage. The size of an earthing conductor is arrived at in basically the same way as for a circuit protective conductor, except that Table 54A of BS 7671 must be applied to any buried earthing conductor. For a TN-C-S (PME) supply, it should be no smaller than the main bonding conductors.

In TN-C and TN-C-S systems, any connection between the combined neutral-and-earth core and the body of the earth could end up carrying significant current under normal conditions, and could carry even more under a broken neutral situation. Therefore, main equipotential bonding conductors must be sized with this in mind; use of TN-C-S is inadvisable in situations such as petrol stations, where there is a combination of lots of buried metalwork and explosive gases.