Earthing categorizes
Mains electricity systems are categorized 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 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: 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.
IT Supply is generator with no earth
connection, installation has it's own
earth rod.
10.7 The qualities of a good earthing system are:
1) Must be of low electrical resistance (2-5) ohm
2) Must be of good corrosion resistance
3) Must be able to dissipate high fault current repeatedly
Conclusion:
In our factory we will design a protective earthing system of type TN – S which provides
a separate earth conductor back to the station.
Design steps and system components:-
1- we will select a suitable place for the earthing rode installation.
2- Measure the earthing and it will be accepted if it is with in ( 2 – 5 ) Ω range if it grater, more than one rode will be used or we could make a chemical treatment for the effective earth ( soil) a round the earthing rode.
3-The last step, we will use an earthing bar which is connected to the earth through the earthing rode and connect all exposed conductive parts of the equipment's to it.
