It appears my website is down at the moment (can't complain. It cost next to nothing) so here is a short explanation:
{Edit to insert my
website }
The first letter is the transformer side:
I (isolated) Neutral not connected to earth
T (terra) Neutral connected to earth
The second is how the earth fault current is sent back to the transformer:
N (neutral) Through the neutral through some connection point between the transformer and the equipment served.
T (terra) Trough the earth
The third letter is only used for TN-systems and explains where the connection point between the transformer neutral and protective bonding of the equipment is.
-C (Common) The neutral is brought all the way to the equipment and serves as protective bonding/earthing
-S (Separate) The neutral and bonding/earthing conductors are separated all the way
-C-S (Common-Separate) Starts out as a common conductor which is then separated somewhere, commonly at the point of entry to a building.
Therefore there now are:
IT Transformer or generator not connected to earth, equipment locally bonded toghether and to earth rod.
TT Transformer and equipment connected to local earth rods. Fault trips earth fault protection device (e.g. RCD/GFCI)
TN-C / TN-S / TN-C-S
If fault occurs, the overcurrent protection sees a short circuit and trips.
(A system where you didn't have any earth connection at all would be II but I don't think this term is ever used)
[This message has been edited by C-H (edited 01-23-2006).]
