Refer to this Topic, I would mention to the following points :
– About “ TN-C ” :
The following remark should be also mentioned :
– If the supply of a load is “ 3Ph + N ” and we will use only “ 1 cable of PEN ” between the network and that load, the connection of PEN should be done as follow “ the PEN cable should be firstly connected to the earth point at Load, then to the neutral point ”.
– About “ TN-C-S ” :
It’s mentioned that “ Usually, the combined PEN conductor is used near the source of the system ”, but it’s better to say :
1- For the big networks where the installed power is big or too big, we use TN-C at the 1st and maybe also at the 2nd levels of the network, then we use TN-S.
2- By this way we can decrease the installation’s costs for these 2 levels.
3- We can’t never ever use firstly “ TN-S ” then “ TN-C ”.
– About “ TT ” :
Also, it’s better to add :
– As we can use many independent Earthing points at consumers side only, this system is approximately used all around the world in LV Distribution Networks to supply Housings and buildings.
– About “ IT ” :
We should also mention to the following :
1- If the connection of the transformer is “ ∆ ” ( no neutral point ) we can connect one phase to earth via high impedance.
2- Two main types of “ IT ” that are :
– IT with distributed Neutral
– IT without distributed Neutral.
3- With this Earthing system, It’s absolutely necessary to use special devices called “ Insulation Monitoring Devices ” to detect and indicate any insulation fault .