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there are 5 earthing systems configurations as defined by the IEC:
TN-S system: where the network is earthed at the source and a dedicated earthing cable runs from the source to the load
TN-C: earthing at the source, and a combined Neutral+earth conductor runs from the source to load
TN-C-S: a combination of the previous two, where the circuit near the source is TN-C and the part near the load is TN-S
TT: earthing is done at both the source and load sides seperately and no earthing connection exists between them.
IT: the load side is either unearthed or earthed via large impedence, load is earthed separately.
TT, TN-S, and TN-C-S are all used in LV distribution networks, with TT being most cost effective, and TN-S most expensive but most reliable. the selection depends on prevailing codes.
IT is used for large industrial, loads, plant level circuits, and cirtical equipment.
TN-C is rarely used.
Depending of this neutral condition (Earthed-not Earthed-access-not access) there are various type of earthing System.
