Which are the earthing system used in low-voltage distribution networks ? which earthing system is more preferred ? Why ?

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.