There are at least six types of earthing system, identified as follows:
The first letter indicates the type of earthing at the distribution transformer, with T (= 'terre') indicating that one or more points of the supply are directly or 'solidly' earthed (e.g. an earthed neutral), and I indicating that either there is no earth at all, or earthing is achieved via an impedance.
The second letter indicates the earthing arrangement at the electrical installation, with N indicating that all conductive parts are connected to an earth terminal provided by the utility company, and T indicating that the conductive parts are connected directly to a buried earth electrode.
The third letter indicates the arrangement for the earthed supply conductor, with C indicating that the supply's neutral and earthing conductors are combined, and Sindicating that the supply's neutral and earthing conductors are separate all the way from the transformer.
The fourth letter, S, indicates that within the installation, the neutral and earth conductors are separate.
(The TN-C-S system is also commonly known as 'protective multiple earthing'.)
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