title
A TT system has a solidly earthed source; the exposed-conductive-parts of the consumer’s installation are earthed through an installation earth electrode which is electrically independent of the source earth. [British Standard]
title
In a TN-C-S system, the neutral and protective functions should be combined in a single conductor (PEN) from the source (solidly earthed) up to the consumers intake. Multiple connections to earth are recommended along the PEN conductor. Within the consumer’s installation the neutral and protective conductors should be kept separate. [British Standard]
TN-S system
In a TN-S, the Neutral and Protective conductors should be kept separate throughout the system and the source is solidly earthed. [British Standard]