A TN system earths the source neutral solidly and runs a protective conductor from every exposed metal part of the installation back to that neutral point, so an earth fault becomes a low-impedance short circuit back to the source. Fault current is then high enough for ordinary fuses and breakers to disconnect within the required time, typically 0.4 s for final circuits at 230 V. Variants are TN-S with separate neutral and protective conductors throughout, TN-C with a combined PEN conductor, and TN-C-S.
Supply is TN-S off the cable sheath, loop impedance is low, so the MCBs will clear an earth fault without help from an RCD.
Related terms
Earthing system
Arrangement describing how a supply's neutral and an installation's exposed metalwork are connected to earth.
TN-C-S system
Supply earthing in which a combined neutral and earth conductor splits into separate ones at the installation.
PEN conductor
Single conductor that serves as both the neutral and the protective earth in a TN-C network.
Earth fault loop impedance
Total impedance of the path a fault current takes from the source, through an earth fault and back again.