TN-C System

An earthing system in which the neutral and protective functions are combined into a single conductor (the PEN conductor, Protective Earthed Neutral) throughout the entire installation, from the supply to the appliance, as opposed to the widely used TN-C-S system, where this combination only applies outside the installation and the PEN is split into separate N and PE conductors before the main intake. Because a break in the PEN conductor under TN-C can directly cause a dangerous rise in potential on all connected exposed-conductive-parts (there is no separate, always-intact PE conductor), NEN 1010 does not permit the use of a pure TN-C system within building installations, restricting its application to specific, fixed industrial supply networks with conductors of sufficient cross-section (typically ≥10 mm² Cu or ≥16 mm² Al) where the PEN is never split further downstream. Furthermore, a TN-C system is not permitted downstream of any point where the PEN has already been split once (at that point it becomes TN-C-S by definition), and an RCD must never be fitted in a pure PEN circuit, since the combined conductor is simultaneously current-carrying and protective.