With a TNC system, the phase(s) + a combined PE/N are fed in from the energy supplier. After the main switch, the N is often fed in separately from the PE/N line, so that in the sub-distribution boards it is installed on the basis of TNS.
With a TNS system, the phase(n)+N+PE are fed in from the energy supplier. The PE is connected to the potential equalisation rail.
In a TT system, the phase(n) + N are fed in from the energy supplier. The low-voltage distributor is earthed separately. This creates an increased potential difference between the earth electrodes in the event of an indirect lightning strike. This potential difference is routed via the N conductor. This is the reason for a 3+1 circuit with extra conductivity between the N- and PE-leads.